parisc: Revise __get_user() to probe user read access

Because of the way read access support is implemented, read access
interruptions are only triggered at privilege levels 2 and 3. The
kernel executes at privilege level 0, so __get_user() never triggers
a read access interruption (code 26). Thus, it is currently possible
for user code to access a read protected address via a system call.

Fix this by probing read access rights at privilege level 3 (PRIV_USER)
and setting __gu_err to -EFAULT (-14) if access isn't allowed.

Note the cmpiclr instruction does a 32-bit compare because COND macro
doesn't work inside asm.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v5.12+

Author: danglin44

arch/parisc/include/asm/uaccess.h

@@ -42,9 +42,24 @@
 	__gu_err;					\
 })
 
-#define __get_user(val, ptr)				\
-({							\
-	__get_user_internal(SR_USER, val, ptr);	\
+#define __probe_user_internal(sr, error, ptr)			\
+({								\
+	__asm__("\tproberi (%%sr%1,%2),%3,%0\n"			\
+		"\tcmpiclr,= 1,%0,%0\n"				\
+		"\tldi %4,%0\n"					\
+		: "=r"(error)					\
+		: "i"(sr), "r"(ptr), "i"(PRIV_USER),		\
+		  "i"(-EFAULT));				\
+})
+
+#define __get_user(val, ptr)					\
+({								\
+	register long __gu_err;					\
+								\
+	__gu_err = __get_user_internal(SR_USER, val, ptr);	\
+	if (likely(!__gu_err))					\
+		__probe_user_internal(SR_USER, __gu_err, ptr);	\
+	__gu_err;						\
 })
 
 #define __get_user_asm(sr, val, ldx, ptr)		\