monitor: report entirety of hmp command on error

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info skeys", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help skeys" for more information", which is incorrect).

Let's correct this by capturing the full name of the command as we recurse through the function monitor_parse_command.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
  monitor.c | 15 +++++++++++----
  1 file changed, 11 insertions(+), 4 deletions(-)

monitor.c +0 -0

index 39f8ee1..d4844b4 100644

Quoted 6 lines · Collin Walling

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info skeys", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help skeys" for more information", which is incorrect).

What's the exact formula for reproducing this? I tried:

$ ./x86_64-softmmu/qemu-system-x86_64 -nodefaults -nographic --monitor stdio
QEMU 2.12.50 monitor - type 'help' for more information
(qemu) info skeys
unknown command: 'info skeys'

Oh, I see now:

(qemu) info uuid blah
uuid: extraneous characters at the end of line
Try "help uuid" for more information

(qemu) help uuid
(qemu)

You'll want to update your commit message to document something that is reproducible (you may be adding an 'info skeys', but until that is in, it doesn't make a good example).

Quoted 15 lines

Let's correct this by capturing the full name of the command as we recurse through the function monitor_parse_command.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
  monitor.c | 15 +++++++++++----
  1 file changed, 11 insertions(+), 4 deletions(-)

monitor.c +0 -0

index 39f8ee1..d4844b4 100644

monitor.c static const mon_cmd_t *search_dispatch_table(const mon_cmd_t *disp_table, · line 2968

Quoted 6 lines

monitor.c +2 -1

@@ -2964,7 +2964,8 @@ static const mon_cmd_t *search_dispatch_table(const mon_cmd_t *disp_table,
  static const mon_cmd_t *monitor_parse_command(Monitor *mon,
                                                const char *cmdp_start,
                                                const char **cmdp,
-                                              mon_cmd_t *table)
+                                              mon_cmd_t *table,
+                                              char *fullname)

Umm, how is fullname any better than cmdp_start that we already have?

monitor.c static const mon_cmd_t *monitor_parse_command(Monitor *mon, · line 2990

Quoted 7 lines

monitor.c +1 -0

  {
      const char *p;
      const mon_cmd_t *cmd;

@@ -2987,10 +2988,14 @@ static const mon_cmd_t *monitor_parse_command(Monitor *mon,
          p++;
      }
+    strncat(fullname, cmdname, strlen(cmdname));

gcc 8 is pickier about using strncat() [perhaps too picky - see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602], but it is generally NOT the function you want to be using.

monitor.c static const mon_cmd_t *monitor_parse_command(Monitor *mon, · line 2997

Quoted 8 lines

monitor.c +4 -1

+
      *cmdp = p;
      /* search sub command */
      if (cmd->sub_table != NULL && *p != '\0') {
-        return monitor_parse_command(mon, cmdp_start, cmdp, cmd->sub_table);
+        strncat(fullname, " ", 1);
+        return monitor_parse_command(mon, cmdp_start, cmdp, cmd->sub_table,
+                                     fullname);

See, you're reconstructing a command into fullname, which already matches the original command in cmdp_start, so I see no reason to change the signature.

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3379

Quoted 7 lines

monitor.c +1 -0

      }
      return cmd;

@@ -3371,10 +3376,12 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
  {
      QDict *qdict;
      const mon_cmd_t *cmd;
+    char fullname[256];

EWWW. Don't do that. You are just ASKING for a buffer overflow exploit that prints the wrong thing or causes a security hole, when I intentionally type a super-long garbage command into HMP.

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3382

Quoted 4 lines

monitor.c +2 -1

      trace_handle_hmp_command(mon, cmdline);
-    cmd = monitor_parse_command(mon, cmdline, &cmdline, mon->cmd_table);
+    cmd = monitor_parse_command(mon, cmdline, &cmdline, mon->cmd_table,
+                                fullname);

Note that even without your patch, this call updates 'cmdline' to point to the position within the original string (although that position has already skipped spaces).

