# \[PATCH 16/52\] arm64: futex: Mask \_\_user pointers prior to dereference

- From: Alex Shi \<hidden\>
- Date: 2018-02-26 08:36:37 
- List: linux-arm-kernel
- Page: https://ratatoskr.run/linux-arm-kernel/2018/02/5194877
- Thread: https://ratatoskr.run/stable/2018/02/6824677/t.md

```text
From: Will Deacon <redacted>

commit 91b2d3442f6a upstream.

The arm64 futex code has some explicit dereferencing of user pointers
where performing atomic operations in response to a futex command. This
patch uses masking to limit any speculative futex operations to within
the user address space.

Signed-off-by: Will Deacon <redacted>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Alex Shi <redacted>

Conflicts:
	change on old futex_atomic_op_inuser function instead of
	arch_futex_atomic_op_inuser in arch/arm64/include/asm/futex.h
---
 arch/arm64/include/asm/futex.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index 85c4a89..1943cf6 100644
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -48,13 +48,14 @@ do {									\
 } while (0)
 
 static inline int
-futex_atomic_op_inuser (int encoded_op, u32 __user *uaddr)
+futex_atomic_op_inuser (int encoded_op, u32 __user *_uaddr)
 {
 	int op = (encoded_op >> 28) & 7;
 	int cmp = (encoded_op >> 24) & 15;
 	int oparg = (encoded_op << 8) >> 20;
 	int cmparg = (encoded_op << 20) >> 20;
 	int oldval = 0, ret, tmp;
+	u32 __user *uaddr = __uaccess_mask_ptr(_uaddr);
 
 	if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28))
 		oparg = 1 << oparg;
@@ -106,15 +107,17 @@ futex_atomic_op_inuser (int encoded_op, u32 __user *uaddr)
 }
 
 static inline int
-futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
+futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *_uaddr,
 			      u32 oldval, u32 newval)
 {
 	int ret = 0;
 	u32 val, tmp;
+	u32 __user *uaddr;
 
-	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
+	if (!access_ok(VERIFY_WRITE, _uaddr, sizeof(u32)))
 		return -EFAULT;
 
+	uaddr = __uaccess_mask_ptr(_uaddr);
 	uaccess_enable();
 	asm volatile("// futex_atomic_cmpxchg_inatomic\n"
 "	prfm	pstl1strm, %2\n"
-- 
2.7.4
```