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SubjectRe: [RFC PATCH] mm: fork: Prevent a NULL deref by getting mm only if the refcount isn't 0
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On 3/10/21 5:37 AM, Filippo Sironi wrote:
> We've seen a number of crashes with the following signature:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> ...
> Oops: 0000 [#1] SMP PTI
> ...
> RIP: 0010:__rb_erase_color+0xc2/0x260
> ...
> Call Trace:
> unlink_file_vma+0x36/0x50
> free_pgtables+0x62/0x110
> exit_mmap+0xd5/0x160
> ? put_dec+0x3a/0x90
> ? num_to_str+0xa8/0xc0
> mmput+0x11/0xb0
> do_task_stat+0x940/0xc80
> proc_single_show+0x49/0x80
> ? __check_object_size+0xcc/0x1a0
> seq_read+0xd3/0x400
> vfs_read+0x72/0xb0
> ksys_read+0x9c/0xd0
> do_syscall_64+0x69/0x400
> ? schedule+0x2a/0x90
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
> ...
>
> This happens when a process goes through the tasks stats in procfs while
> another is exiting. This looks like a race where the process that's
> exiting drops the last reference on the mm (with mmput) while the other
> increases it (with mmget). By only increasing when the reference isn't
> 0 to begin with, we prevent this from happening.

From a quick look it looks reasonable, but I don't quite see how we get
in the situation of finding a valid ->mm under task_lock() and the
mm_users count being 0? I'd like to understand that, because it may just
be that your patch just narrows the gap but it's still possible to
trigger a use-after-free. Doesn't seem like that would be possible under
exit_mm().

--
Jens Axboe

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