MIPS: math-emu: Write-protect delay slot emulation pages

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 <[email protected]>
Signed-off-by: Paul Burton <[email protected]>
Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")
Cc: [email protected] # v4.8+
Cc: [email protected]
Cc: [email protected]
Cc: Rich Felker <[email protected]>
Cc: David Daney <[email protected]>

0 files changed, 0 insertions, 0 deletions