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@kernel.org Signed-off-by: Paul Burton paul.burton@mips.com Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions") Cc: stable@vger.kernel.org # v4.8+ Cc: linux-mips@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: Rich Felker dalias@libc.org Cc: David Daney david.daney@cavium.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