Re: [PATCH] signal: Extend exec_id to 64bits

[Jann Horn <jannh@google.com>]

+memory model folks because this seems like a pretty sharp corner

On Wed, Apr 1, 2020 at 10:50 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> Replace the 32bit exec_id with a 64bit exec_id to make it impossible
> to wrap the exec_id counter.  With care an attacker can cause exec_id
> wrap and send arbitrary signals to a newly exec'd parent.  This
> bypasses the signal sending checks if the parent changes their
> credentials during exec.
>
> The severity of this problem can been seen that in my limited testing
> of a 32bit exec_id it can take as little as 19s to exec 65536 times.
> Which means that it can take as little as 14 days to wrap a 32bit
> exec_id.  Adam Zabrocki has succeeded wrapping the self_exe_id in 7
> days.  Even my slower timing is in the uptime of a typical server.
> Which means self_exec_id is simply a speed bump today, and if exec
> gets noticably faster self_exec_id won't even be a speed bump.
>
> Extending self_exec_id to 64bits introduces a problem on 32bit
> architectures where reading self_exec_id is no longer atomic and can
> take two read instructions.  Which means that is is possible to hit
> a window where the read value of exec_id does not match the written
> value.  So with very lucky timing after this change this still
> remains expoiltable.
>
> I have updated the update of exec_id on exec to use WRITE_ONCE
> and the read of exec_id in do_notify_parent to use READ_ONCE
> to make it clear that there is no locking between these two
> locations.
>
> Link: https://lore.kernel.org/kernel-hardening/20200324215049.GA3710@pi3.com.pl
> Fixes: 2.3.23pre2
> Cc: stable@vger.kernel.org
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>
> Linus would you prefer to take this patch directly or I could put it in
> a brach and send you a pull request.
>
>  fs/exec.c             | 2 +-
>  include/linux/sched.h | 4 ++--
>  kernel/signal.c       | 2 +-
>  3 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 0e46ec57fe0a..d55710a36056 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1413,7 +1413,7 @@ void setup_new_exec(struct linux_binprm * bprm)
>
>         /* An exec changes our domain. We are no longer part of the thread
>            group */
> -       current->self_exec_id++;
> +       WRITE_ONCE(current->self_exec_id, current->self_exec_id + 1);

GCC will generate code for this without complaining, but I think it'll
probably generate a tearing store on 32-bit platforms:

$ cat volatile-8.c
typedef unsigned long long u64;
static volatile u64 n;
void blah(void) {
  n = n + 1;
}
$ gcc -O2 -m32 -c -o volatile-8.o volatile-8.c -Wall
$ objdump --disassemble=blah volatile-8.o
[...]
   b: 8b 81 00 00 00 00    mov    0x0(%ecx),%eax
  11: 8b 91 04 00 00 00    mov    0x4(%ecx),%edx
  17: 83 c0 01              add    $0x1,%eax
  1a: 83 d2 00              adc    $0x0,%edx
  1d: 89 81 00 00 00 00    mov    %eax,0x0(%ecx)
  23: 89 91 04 00 00 00    mov    %edx,0x4(%ecx)
[...]
$

You could maybe use atomic64_t to work around that? atomic64_read()
and atomic64_set() should be straightforward READ_ONCE() /
WRITE_ONCE() on 64-bit systems while compiling into something more
complicated on 32-bit.

>         flush_signal_handlers(current, 0);
>  }
>  EXPORT_SYMBOL(setup_new_exec);
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 04278493bf15..0323e4f0982a 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -939,8 +939,8 @@ struct task_struct {
>         struct seccomp                  seccomp;
>
>         /* Thread group tracking: */
> -       u32                             parent_exec_id;
> -       u32                             self_exec_id;
> +       u64                             parent_exec_id;
> +       u64                             self_exec_id;
>
>         /* Protection against (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed, mempolicy: */
>         spinlock_t                      alloc_lock;
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9ad8dea93dbb..5383b562df85 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1926,7 +1926,7 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
>                  * This is only possible if parent == real_parent.
>                  * Check if it has changed security domain.
>                  */
> -               if (tsk->parent_exec_id != tsk->parent->self_exec_id)
> +               if (tsk->parent_exec_id != READ_ONCE(tsk->parent->self_exec_id))
>                         sig = SIGCHLD;
>         }
>
> --
> 2.20.1
>