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3392

Quoted 9 lines

monitor.c +1 -1

      if (!cmd) {
          return;
      }

@@ -3382,7 +3389,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
      qdict = monitor_parse_arguments(mon, &cmdline, cmd);
      if (!qdict) {
          monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+                       fullname);

So rather than trying to reconstruct a string, you could reuse what you already have. This is a shorter patch that I think accomplishes the same goal:

monitor.c +6 -2

index 39f8ee17ba7..38736b3a20d 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const
 char *cmdline)
 {
     QDict *qdict;
     const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
     trace_handle_hmp_command(mon, cmdline);

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const
 char *cmdline)
 
     qdict = monitor_parse_arguments(mon, &cmdline, cmd);
     if (!qdict) {
-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
         return;
     }

Signature · 4 lines

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Quoted 6 lines · Collin Walling

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info skeys", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help skeys" for more information", which is incorrect).

What's the exact formula for reproducing this?  I tried:

$ ./x86_64-softmmu/qemu-system-x86_64 -nodefaults -nographic --monitor stdio
QEMU 2.12.50 monitor - type 'help' for more information
(qemu) info skeys
unknown command: 'info skeys'

Oh, I see now:

(qemu) info uuid blah
uuid: extraneous characters at the end of line
Try "help uuid" for more information
(qemu) help uuid
(qemu)

You'll want to update your commit message to document something that is reproducible (you may be adding an 'info skeys', but until that is in, it doesn't make a good example).

Quoted 15 lines

Let's correct this by capturing the full name of the command as we recurse through the function monitor_parse_command.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
  monitor.c | 15 +++++++++++----

  1 file changed, 11 insertions(+), 4 deletions(-)

monitor.c +0 -0

index 39f8ee1..d4844b4 100644

monitor.c static const mon_cmd_t *search_dispatch_table(const mon_cmd_t *disp_table, · line 2968

Quoted 6 lines

monitor.c +2 -1

@@ -2964,7 +2964,8 @@ static const mon_cmd_t *search_dispatch_table(const mon_cmd_t *disp_table,
   static const mon_cmd_t *monitor_parse_command(Monitor *mon,
                                                 const char *cmdp_start,
                                                 const char **cmdp,
-                                              mon_cmd_t *table)
+                                              mon_cmd_t *table,
+                                              char *fullname)

Umm, how is fullname any better than cmdp_start that we already have?

monitor.c static const mon_cmd_t *monitor_parse_command(Monitor *mon, · line 2990

Quoted 7 lines

monitor.c +0 -0

   {
       const char *p;
       const mon_cmd_t *cmd;

@@ -2987,10 +2988,14 @@ static const mon_cmd_t *monitor_parse_command(Monitor *mon,
           p++;
       }
   +    strncat(fullname, cmdname, strlen(cmdname));

gcc 8 is pickier about using strncat() [perhaps too picky - see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602], but it is generally NOT the function you want to be using.

monitor.c static const mon_cmd_t *monitor_parse_command(Monitor *mon, · line 2997

Quoted 8 lines

monitor.c +4 -1

+
       *cmdp = p;
       /* search sub command */
       if (cmd->sub_table != NULL && *p != '\0') {
-        return monitor_parse_command(mon, cmdp_start, cmdp, cmd->sub_table);
+        strncat(fullname, " ", 1);
+        return monitor_parse_command(mon, cmdp_start, cmdp, cmd->sub_table,
+                                     fullname);

See, you're reconstructing a command into fullname, which already matches the original command in cmdp_start, so I see no reason to change the signature.

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3379

Quoted 7 lines

monitor.c +1 -0

       }
         return cmd;

@@ -3371,10 +3376,12 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
   {
       QDict *qdict;
       const mon_cmd_t *cmd;
+    char fullname[256];

EWWW. Don't do that.  You are just ASKING for a buffer overflow exploit that prints the wrong thing or causes a security hole, when I intentionally type a super-long garbage command into HMP.

