exec: cleanup search_binary_handler()

PATCH New Oleg Nesterov 1 participant 9 messages Aug 5, 2013

Subsystems (2)

VFS exec and binfmt

lore

Share & tools

lore

Oleg Nesterov

author

2013-08-05 13:46 UTC #0 lore raw

v2 cover exec: cleanup search_binary_handler()

To remind, by discussion with Zach

fs-binfmts-add-a-name-field-to-the-binfmt-struct.patch fs-binfmts-better-handling-of-binfmt-loops.patch fs-binfmts-whitespace-fixes-with-scripts-cleanfile.patch

should be dropped. Zach is working on the new version on top of these changes.

Changes:

2/8: add the comment about recursion_depth

4/8: nullify bprm->file to catch the potential use after free

Add the acks from Kees (thanks!)

Oleg.

 fs/exec.c               |  120 +++++++++++++++++++++++------------------------
 include/linux/binfmts.h |    2 +-
 2 files changed, 60 insertions(+), 62 deletions(-)

Oleg Nesterov

author

2013-08-05 13:47 UTC #1 lore raw

PATCH v2 1/8 exec: introduce exec_binprm() for "depth == 0" code

task_pid_nr_ns() and trace/ptrace code in the middle of the recursive search_binary_handler() looks confusing and imho annoying. We only need this code if "depth == 0", lets add a simple helper which calls search_binary_handler() and does trace_sched_process_exec() + ptrace_event().

The patch also moves the setting of task->did_exec, we need to do this only once.

Note: we can kill either task->did_exec or PF_FORKNOEXEC.

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c |   36 ++++++++++++++++++++++--------------
 1 files changed, 22 insertions(+), 14 deletions(-)

fs/exec.c +22 ~~-14~~

index 9c73def..a9ae4f2 100644
@@ -1373,7 +1373,6 @@ int search_binary_handler(struct linux_binprm *bprm)
 	unsigned int depth = bprm->recursion_depth;
 	int try,retval;
 	struct linux_binfmt *fmt;
-	pid_t old_pid, old_vpid;
 
 	/* This allows 4 levels of binfmt rewrites before failing hard. */
 	if (depth > 5)

@@ -1387,12 +1386,6 @@ int search_binary_handler(struct linux_binprm *bprm)
 	if (retval)
 		return retval;
 
-	/* Need to fetch pid before load_binary changes it */
-	old_pid = current->pid;
-	rcu_read_lock();
-	old_vpid = task_pid_nr_ns(current, task_active_pid_ns(current->parent));
-	rcu_read_unlock();
-
 	retval = -ENOENT;
 	for (try=0; try<2; try++) {
 		read_lock(&binfmt_lock);

@@ -1407,16 +1400,11 @@ int search_binary_handler(struct linux_binprm *bprm)
 			retval = fn(bprm);
 			bprm->recursion_depth = depth;
 			if (retval >= 0) {
-				if (depth == 0) {
-					trace_sched_process_exec(current, old_pid, bprm);
-					ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
-				}
 				put_binfmt(fmt);
 				allow_write_access(bprm->file);
 				if (bprm->file)
 					fput(bprm->file);
 				bprm->file = NULL;
-				current->did_exec = 1;
 				proc_exec_connector(current);
 				return retval;
 			}

@@ -1450,9 +1438,29 @@ int search_binary_handler(struct linux_binprm *bprm)
 	}
 	return retval;
 }
-
 EXPORT_SYMBOL(search_binary_handler);
 
+static int exec_binprm(struct linux_binprm *bprm)
+{
+	pid_t old_pid, old_vpid;
+	int ret;
+
+	/* Need to fetch pid before load_binary changes it */
+	old_pid = current->pid;
+	rcu_read_lock();
+	old_vpid = task_pid_nr_ns(current, task_active_pid_ns(current->parent));
+	rcu_read_unlock();
+
+	ret = search_binary_handler(bprm);
+	if (ret >= 0) {
+		trace_sched_process_exec(current, old_pid, bprm);
+		ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
+		current->did_exec = 1;
+	}
+
+	return ret;
+}
+
 /*
  * sys_execve() executes a new program.
  */

@@ -1541,7 +1549,7 @@ static int do_execve_common(const char *filename,
 	if (retval < 0)
 		goto out;
 
-	retval = search_binary_handler(bprm);
+	retval = exec_binprm(bprm);
 	if (retval < 0)
 		goto out;

Signature · 1 line

1.5.5.1

Oleg Nesterov

author

2013-08-05 13:47 UTC #2 lore raw

PATCH v2 2/8 exec: kill "int depth" in search_binary_handler()

In reply to Oleg Nesterov

Nobody except search_binary_handler() should touch ->recursion_depth, "int depth" buys nothing but complicates the code, kill it.

