Re: [PATCH v3 2/3] mm: add LSM hook for writes to readonly memory

From: Jann Horn
Date: Wed Nov 02 2016 - 23:10:19 EST

On Thu, Nov 03, 2016 at 04:04:45AM +0100, Jann Horn wrote:
> SELinux attempts to make it possible to whitelist trustworthy sources of
> code that may be mapped into memory, and Android makes use of this feature.
> To prevent an attacker from bypassing this by modifying R+X memory through
> /proc/$pid/mem, PTRACE_POKETEXT or DMA, it is necessary to call a security
> hook in check_vma_flags().
> PTRACE_POKETEXT can also be mitigated by blocking ptrace access, and
> /proc/$pid/mem can also be blocked at the VFS layer, but DMA is harder to
> deal with: Some driver functions (e.g. videobuf_dma_init_user_locked)
> write to user-specified DMA mappings even if those mappings are readonly
> or R+X.

Whoops, sorry for sending that twice. :/

A comment regarding the whole series: I'm not entirely sure whether this is
the best way to fix this after all. It's quite a bit of code churn, but it
has the benefit of having a single check in a central place.

As an alternative to this patch, it might be possible to break the ABIs
of the drivers that access DMA buffers with FOLL_FORCE by simply removing
FOLL_FORCE from those drivers. However, I'm not sure how much that would
break existing userspace code.

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