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

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. )

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kostya Serebryany <kcc@google.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: kasan-dev <kasan-dev@googlegroups.com>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/20150930135917.GA3285@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index d411ca63c8b6ce1f00ba624ace1fe5e0045da554..3a9d65c912e780977c12102d7719a0374241b962 100644 (file)
--- 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 f60f0121e3319ec616d0a0151d3e4df146637c88..eed2050db9be9c7795acd2153f976d4742e2fe82 100644 (file)
--- 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 b25eee4cead5398b69889c95c29480ba9862c397..29595af328669543a70115a7613539a48f6e4226 100644 (file)
--- 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;
 }