Probably we should also kill "fn" and the !NULL check, ->load_binary should be always defined. And it can not go away after read_unlock() or this code is buggy anyway.

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c               |    9 ++++-----
 include/linux/binfmts.h |    2 +-
 2 files changed, 5 insertions(+), 6 deletions(-)

fs/exec.c +4 -5

index a9ae4f2..f32079c 100644
@@ -1370,12 +1370,11 @@ EXPORT_SYMBOL(remove_arg_zero);
  */
 int search_binary_handler(struct linux_binprm *bprm)
 {
-	unsigned int depth = bprm->recursion_depth;
-	int try,retval;
+	int try, retval;
 	struct linux_binfmt *fmt;
 
 	/* This allows 4 levels of binfmt rewrites before failing hard. */
-	if (depth > 5)
+	if (bprm->recursion_depth > 5)
 		return -ELOOP;
 
 	retval = security_bprm_check(bprm);

@@ -1396,9 +1395,9 @@ int search_binary_handler(struct linux_binprm *bprm)
 			if (!try_module_get(fmt->module))
 				continue;
 			read_unlock(&binfmt_lock);
-			bprm->recursion_depth = depth + 1;
+			bprm->recursion_depth++;
 			retval = fn(bprm);
-			bprm->recursion_depth = depth;
+			bprm->recursion_depth--;
 			if (retval >= 0) {
 				put_binfmt(fmt);
 				allow_write_access(bprm->file);

include/linux/binfmts.h +1 -1

index 70cf138..e8112ae 100644
@@ -31,7 +31,7 @@ struct linux_binprm {
 #ifdef __alpha__
 	unsigned int taso:1;
 #endif
-	unsigned int recursion_depth;
+	unsigned int recursion_depth; /* only for search_binary_handler() */
 	struct file * file;
 	struct cred *cred;	/* new credentials */
 	int unsafe;		/* how unsafe this exec is (mask of LSM_UNSAFE_*) */

Signature · 1 line

1.5.5.1

Oleg Nesterov

author

2013-08-05 13:48 UTC #3 lore raw

PATCH v2 3/8 exec: proc_exec_connector() should be called only once

In reply to Oleg Nesterov

A separate one-liner with the minor fix.

PROC_EVENT_EXEC reports the "exec" event, but this message is sent at least twice if search_binary_handler() is called by ->load_binary() recursively, say, load_script().

Move it to exec_binprm(), this is "depth == 0" code too.

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

fs/exec.c +1 -1

index f32079c..ad7d624 100644
@@ -1404,7 +1404,6 @@ int search_binary_handler(struct linux_binprm *bprm)
 				if (bprm->file)
 					fput(bprm->file);
 				bprm->file = NULL;
-				proc_exec_connector(current);
 				return retval;
 			}
 			read_lock(&binfmt_lock);

@@ -1455,6 +1454,7 @@ static int exec_binprm(struct linux_binprm *bprm)
 		trace_sched_process_exec(current, old_pid, bprm);
 		ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
 		current->did_exec = 1;
+		proc_exec_connector(current);
 	}
 
 	return ret;

Signature · 1 line

1.5.5.1

Oleg Nesterov

author

2013-08-05 13:47 UTC #4 lore raw

PATCH v2 4/8 exec: move allow_write_access/fput to exec_binprm()

In reply to Oleg Nesterov

When search_binary_handler() succeeds it does allow_write_access() and fput(), then it clears bprm->file to ensure the caller will not do the same.

We can simply move this code to exec_binprm() which is called only once. In fact we could move this to free_bprm() and remove the same code in do_execve_common's error path.

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

fs/exec.c +6 -4

index ad7d624..577e98b 100644
@@ -1400,10 +1400,6 @@ int search_binary_handler(struct linux_binprm *bprm)
 			bprm->recursion_depth--;
 			if (retval >= 0) {
 				put_binfmt(fmt);
-				allow_write_access(bprm->file);
-				if (bprm->file)
-					fput(bprm->file);
-				bprm->file = NULL;
 				return retval;
 			}
 			read_lock(&binfmt_lock);

@@ -1455,6 +1451,12 @@ static int exec_binprm(struct linux_binprm *bprm)
 		ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
 		current->did_exec = 1;
 		proc_exec_connector(current);
+
+		if (bprm->file) {
+			allow_write_access(bprm->file);
+			fput(bprm->file);
+			bprm->file = NULL; /* to catch use-after-free */
+		}
 	}
 
 	return ret;

Signature · 1 line

1.5.5.1

Oleg Nesterov

author

2013-08-05 13:47 UTC #5 lore raw

PATCH v2 5/8 exec: kill ->load_binary != NULL check in search_binary_handler()

In reply to Oleg Nesterov

search_binary_handler() checks ->load_binary != NULL for no reason, this method should be always defined. Turn this check into WARN_ON() and move it into __register_binfmt().

