linux-kernel - [GIT PULL] procfs: Don't expose absolute kernel addresses via /proc/PID/stat

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Message-ID: <20151103085236.GA21868@gmail.com>
Date:	Tue, 3 Nov 2015 09:52:36 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [GIT PULL] procfs: Don't expose absolute kernel addresses via
 /proc/PID/stat

Linus,

Please pull this single commit from:

   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-debug-for-linus

   # HEAD: b2f73922d119686323f14fbbe46587f863852328 fs/proc, core/debug: Don't expose absolute kernel addresses via wchan

This fixes a wchan related information leak in /proc/PID/stat.

There's a bit of an ABI twist to it: instead of setting the wchan field to 0 
(which is our usual technique) we set it conditionally to a 0/1 flag to keep ABI 
compatibility with older procps versions that only fetches /proc/PID/wchan 
(symbolic names) if the absolute wchan address is nonzero.

 Thanks,

	Ingo

------------------>
Ingo Molnar (1):
      fs/proc, core/debug: Don't expose absolute kernel addresses via wchan


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

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index d411ca63c8b6..3a9d65c912e7 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -140,7 +140,8 @@ Table 1-1: Process specific entries in /proc
  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
@@ -310,7 +311,7 @@ Table 1-4: Contents of the stat files (as of 2.6.30-rc7)
   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
diff --git a/fs/proc/array.c b/fs/proc/array.c
index f60f0121e331..eed2050db9be 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -375,7 +375,7 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
 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;
@@ -507,7 +507,19 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	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);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index b25eee4cead5..29595af32866 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -430,13 +430,10 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
 
 	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;
 }
--
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