Re: CVE-2022-2590: Linux kernel: Modifying shmem/tmpfs files without write permissions

[Demi Marie Obenour <demi () invisiblethingslab com>]

On Mon, Aug 08, 2022 at 09:18:27AM +0200, David Hildenbrand wrote:
> Hi,
>
> I found a security issue (CVE-2022-2590) in the Linux kernel similar to
> Dirty COW (CVE-2016-5195), however, restricted to shared memory (shmem /
> tmpfs). I notified distributions one week ago and the embargo ended today.
>
> An unprivileged user can modify file content of a shmem (tmpfs) file,
> even if that user does not have write permissions to the file. The file
> could be an executable.

Is Android affected by this, or do other protections (such as SELinux)
prevent an exploit from succeeding?  Also, is read access to the file
necessary?  Are sealed memfds impacted?

> The introducing upstream commit ID is:
>   9ae0f87d009c ("mm/shmem: unconditionally set pte dirty in
>   mfill_atomic_install_pte")
>
> Linux >= v5.16 is affected on x86-64 and aarch64 if the kernel is
> compiled with CONFIG_USERFAULTFD=y. For Linux < v5.19 it's sufficient to
> revert the problematic commit, which is possible with minor contextual
> conflicts. For Linux >= v5.19 I'll send a proposal fix today.
>
> I have a working reproducer that I will post as reply to this mail in
> one week (August 15).

Can you try to make sure that a patch has made it into Greg’s stable
trees by then?  Also, would it be possible to include a regression test?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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