linux-kernel - Re: [4.9 PATCH] MIPS: math-emu: Write-protect delay slot emulation pages

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Date:   Thu, 10 Jan 2019 20:09:21 +0100
From:   "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To:     Paul Burton <paul.burton@...s.com>
Cc:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Paul Burton <pburton@...ecomp.com>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rich Felker <dalias@...c.org>,
        David Daney <david.daney@...ium.com>
Subject: Re: [4.9 PATCH] MIPS: math-emu: Write-protect delay slot emulation
 pages

On Thu, Jan 10, 2019 at 05:47:42PM +0000, Paul Burton wrote:
> commit adcc81f148d733b7e8e641300c5590a2cdc13bf3 upstream.
> 
> Mapping the delay slot emulation page as both writeable & executable
> presents a security risk, in that if an exploit can write to & jump into
> the page then it can be used as an easy way to execute arbitrary code.
> 
> Prevent this by mapping the page read-only for userland, and using
> access_process_vm() with the FOLL_FORCE flag to write to it from
> mips_dsemul().
> 
> This will likely be less efficient due to copy_to_user_page() performing
> cache maintenance on a whole page, rather than a single line as in the
> previous use of flush_cache_sigtramp(). However this delay slot
> emulation code ought not to be running in any performance critical paths
> anyway so this isn't really a problem, and we can probably do better in
> copy_to_user_page() anyway in future.
> 
> A major advantage of this approach is that the fix is small & simple to
> backport to stable kernels.
> 
> Reported-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Paul Burton <paul.burton@...s.com>
> Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")
> Cc: stable@...r.kernel.org # v4.8+
> Cc: linux-mips@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: Rich Felker <dalias@...c.org>
> Cc: David Daney <david.daney@...ium.com>
> ---
>  arch/mips/kernel/vdso.c     |  4 ++--
>  arch/mips/math-emu/dsemul.c | 38 +++++++++++++++++++------------------
>  2 files changed, 22 insertions(+), 20 deletions(-)
> 

Thanks for the backport, now queued up.

greg k-h