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SubjectRe: [PATCHv2] perf: Fix race in perf_mmap_close function
On Wed, Sep 16, 2020 at 03:54:02PM +0200, peterz@infradead.org wrote:
> On Wed, Sep 16, 2020 at 01:53:11PM +0200, Jiri Olsa 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.
>
> I'm tempted to remove that line; I can never seem to find anything
> useful in a CVE.

I was asked by security guys to add this, Wade?

>
> Fixes: ?

right, sry..

Fixes: 9bb5d40cd93c ("perf: Fix mmap() accounting hole");

thanks,
jirka

>
> > Acked-by: Namhyung Kim <namhyung@kernel.org>
> > Tested-by: Michael Petlan <mpetlan@redhat.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>

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