Also, kill the function pointer. The current code looks confusing, as if ->load_binary can go away after read_unlock(&binfmt_lock). But we rely on module_get(fmt->module), this fmt can't be changed or unregistered, otherwise this code is buggy anyway.

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c |    7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)

fs/exec.c +3 -4

index 577e98b..f0112f9 100644
@@ -74,6 +74,8 @@ static DEFINE_RWLOCK(binfmt_lock);
 void __register_binfmt(struct linux_binfmt * fmt, int insert)
 {
 	BUG_ON(!fmt);
+	if (WARN_ON(!fmt->load_binary))
+		return;
 	write_lock(&binfmt_lock);
 	insert ? list_add(&fmt->lh, &formats) :
 		 list_add_tail(&fmt->lh, &formats);

@@ -1389,14 +1391,11 @@ int search_binary_handler(struct linux_binprm *bprm)
 	for (try=0; try<2; try++) {
 		read_lock(&binfmt_lock);
 		list_for_each_entry(fmt, &formats, lh) {
-			int (*fn)(struct linux_binprm *) = fmt->load_binary;
-			if (!fn)
-				continue;
 			if (!try_module_get(fmt->module))
 				continue;
 			read_unlock(&binfmt_lock);
 			bprm->recursion_depth++;
-			retval = fn(bprm);
+			retval = fmt->load_binary(bprm);
 			bprm->recursion_depth--;
 			if (retval >= 0) {
 				put_binfmt(fmt);

Signature · 1 line

1.5.5.1

Oleg Nesterov

author

2013-08-05 13:47 UTC #6 lore raw

PATCH v2 6/8 exec: cleanup the CONFIG_MODULES logic

In reply to Oleg Nesterov

search_binary_handler() uses "for (try=0; try<2; try++)" to avoid "goto" but the code looks too complicated and horrible imho. We still need to check "try == 0" before request_module() and add the additional "break" for !CONFIG_MODULES case.

Kill this loop and use a simple "bool need_retry" + "goto retry". The code looks much simpler and we do not even need ifdef's, gcc can optimize out the "if (need_retry)" block if !IS_ENABLED().

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c |   68 +++++++++++++++++++++++++++---------------------------------
 1 files changed, 31 insertions(+), 37 deletions(-)

fs/exec.c +31 ~~-37~~

index f0112f9..f69cf54 100644
@@ -1367,13 +1367,15 @@ out:
 }
 EXPORT_SYMBOL(remove_arg_zero);
 
+#define printable(c) (((c)=='\t') || ((c)=='\n') || (0x20<=(c) && (c)<=0x7e))
 /*
  * cycle the list of binary formats handler, until one recognizes the image
  */
 int search_binary_handler(struct linux_binprm *bprm)
 {
-	int try, retval;
+	bool need_retry = IS_ENABLED(CONFIG_MODULES);
 	struct linux_binfmt *fmt;
+	int retval;
 
 	/* This allows 4 levels of binfmt rewrites before failing hard. */
 	if (bprm->recursion_depth > 5)

@@ -1388,47 +1390,39 @@ int search_binary_handler(struct linux_binprm *bprm)
 		return retval;
 
 	retval = -ENOENT;
-	for (try=0; try<2; try++) {
-		read_lock(&binfmt_lock);
-		list_for_each_entry(fmt, &formats, lh) {
-			if (!try_module_get(fmt->module))
-				continue;
-			read_unlock(&binfmt_lock);
-			bprm->recursion_depth++;
-			retval = fmt->load_binary(bprm);
-			bprm->recursion_depth--;
-			if (retval >= 0) {
-				put_binfmt(fmt);
-				return retval;
-			}
-			read_lock(&binfmt_lock);
+ retry:
+	read_lock(&binfmt_lock);
+	list_for_each_entry(fmt, &formats, lh) {
+		if (!try_module_get(fmt->module))
+			continue;
+		read_unlock(&binfmt_lock);
+		bprm->recursion_depth++;
+		retval = fmt->load_binary(bprm);
+		bprm->recursion_depth--;
+		if (retval >= 0) {
 			put_binfmt(fmt);
-			if (retval != -ENOEXEC || bprm->mm == NULL)
-				break;
-			if (!bprm->file) {
-				read_unlock(&binfmt_lock);
-				return retval;
-			}
+			return retval;
 		}
-		read_unlock(&binfmt_lock);
-#ifdef CONFIG_MODULES
-		if (retval != -ENOEXEC || bprm->mm == NULL) {
+		read_lock(&binfmt_lock);
+		put_binfmt(fmt);
+		if (retval != -ENOEXEC || bprm->mm == NULL)
 			break;
-		} else {
-#define printable(c) (((c)=='\t') || ((c)=='\n') || (0x20<=(c) && (c)<=0x7e))
-			if (printable(bprm->buf[0]) &&
-			    printable(bprm->buf[1]) &&
-			    printable(bprm->buf[2]) &&
-			    printable(bprm->buf[3]))
-				break; /* -ENOEXEC */
-			if (try)
-				break; /* -ENOEXEC */
-			request_module("binfmt-%04x", *(unsigned short *)(&bprm->buf[2]));
+		if (!bprm->file) {
+			read_unlock(&binfmt_lock);
+			return retval;
 		}
-#else
-		break;
-#endif
 	}
+	read_unlock(&binfmt_lock);
+
+	if (need_retry && retval == -ENOEXEC && bprm->mm) {
+		if (printable(bprm->buf[0]) && printable(bprm->buf[1]) &&
+		    printable(bprm->buf[2]) && printable(bprm->buf[3]))
+			return retval;
+		request_module("binfmt-%04x", *(ushort *)(bprm->buf + 2));
+		need_retry = false;
+		goto retry;
+	}
+
 	return retval;
 }
 EXPORT_SYMBOL(search_binary_handler);

