arm64: futex: Avoid copying out uninitialised stack in failed cmpxchg()

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@arm.com>

diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index 2d78ea6..bdb3c05 100644 (file)
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -134,7 +134,9 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *_uaddr,
        : "memory");
        uaccess_disable();
 
-       *uval = val;
+       if (!ret)
+               *uval = val;
+
        return ret;
 }