From: Will Deacon <will.deacon@arm.com>

Date: Mon, 8 Apr 2019 12:45:09 +0100

Subject: arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value

Git-commit: 045afc24124d80c6998d9c770844c67912083506

Patch-mainline: v5.1-rc5

References: git-fixes

Rather embarrassingly, our futex() FUTEX_WAKE_OP implementation doesn't

explicitly set the return value on the non-faulting path and instead

leaves it holding the result of the underlying atomic operation. This

means that any FUTEX_WAKE_OP atomic operation which computes a non-zero

value will be reported as having failed. Regrettably, I wrote the buggy

code back in 2011 and it was upstreamed as part of the initial arm64

support in 2012.

The reasons we appear to get away with this are:

  1. FUTEX_WAKE_OP is rarely used and therefore doesn't appear to get

     exercised by futex() test applications

  2. If the result of the atomic operation is zero, the system call

     behaves correctly

  3. Prior to version 2.25, the only operation used by GLIBC set the

     futex to zero, and therefore worked as expected. From 2.25 onwards,

     FUTEX_WAKE_OP is not used by GLIBC at all.

Fix the implementation by ensuring that the return value is either 0

to indicate that the atomic operation completed successfully, or -EFAULT

if we encountered a fault when accessing the user mapping.

Cc: <stable@kernel.org>

Fixes: 6170a97460db ("arm64: Atomic operations")

Signed-off-by: Will Deacon <will.deacon@arm.com>

Acked-by: Ivan T. Ivanov <iivanov@suse.de>

---

 arch/arm64/include/asm/futex.h |   16 ++++++++--------

 1 file changed, 8 insertions(+), 8 deletions(-)

--- a/arch/arm64/include/asm/futex.h

+++ b/arch/arm64/include/asm/futex.h

@@ -30,8 +30,8 @@ do {									\

 "	prfm	pstl1strm, %2\n"					\

 "1:	ldxr	%w1, %2\n"						\

 	insn "\n"							\

-"2:	stlxr	%w3, %w0, %2\n"						\

-"	cbnz	%w3, 1b\n"						\

+"2:	stlxr	%w0, %w3, %2\n"						\

+"	cbnz	%w0, 1b\n"						\

 "	dmb	ish\n"							\

 "3:\n"									\

 "	.pushsection .fixup,\"ax\"\n"					\

@@ -50,30 +50,30 @@ do {									\

 static inline int

 arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *_uaddr)

 {

-	int oldval = 0, ret, tmp;

+	int oldval, ret, tmp;

 	u32 __user *uaddr = __uaccess_mask_ptr(_uaddr);

 	pagefault_disable();

 	switch (op) {

 	case FUTEX_OP_SET:

-		__futex_atomic_op("mov	%w0, %w4",

+		__futex_atomic_op("mov	%w3, %w4",

 				  ret, oldval, uaddr, tmp, oparg);

 		break;

 	case FUTEX_OP_ADD:

-		__futex_atomic_op("add	%w0, %w1, %w4",

+		__futex_atomic_op("add	%w3, %w1, %w4",

 				  ret, oldval, uaddr, tmp, oparg);

 		break;

 	case FUTEX_OP_OR:

-		__futex_atomic_op("orr	%w0, %w1, %w4",

+		__futex_atomic_op("orr	%w3, %w1, %w4",

 				  ret, oldval, uaddr, tmp, oparg);

 		break;

 	case FUTEX_OP_ANDN:

-		__futex_atomic_op("and	%w0, %w1, %w4",

+		__futex_atomic_op("and	%w3, %w1, %w4",

 				  ret, oldval, uaddr, tmp, ~oparg);

 		break;

 	case FUTEX_OP_XOR:

-		__futex_atomic_op("eor	%w0, %w1, %w4",

+		__futex_atomic_op("eor	%w3, %w1, %w4",

 				  ret, oldval, uaddr, tmp, oparg);

 		break;

 	default: