[OpenWrt-Devel] [PATCH] kernel: MIPS: math-emu Write-protect delay slot emulation pages

[Rosen Penev]

On Fri, Dec 21, 2018 at 8:05 PM Yousong Zhou <yszhou4tech at gmail.com> wrote:
>
> On Sat, 22 Dec 2018 at 01:21, Kevin 'ldir' Darbyshire-Bryant
> <ldir at darbyshire-bryant.me.uk> wrote:
> >
> > Backport https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git/commit/?id=adcc81f148d733b7e8e641300c5590a2cdc13bf3
> >
> > "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 at kernel.org>
> > Signed-off-by: Paul Burton <paul.burton at mips.com>
> > Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")"
> >
> > Without patch:
> >
> > cat /proc/self/maps
> > 00400000-0047a000 r-xp 00000000 1f:03 1823       /bin/busybox
> > 00489000-0048a000 r-xp 00079000 1f:03 1823       /bin/busybox
> > 0048a000-0048b000 rwxp 0007a000 1f:03 1823       /bin/busybox
> > 77ec8000-77eed000 r-xp 00000000 1f:03 2296       /lib/libgcc_s.so.1
> > 77eed000-77eee000 rwxp 00015000 1f:03 2296       /lib/libgcc_s.so.1
> > 77eee000-77f81000 r-xp 00000000 1f:03 2470       /lib/libc.so
> > 77f90000-77f92000 rwxp 00092000 1f:03 2470       /lib/libc.so
> > 77f92000-77f94000 rwxp 00000000 00:00 0
> > 7f946000-7f967000 rw-p 00000000 00:00 0          [stack]
> > 7fefb000-7fefc000 rwxp 00000000 00:00 0
> > 7ffac000-7ffad000 r--p 00000000 00:00 0          [vvar]
> > 7ffad000-7ffae000 r-xp 00000000 00:00 0          [vdso]
>
> Hi,
>
> I must miss something.  After reading another thread on mips security,
> I was thinking that all segments with w and x permission set were
> problematic:  the same attacker can write and execute shellcode there,
> right?  Sorry, if the answer is too apparent ;(
Right. This is a fix for one of those sections. Not all of them.

It will take quite a bit of work to fix all of them.
>
> Regards,
>                 yousong
>
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