Signature · 1 line

1.5.5.1

Oleg Nesterov

author

2013-08-05 13:47 UTC #7 lore raw

PATCH v2 7/8 exec: don't retry if request_module() fails

In reply to Oleg Nesterov

A separate one-liner for better documentation.

It doesn't make sense to retry if request_module() fails to exec /sbin/modprobe, add the additional "request_module() < 0" check.

However, this logic still doesn't look exactly right:

1. It would be better to check "request_module() != 0", the user space modprobe process should report the correct exit code. But I didn't dare to add the user-visible change.

2. The whole ENOEXEC logic looks suboptimal. Suppose that we try to exec a "#!path-to-unsupported-binary" script. In this case request_module() + "retry" will be done twice: first by the "depth == 1" code, and then again by the "depth == 0" caller which doesn't make sense.

3. And note that in the case above bprm->buf was already changed by load_script()->prepare_binprm(), so this looks even more ugly.

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

fs/exec.c +2 -1

index f69cf54..682895d 100644
@@ -1418,7 +1418,8 @@ int search_binary_handler(struct linux_binprm *bprm)
 		if (printable(bprm->buf[0]) && printable(bprm->buf[1]) &&
 		    printable(bprm->buf[2]) && printable(bprm->buf[3]))
 			return retval;
-		request_module("binfmt-%04x", *(ushort *)(bprm->buf + 2));
+		if (request_module("binfmt-%04x", *(ushort *)(bprm->buf + 2)) < 0)
+			return retval;
 		need_retry = false;
 		goto retry;
 	}

Signature · 1 line

1.5.5.1

Oleg Nesterov

author

2013-08-05 13:48 UTC #8 lore raw

PATCH v2 8/8 exec: cleanup the error handling in search_binary_handler()

In reply to Oleg Nesterov

The error hanling and ret-from-loop look confusing and inconsistent.

- "retval >= 0" simply returns

- "!bprm->file" returns too but with read_unlock() because binfmt_lock was already re-acquired

- "retval != -ENOEXEC || bprm->mm == NULL" does "break" and relies on the same check after the main loop

Consolidate these checks into a single if/return statement.

need_retry still checks "retval == -ENOEXEC", but this and -ENOENT before the main loop are not needed. This is only for pathological and impossible list_empty(&formats) case.

It is not clear why do we check "bprm->mm == NULL", probably this should be removed.

Signed-off-by Oleg Nesterov <[email protected]>

Acked-by Kees Cook <[email protected]>

---
 fs/exec.c |   11 +++--------
 1 files changed, 3 insertions(+), 8 deletions(-)

fs/exec.c +3 -8

index 682895d..eb2f05a 100644
@@ -1399,22 +1399,17 @@ int search_binary_handler(struct linux_binprm *bprm)
 		bprm->recursion_depth++;
 		retval = fmt->load_binary(bprm);
 		bprm->recursion_depth--;
-		if (retval >= 0) {
+		if (retval >= 0 || retval != -ENOEXEC ||
+		    bprm->mm == NULL || bprm->file == NULL) {
 			put_binfmt(fmt);
 			return retval;
 		}
 		read_lock(&binfmt_lock);
 		put_binfmt(fmt);
-		if (retval != -ENOEXEC || bprm->mm == NULL)
-			break;
-		if (!bprm->file) {
-			read_unlock(&binfmt_lock);
-			return retval;
-		}
 	}
 	read_unlock(&binfmt_lock);
 
-	if (need_retry && retval == -ENOEXEC && bprm->mm) {
+	if (need_retry && retval == -ENOEXEC) {
 		if (printable(bprm->buf[0]) && printable(bprm->buf[1]) &&
 		    printable(bprm->buf[2]) && printable(bprm->buf[3]))
 			return retval;

Signature · 1 line

1.5.5.1