Patch "fs/proc, core/debug: Don't expose absolute kernel addresses via wchan" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

fs/proc, core/debug: Don't expose absolute kernel addresses via wchan

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fs-proc-core-debug-don-t-expose-absolute-kernel-addresses-via-wchan.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ingo Molnar <[email protected]>
Date: Wed, 30 Sep 2015 15:59:17 +0200
Subject: fs/proc, core/debug: Don't expose absolute kernel addresses via wchan

From: Ingo Molnar <[email protected]>

commit b2f73922d119686323f14fbbe46587f863852328 upstream.

So the /proc/PID/stat 'wchan' field (the 30th field, which contains the absolute kernel address of the kernel function a task is blocked in) leaks absolute kernel addresses to unprivileged user-space:

        seq_put_decimal_ull(m, ' ', wchan);

The absolute address might also leak via /proc/PID/wchan as well, if KALLSYMS is turned off or if the symbol lookup fails for some reason:

static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *task)

{
        unsigned long wchan;
        char symname[KSYM_NAME_LEN];

        wchan = get_wchan(task);

        if (lookup_symbol_name(wchan, symname) < 0) {
                if (!ptrace_may_access(task, PTRACE_MODE_READ))
                        return 0;
                seq_printf(m, "%lu", wchan);
        } else {
                seq_printf(m, "%s", symname);
        }

        return 0;
}

This isn't ideal, because for example it trivially leaks the KASLR offset to any local attacker:

  fomalhaut:~> printf "%016lx\n" $(cat /proc/$$/stat | cut -d' ' -f35)
  ffffffff8123b380

Most real-life uses of wchan are symbolic:

ps -eo pid:10,tid:10,wchan:30,comm

and procps uses /proc/PID/wchan, not the absolute address in /proc/PID/stat:

triton:~/tip> strace -f ps -eo pid:10,tid:10,wchan:30,comm 2>&1 | grep wchan | tail -1
open("/proc/30833/wchan", O_RDONLY) = 6

There's one compatibility quirk here: procps relies on whether the absolute value is non-zero - and we can provide that functionality by outputing "0" or "1" depending on whether the task is blocked (whether there's a wchan address).

These days there appears to be very little legitimate reason user-space would be interested in the absolute address. The absolute address is mostly historic: from the days when we didn't have kallsyms and user-space procps had to do the decoding itself via the System.map.

So this patch sets all numeric output to "0" or "1" and keeps only symbolic output, in /proc/PID/wchan.

( The absolute sleep address can generally still be profiled via
perf, by tasks with sufficient privileges. )

---
 Documentation/filesystems/proc.txt |    5 +++--
 fs/proc/array.c                    |   16 ++++++++++++++--
 fs/proc/base.c                     |    9 +++------
 3 files changed, 20 insertions(+), 10 deletions(-)

@@ -140,7 +140,8 @@ Table 1-1: Process specific entries in /
  stat		Process status
  statm		Process memory status information
  status		Process status in human readable form
- wchan		If CONFIG_KALLSYMS is set, a pre-decoded wchan
+ wchan		Present with CONFIG_KALLSYMS=y: it shows the kernel function
+		symbol the task is blocked in - or "0" if not blocked.
  pagemap	Page table
  stack		Report full stack trace, enable via CONFIG_STACKTRACE
  smaps		a extension based on maps, showing the memory consumption of

@@ -309,7 +310,7 @@ Table 1-4: Contents of the stat files (a
   blocked       bitmap of blocked signals
   sigign        bitmap of ignored signals
   sigcatch      bitmap of caught signals
-  wchan         address where process went to sleep
+  0		(place holder, used to be the wchan address, use /proc/PID/wchan instead)
   0             (place holder)
   0             (place holder)
   exit_signal   signal to send to parent thread on exit

@@ -364,7 +364,7 @@ int proc_pid_status(struct seq_file *m,
 static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 			struct pid *pid, struct task_struct *task, int whole)
 {
-	unsigned long vsize, eip, esp, wchan = ~0UL;
+	unsigned long vsize, eip, esp, wchan = 0;
 	int priority, nice;
 	int tty_pgrp = -1, tty_nr = 0;
 	sigset_t sigign, sigcatch;

@@ -496,7 +496,19 @@ static int do_task_stat(struct seq_file
 	seq_put_decimal_ull(m, ' ', task->blocked.sig[0] & 0x7fffffffUL);
 	seq_put_decimal_ull(m, ' ', sigign.sig[0] & 0x7fffffffUL);
 	seq_put_decimal_ull(m, ' ', sigcatch.sig[0] & 0x7fffffffUL);
-	seq_put_decimal_ull(m, ' ', wchan);
+
+	/*
+	 * We used to output the absolute kernel address, but that's an
+	 * information leak - so instead we show a 0/1 flag here, to signal
+	 * to user-space whether there's a wchan field in /proc/PID/wchan.
+	 *
+	 * This works with older implementations of procps as well.
+	 */
+	if (wchan)
+		seq_puts(m, " 1");
+	else
+		seq_puts(m, " 0");
+
 	seq_put_decimal_ull(m, ' ', 0);
 	seq_put_decimal_ull(m, ' ', 0);
 	seq_put_decimal_ll(m, ' ', task->exit_signal);

@@ -238,13 +238,10 @@ static int proc_pid_wchan(struct seq_fil
 
 	wchan = get_wchan(task);
 
-	if (lookup_symbol_name(wchan, symname) < 0) {
-		if (!ptrace_may_access(task, PTRACE_MODE_READ))
-			return 0;
-		seq_printf(m, "%lu", wchan);
-	} else {
+	if (wchan && ptrace_may_access(task, PTRACE_MODE_READ) && !lookup_symbol_name(wchan, symname))
 		seq_printf(m, "%s", symname);
-	}
+	else
+		seq_putc(m, '0');
 
 	return 0;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/x86-setup-extend-low-identity-map-to-cover-whole-kernel-range.patch queue-4.1/fs-proc-core-debug-don-t-expose-absolute-kernel-addresses-via-wchan.patch