x86/uaccess: Dont leak the AC flag into __put_user() argument evaluation

x86/uaccess: Dont leak the AC flag into __put_user() argument evaluation

commit 6ae865615fc43d014da2fd1f1bba7e81ee622d1b upstream.

The __put_user() macro evaluates it's @ptr argument inside the
__uaccess_begin() / __uaccess_end() region. While this would normally
not be expected to be an issue, an UBSAN bug (it ignored -fwrapv,
fixed in GCC 8+) would transform the @ptr evaluation for:

  drivers/gpu/drm/i915/i915_gem_execbuffer.c: if (unlikely(__put_user(offset, &urelocs[r-stack].presumed_offset))) {

into a signed-overflow-UB check and trigger the objtool AC validation.

Finish this commit:

  2a418cf3f5f1 ("x86/uaccess: Don't leak the AC flag into __put_user() value evaluation")

and explicitly evaluate all 3 arguments early.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: luto@kernel.org
Fixes: 2a418cf3f5f1 ("x86/uaccess: Don't leak the AC flag into __put_user() value evaluation")
Link: http://lkml.kernel.org/r/20190424072208.695962771@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>