[PATCH] x86: Fix off-by-one error in __access_ok

[Mikel Rychliski <mikel@mikelr.com>]

We were checking one byte beyond the actual range that would be accessed.
Originally, valid_user_address would consider the user guard page to be
valid, so checks including the final accessible byte would still succeed.
However, after commit 86e6b1547b3d ("x86: fix user address masking
non-canonical speculation issue") this is no longer the case.

Update the logic to always consider the final address in the range.

Fixes: 86e6b1547b3d ("x86: fix user address masking non-canonical speculation issue")
Signed-off-by: Mikel Rychliski <mikel@mikelr.com>
---
 arch/x86/include/asm/uaccess_64.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index b0a887209400..3e0eb72c036f 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -100,9 +100,11 @@ static inline bool __access_ok(const void __user *ptr, unsigned long size)
 	if (__builtin_constant_p(size <= PAGE_SIZE) && size <= PAGE_SIZE) {
 		return valid_user_address(ptr);
 	} else {
-		unsigned long sum = size + (__force unsigned long)ptr;
+		unsigned long end = (__force unsigned long)ptr;
 
-		return valid_user_address(sum) && sum >= (__force unsigned long)ptr;
+		if (size)
+			end += size - 1;
+		return valid_user_address(end) && end >= (__force unsigned long)ptr;
 	}
 }
 #define __access_ok __access_ok
-- 
2.47.0