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3383

Quoted 4 lines

monitor.c +2 -0

         trace_handle_hmp_command(mon, cmdline);
   -    cmd = monitor_parse_command(mon, cmdline, &cmdline, mon->cmd_table);
+    cmd = monitor_parse_command(mon, cmdline, &cmdline, mon->cmd_table,
+                                fullname);

Note that even without your patch, this call updates 'cmdline' to point to the position within the original string (although that position has already skipped spaces).

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3392

Quoted 9 lines

monitor.c +1 -1

       if (!cmd) {
           return;
       }

@@ -3382,7 +3389,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
       qdict = monitor_parse_arguments(mon, &cmdline, cmd);
       if (!qdict) {
           monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+                       fullname);

So rather than trying to reconstruct a string, you could reuse what you already have.  This is a shorter patch that I think accomplishes the same goal:

monitor.c +1 -0

index 39f8ee17ba7..38736b3a20d 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
  {
      QDict *qdict;
      const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
      trace_handle_hmp_command(mon, cmdline);
 

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 
      qdict = monitor_parse_arguments(mon, &cmdline, cmd);
      if (!qdict) {

Quoted 20 lines

So rather than trying to reconstruct a string, you could reuse what you already have.  This is a shorter patch that I think accomplishes the same goal:

monitor.c +1 -0

index 39f8ee17ba7..38736b3a20d 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
  {
      QDict *qdict;
      const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
      trace_handle_hmp_command(mon, cmdline);
 

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 
      qdict = monitor_parse_arguments(mon, &cmdline, cmd);
      if (!qdict) {

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3392

Quoted 10 lines

monitor.c +5 -2

-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
          return;
      }
 

Very interesting... you managed to reuse what was in cmdline without printing anything extraneous that the user might have provided... nicely done!

Your print statement is intriguing to me... I'm not entirely sure how it works.

How would you like to move forward with this patch?

Quoted 34 lines · Collin Walling

Quoted 20 lines

So rather than trying to reconstruct a string, you could reuse what you already have.  This is a shorter patch that I think accomplishes the same goal:

monitor.c +1 -0

index 39f8ee17ba7..38736b3a20d 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
   {
       QDict *qdict;
       const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
       trace_handle_hmp_command(mon, cmdline);
 

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 
       qdict = monitor_parse_arguments(mon, &cmdline, cmd);
       if (!qdict) {

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3392

Quoted 10 lines

monitor.c +5 -2

-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
           return;
       }
 

Very interesting... you managed to reuse what was in cmdline without printing anything extraneous that the user might have provided... nicely done!

Quoted 1 line

Your print statement is intriguing to me... I'm not entirely sure how it works.

The format specifiers in printf are %[flags][width][.precision]format. So I'm requesting a precision-limited string print (which says the maximum number of characters to print, rather than the usual semantics of printing until the trailing NUL is found), and the precision of .* (instead of a more typical .1 or similar) says that the precision will be an int argument to printf rather than inline (the width argument can also be passed via *).  The cast to int is annoyingly part of the specification (subtracting two pointers within a string results in a ptrdiff_t, but on 64-bit platforms, ptrdiff_t and int are not equally handled through vararg functions).  And that exact style of printf() magic was already in use in monitor_parse_command() that your patch attempt was touching (see where cmdp_start is used).  And if you really want weird, 'man 3 printf' states that "%.*s" is equivalent to "%2$.*1$s" (the $ syntax is for cases cases where you need to consume printf arguments out-of-order, often when dealing with translated strings).

Quoted 2 lines

How would you like to move forward with this patch?

I'm more than happy to let you post a v2 of the patch, incorporating the ideas you just learned from me.  (Or, if you do nothing, then in a week or so, I'll probably notice the patch is still sitting unapplied in my local repository and submit it myself - but then I wouldn't be helping the qemu community grow...)

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info uuid xyz", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help uuid" for more information", which is incorrect).

Let's correct this by capturing the entirety of the command from the command line -- excluding any extraneous characters.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
  monitor.c | 8 ++++++--
  1 file changed, 6 insertions(+), 2 deletions(-)

Quoted 10 lines · Collin Walling

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info uuid xyz", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help uuid" for more information", which is incorrect).

