parisc: Check region is readable by user in raw_copy_from_user()

Because of the way the _PAGE_READ is handled in the parisc PTE, an
access interruption is not generated when the kernel reads from a
region where the _PAGE_READ is zero. The current code was written
assuming read access faults would also occur in the kernel.

This change adds user access checks to raw_copy_from_user().  The
prober_user() define checks whether user code has read access to
a virtual address. Note that page faults are not handled in the
exception support for the probe instruction. For this reason, we
precede the probe by a ldb access check.

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+

arch/parisc/include/asm/special_insns.h

@@ -32,6 +32,34 @@
 	pa;						\
 })
 
+/**
+ * prober_user() - Probe user read access
+ * @sr:		Space regster.
+ * @va:		Virtual address.
+ *
+ * Return: Non-zero if address is accessible.
+ *
+ * Due to the way _PAGE_READ is handled in TLB entries, we need
+ * a special check to determine whether a user address is accessible.
+ * The ldb instruction does the initial access check. If it is
+ * successful, the probe instruction checks user access rights.
+ */
+#define prober_user(sr, va)	({			\
+	unsigned long read_allowed;			\
+	__asm__ __volatile__(				\
+		"copy %%r0,%0\n"			\
+		"8:\tldb 0(%%sr%1,%2),%%r0\n"		\
+		"\tproberi (%%sr%1,%2),%3,%0\n"		\
+		"9:\n"					\
+		ASM_EXCEPTIONTABLE_ENTRY(8b, 9b,	\
+				"or %%r0,%%r0,%%r0")	\
+		: "=&r" (read_allowed)			\
+		: "i" (sr), "r" (va), "i" (PRIV_USER)	\
+		: "memory"				\
+	);						\
+	read_allowed;					\
+})
+
 #define CR_EIEM 15	/* External Interrupt Enable Mask */
 #define CR_CR16 16	/* CR16 Interval Timer */
 #define CR_EIRR 23	/* External Interrupt Request Register */

arch/parisc/lib/memcpy.c

@@ -12,6 +12,7 @@
 #include <linux/module.h>
 #include <linux/compiler.h>
 #include <linux/uaccess.h>
+#include <linux/mm.h>
 
 #define get_user_space()	mfsp(SR_USER)
 #define get_kernel_space()	SR_KERNEL
@@ -32,9 +33,25 @@ EXPORT_SYMBOL(raw_copy_to_user);
 unsigned long raw_copy_from_user(void *dst, const void __user *src,
 			       unsigned long len)
 {
+	unsigned long start = (unsigned long) src;
+	unsigned long end = start + len;
+	unsigned long newlen = len;
+
 	mtsp(get_user_space(), SR_TEMP1);
 	mtsp(get_kernel_space(), SR_TEMP2);
-	return pa_memcpy(dst, (void __force *)src, len);
+
+	/* Check region is user accessible */
+	if (start)
+	while (start < end) {
+		if (!prober_user(SR_TEMP1, start)) {
+			newlen = (start - (unsigned long) src);
+			break;
+		}
+		start += PAGE_SIZE;
+		/* align to page boundry which may have different permission */
+		start = PAGE_ALIGN_DOWN(start);
+	}
+	return len - newlen + pa_memcpy(dst, (void __force *)src, newlen);
 }
 EXPORT_SYMBOL(raw_copy_from_user);