Re: [PATCH] perf: Fix race in perf_mmap_close function

[Namhyung Kim]

On Thu, Sep 10, 2020 at 7:42 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> There's a possible race in perf_mmap_close when checking ring buffer's
> mmap_count refcount value. The problem is that the mmap_count check is
> not atomic because we call atomic_dec and atomic_read separately.
>
>   perf_mmap_close:
>   ...
>    atomic_dec(&rb->mmap_count);
>    ...
>    if (atomic_read(&rb->mmap_count))
>       goto out_put;
>
>    <ring buffer detach>
>    free_uid
>
> out_put:
>   ring_buffer_put(rb); /* could be last */
>
> The race can happen when we have two (or more) events sharing same ring
> buffer and they go through atomic_dec and then they both see 0 as refcount
> value later in atomic_read. Then both will go on and execute code which
> is meant to be run just once.
>
> The code that detaches ring buffer is probably fine to be executed more
> than once, but the problem is in calling free_uid, which will later on
> demonstrate in related crashes and refcount warnings, like:
>
>   refcount_t: addition on 0; use-after-free.
>   ...
>   RIP: 0010:refcount_warn_saturate+0x6d/0xf
>   ...
>   Call Trace:
>   prepare_creds+0x190/0x1e0
>   copy_creds+0x35/0x172
>   copy_process+0x471/0x1a80
>   _do_fork+0x83/0x3a0
>   __do_sys_wait4+0x83/0x90
>   __do_sys_clone+0x85/0xa0
>   do_syscall_64+0x5b/0x1e0
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Using atomic decrease and check instead of separated calls.
> This fixes CVE-2020-14351.
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  kernel/events/core.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 7ed5248f0445..29313cc54d9e 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5903,8 +5903,6 @@ static void perf_mmap_close(struct vm_area_struct *vma)
>                 mutex_unlock(&event->mmap_mutex);
>         }
>
> -       atomic_dec(&rb->mmap_count);
> -
>         if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
>                 goto out_put;

But when it takes the goto, rb->mmap_count won't decrement anymore..

Thanks
Namhyung

>
> @@ -5912,7 +5910,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
>         mutex_unlock(&event->mmap_mutex);
>
>         /* If there's still other mmap()s of this buffer, we're done. */
> -       if (atomic_read(&rb->mmap_count))
> +       if (!atomic_dec_and_test(&rb->mmap_count))
>                 goto out_put;
>
>         /*
> --
> 2.26.2
>