linux-kernel - Re: [RFC PATCH] mm: fork: Prevent a NULL deref by getting mm only if the refcount isn't 0

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Date:   Wed, 10 Mar 2021 08:57:00 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Filippo Sironi <sironi@...zon.de>, linux-kernel@...r.kernel.org
Cc:     dwmw@...zon.co.uk, christian.brauner@...ntu.com,
        akpm@...ux-foundation.org, ebiederm@...ssion.com,
        peterz@...radead.org, keescook@...omium.org, krisman@...labora.com,
        peterx@...hat.com, surenb@...gle.com, shakeelb@...gle.com,
        guro@...com, elver@...gle.com
Subject: Re: [RFC PATCH] mm: fork: Prevent a NULL deref by getting mm only if
 the refcount isn't 0

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