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

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/mips/kernel/vdso.c b/arch/mips/kernel/vdso.c
index 48a9c6b90e07..9df3ebdc7b0f 100644
--- a/arch/mips/kernel/vdso.c
+++ b/arch/mips/kernel/vdso.c
@@ -126,8 +126,8 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 
 	/* Map delay slot emulation page */
 	base = mmap_region(NULL, STACK_TOP, PAGE_SIZE,
-			   VM_READ|VM_WRITE|VM_EXEC|
-			   VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
+			   VM_READ | VM_EXEC |
+			   VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
 			   0, NULL);
 	if (IS_ERR_VALUE(base)) {
 		ret = base;