Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

David Hildenbrand
2022-08-08T16:02:14Z

On 08.08.22 09:32, David Hildenbrand wrote:
> Ever since the Dirty COW (CVE-2016-5195) security issue happened, we know
> that FOLL_FORCE can be possibly dangerous, especially if there are races
> that can be exploited by user space.
>
> Right now, it would be sufficient to have some code that sets a PTE of
> a R/O-mapped shared page dirty, in order for it to erroneously become
> writable by FOLL_FORCE. The implications of setting a write-protected PTE
> dirty might not be immediately obvious to everyone.
>
> And in fact ever since commit 9ae0f87d009c ("mm/shmem: unconditionally set
> pte dirty in mfill_atomic_install_pte"), we can use UFFDIO_CONTINUE to map
> a shmem page R/O while marking the pte dirty. This can be used by
> unprivileged user space to modify tmpfs/shmem file content even if the user
> does not have write permissions to the file -- Dirty COW restricted to
> tmpfs/shmem (CVE-2022-2590).
>
> To fix such security issues for good, the insight is that we really only
> need that fancy retry logic (FOLL_COW) for COW mappings that are not
> writable (!VM_WRITE). And in a COW mapping, we really only broke COW if
> we have an exclusive anonymous page mapped. If we have something else
> mapped, or the mapped anonymous page might be shared (!PageAnonExclusive),
> we have to trigger a write fault to break COW. If we don't find an
> exclusive anonymous page when we retry, we have to trigger COW breaking
> once again because something intervened.
>
> Let's move away from this mandatory-retry + dirty handling and rely on
> our PageAnonExclusive() flag for making a similar decision, to use the
> same COW logic as in other kernel parts here as well. In case we stumble
> over a PTE in a COW mapping that does not map an exclusive anonymous page,
> COW was not properly broken and we have to trigger a fake write-fault to
> break COW.
>
> Just like we do in can_change_pte_writable() added via
> commit 64fe24a3e05e ("mm/mprotect: try avoiding write faults for exclusive
> anonymous pages when changing protection") and commit 76aefad628aa
> ("mm/mprotect: fix soft-dirty check in can_change_pte_writable()"), take
> care of softdirty and uffd-wp manually.
>
> For example, a write() via /proc/self/mem to a uffd-wp-protected range has
> to fail instead of silently granting write access and bypassing the
> userspace fault handler. Note that FOLL_FORCE is not only used for debug
> access, but also triggered by applications without debug intentions, for
> example, when pinning pages via RDMA.
>
> This fixes CVE-2022-2590. Note that only x86_64 and aarch64 are
> affected, because only those support CONFIG_HAVE_ARCH_USERFAULTFD_MINOR.
>
> Fortunately, FOLL_COW is no longer required to handle FOLL_FORCE. So
> let's just get rid of it.

I have to add here:

"Thanks to Nadav Amit for pointing out that the pte_dirty() check in
FOLL_FORCE code is problematic and might be exploitable."

--
Thanks,

David / dhildenb