Re: Question: consult patch

[Lee Jones]

On Fri, 12 Aug 2022, zhangwensheng (E) wrote:

> Hi
> 
> In CVE list last week, there is a new cve reported in asop 4.14 like below:
> Reference link: https://lore.kernel.org/all/CAODzB9rgMexvLjE=WuTm+SN8SfUggaZgWG-aBcy6cotppju6mw@xxxxxxxxxxxxxx/T/
> 
> ---
> CVE-2022-20158: mm: backing-dev: Take a reference to the bdi in use to
> prevent UAF
> 
> CVSS v3 score is not assigned.
> 
> AOSP kernel 4.14 contains following 2 patches.
> - 69e8f03c5ced3e4e6fb4181f4dac185104e3420b ("mm: backing-dev: Take a
> reference to the bdi in use to prevent UAF")
> - 80d91b86a199798ee2321a0ab0f09e6e12764678 ("fs: explicitly unregister
> per-superblock BDIs")
> 
> The first commit 69e8f03("mm: backing-dev: Take a reference to the bdi
> in use to prevent UAF") is not merged in the mainline and stable
> kernels.
> Commit 80d91b8 was merged in 5.16-rc1(commit hash is
> 0b3ea0926afb8dde70cfab00316ae0a70b93a7cc) which requires commit
> c6fd3ac ("mm: export bdi_unregister") that exports symbol of
> bdi_unregister().
> 
> Fixed status
> mainline: [0b3ea0926afb8dde70cfab00316ae0a70b93a7cc]
> ---
> 
> As mentioned above, patch 69e8f03c5ced ("mm: backing-dev: Take a
> reference to the bdi in use to prevent UAF") in asop 4.14 can fix
> a null dereference problem, form my analysis, may like below:
> 
> blk_cleanup_queue
>     blk_put_queue
>         kobject_put(&q->kobj)
>                 blk_release_queue
>                     blk_exit_queue
>                         bdi_put
>                             release_bdi  // bdi -> null
> del_gendisk
>     bdi_unregister(disk->queue->backing_dev_info) // null -> reference
> 
> From my analysis, In asop 4.14 kernel, in loop_remove function, there is
> such a timing that executing "blk_cleanup_queue" first and then
> "del_gendisk".
> but because of the refcnt of queue will add by hte line
> "WARN_ON_ONCE(!blk_get_queue(disk->queue));" in "device_add_disk", which may
> not result in "bdi_put" releasing bdi  in "blk_cleanup_queue".
> 
> I'm not sure where the problem is, so I want to ask Lee Jones who sent this
> patch
> for clarification.
> 
> Thanks!
> 
> Wensheng
> 
> 在 2022/8/12 13:44, Christoph Hellwig 写道:
> > On Fri, Aug 12, 2022 at 11:34:59AM +0800, zhangwensheng (E) wrote:
> > > Hi Lee ：
> > >      I saw your patch because of CVE-2022-20158, the patch like below:
> > > 
> > > ---
> > >      mm: backing-dev: Take a reference to the bdi in use to prevent UAF
> > I can't see that patch anywhere, and I've not seen an bug report for it.
> > 
> > >      Because of a distinct lack of locking and/or reference taking,
> > >      blk_cleanup_queue() puts the final taken reference to the bdi, which
> > .. and blk_cleanup_queue also is gone upstream.
> > 
> > What am I missing?

The issue reported in the aforementioned CVE was caused by a commit
which was applied to an internal, device specific repository.  One
that has never existed in Mainline.  I failed to reproduce the KASAN
report in any upstream or stable tree without the offending patch
applied.  The issue was fixed in all affected internal trees.

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