ACK: [SRU][CVE-2020-14351][X/B/F][PATCH 0/2] perf/core: Fix race in the perf_mmap_close() function

[Thadeu Lima de Souza Cascardo]

On Thu, Nov 05, 2020 at 12:41:30PM -0500, William Breathitt Gray wrote:
> SRU Justification
> =================
> 
> [Impact]
> 
> 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
> 
> [Regression Potential]
> 
> Regression potentional is very low. Changes only affect the
> perf_mmap_close() function, and the only change in logic is to perform
> the decrement and read together atomically rather than separately one
> after another.
> 
> [Miscellaneous]
> 
> Fix is already available in Groovy, but needed for Focal, Bionic, and
> Xenial.
> 
> Jiri Olsa (1):
>   perf/core: Fix race in the perf_mmap_close() function
> 
>  kernel/events/core.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> -- 
> 2.25.1

Acked-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>