Let's correct this by capturing the entirety of the command from the command line -- excluding any extraneous characters.

It's better to post a v2 patch as a new top-level thread instead of in-reply to an earlier version, as some of our automated tooling is more likely to see it.

Quoted 5 lines

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
  monitor.c | 8 ++++++--

  1 file changed, 6 insertions(+), 2 deletions(-)

Reviewed-by Eric Blake <[email protected]>

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info uuid xyz", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help uuid" for more information", which is incorrect).

Let's correct this by capturing the entirety of the command from the command line -- excluding any extraneous characters.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
 monitor.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

monitor.c +6 -2

index 39f8ee1..38736b3 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 {
     QDict *qdict;
     const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
     trace_handle_hmp_command(mon, cmdline);
 

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 
     qdict = monitor_parse_arguments(mon, &cmdline, cmd);
     if (!qdict) {
-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
         return;
     }

David, looks like your turf.

Quoted 30 lines · Collin Walling

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info uuid xyz", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help uuid" for more information", which is incorrect).

Let's correct this by capturing the entirety of the command from the command line -- excluding any extraneous characters.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
 monitor.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

monitor.c +1 -0

index 39f8ee1..38736b3 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 {
     QDict *qdict;
     const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
     trace_handle_hmp_command(mon, cmdline);
 

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 

monitor.c static void handle_hmp_command(Monitor *mon, const char *cmdline) · line 3391

Quoted 11 lines

monitor.c +5 -2

     qdict = monitor_parse_arguments(mon, &cmdline, cmd);
     if (!qdict) {
-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
         return;
     }

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info uuid xyz", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help uuid" for more information", which is incorrect).

Let's correct this by capturing the entirety of the command from the command line -- excluding any extraneous characters.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

---
 monitor.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

monitor.c +6 -2

index 39f8ee1..38736b3 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 {
     QDict *qdict;
     const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
     trace_handle_hmp_command(mon, cmdline);
 

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 
     qdict = monitor_parse_arguments(mon, &cmdline, cmd);
     if (!qdict) {
-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
         return;
     }
 

Signature · 1 line

2.7.4

Quoted 7 lines · * Collin Walling ([email protected]) wrote:

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info uuid xyz", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help uuid" for more information", which is incorrect).

Quoted 5 lines

Let's correct this by capturing the entirety of the command from the command line -- excluding any extraneous characters.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>

Queued

Quoted 35 lines

---
 monitor.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

monitor.c +6 -2

index 39f8ee1..38736b3 100644
@@ -3371,6 +3371,7 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 {
     QDict *qdict;
     const mon_cmd_t *cmd;
+    const char *cmd_start = cmdline;
 
     trace_handle_hmp_command(mon, cmdline);
 

@@ -3381,8 +3382,11 @@ static void handle_hmp_command(Monitor *mon, const char *cmdline)
 
     qdict = monitor_parse_arguments(mon, &cmdline, cmd);
     if (!qdict) {
-        monitor_printf(mon, "Try \"help %s\" for more information\n",
-                       cmd->name);
+        while (cmdline > cmd_start && qemu_isspace(cmdline[-1])) {
+            cmdline--;
+        }
+        monitor_printf(mon, "Try \"help %.*s\" for more information\n",
+                       (int)(cmdline - cmd_start), cmd_start);
         return;
     }
 

Signature · 1 line

2.7.4

--
Dr. David Alan Gilbert / [email protected] / Manchester, UK

When a user incorrectly provides an hmp command, an error response will be printed that prompts the user to try "help <command name>". However, when the command contains multiple parts e.g. "info skeys", only the last whitespace delimited string will be reported (in this example "info" will be dropped and the message will read "Try "help skeys" for more information", which is incorrect).

Let's correct this by capturing the full name of the command as we recurse through the function monitor_parse_command.

Reported-by Mikhail Fokin <[email protected]>

Signed-off-by Collin Walling <[email protected]>