Re: [PATCH v2] fs/proc: Don't expose absolute kernel addresses via wchan

[Kees Cook]

On Wed, Sep 30, 2015 at 6:59 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
>> On Wed, 30 Sep 2015, Ingo Molnar wrote:
>> > diff --git a/fs/proc/array.c b/fs/proc/array.c
>> > index f60f0121e331..99082730b2ac 100644
>> > --- a/fs/proc/array.c
>> > +++ b/fs/proc/array.c
>> > @@ -507,7 +507,7 @@ 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);
>> > +   seq_puts(m, " 0"); /* Used to be numeric wchan - replaced by /proc/PID/wchan */
>>
>> That should get rid of all wchan usage in do_task_stat()
>
> Indeed - updated patch attached.
>
> Thanks,
>
>         Ingo
>
> ================================>
> From 985037cd05b379240dd381b29c2525758c665bb0 Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@xxxxxxxxxx>
> Date: Wed, 30 Sep 2015 09:15:37 +0200
> Subject: [PATCH] fs/proc: Don't expose absolute kernel addresses via wchan
>
> So wchan leaks absolute kernel addresses to unprivileged
> user-space, of kernel  functions that sleep:
>
> 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;
> }
>
> So 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
>
> These days there's 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 and keeps the
> symbolic output in /proc/PID/wchan.
>
> ( The absolute sleep address can generally still be profiled via
>   perf, by tasks with sufficient privileges. )
>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mike Galbraith <efault@xxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: kasan-dev <kasan-dev@xxxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Link: http://lkml.kernel.org/r/20150930071537.GA19048@xxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  fs/proc/array.c | 6 ++----
>  fs/proc/base.c  | 7 +------
>  2 files changed, 3 insertions(+), 10 deletions(-)
>
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index f60f0121e331..ad5ad1e376ad 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;
>         int priority, nice;
>         int tty_pgrp = -1, tty_nr = 0;
>         sigset_t sigign, sigcatch;
> @@ -454,8 +454,6 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>                 unlock_task_sighand(task, &flags);
>         }
>
> -       if (permitted && (!whole || num_threads < 2))
> -               wchan = get_wchan(task);
>         if (!whole) {
>                 min_flt = task->min_flt;
>                 maj_flt = task->maj_flt;
> @@ -507,7 +505,7 @@ 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);
> +       seq_puts(m, " 0"); /* Used to be numeric wchan - replaced by /proc/PID/wchan */

Probably should also update Documentation/filesystems/proc.txt with
something like:

--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -310,7 +310,7 @@ 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, was wchan, see /proc/PID/wchan instead)
   0             (place holder)
   0             (place holder)
   exit_signal   signal to send to parent thread on exit

>         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..2fdbf303e3eb 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -430,13 +430,8 @@ 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 (!lookup_symbol_name(wchan, symname))
>                 seq_printf(m, "%s", symname);
> -       }
>
>         return 0;
>  }

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

-- 
Kees Cook
Chrome OS Security
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