[PATCH 4.19 68/72] arm64: futex: Avoid copying out uninitialised stack in failed cmpxchg()

[Greg Kroah-Hartman]

From: Will Deacon <will.deacon@xxxxxxx>

commit 8e4e0ac02b449297b86498ac24db5786ddd9f647 upstream.

Returning an error code from futex_atomic_cmpxchg_inatomic() indicates
that the caller should not make any use of *uval, and should instead act
upon on the value of the error code. Although this is implemented
correctly in our futex code, we needlessly copy uninitialised stack to
*uval in the error case, which can easily be avoided.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/arm64/include/asm/futex.h |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -134,7 +134,9 @@ futex_atomic_cmpxchg_inatomic(u32 *uval,
 	: "memory");
 	uaccess_disable();
 
-	*uval = val;
+	if (!ret)
+		*uval = val;
+
 	return ret;
 }