x86/fault: Fix AMD erratum #91 errata fixup for user code

The recent rework of probe_kernel_address() and its conversion to
get_kernel_nofault() inadvertently broke is_prefetch(). Before this
change, probe_kernel_address() was used as a sloppy "read user or
kernel memory" helper, but it doesn't do that any more. The new
get_kernel_nofault() reads *kernel* memory only, which completely broke
is_prefetch() for user access.

Adjust the code to the correct accessor based on access mode. The
manual address bounds check is no longer necessary, since the accessor
helpers (get_user() / get_kernel_nofault()) do the right thing all by
themselves. As a bonus, by using the correct accessor, the open-coded
address bounds check is not needed anymore.

 [ bp: Massage commit message. ]

Fixes: eab0c6089b ("maccess: unify the probe kernel arch hooks")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/b91f7f92f3367d2d3a88eec3b09c6aab1b2dc8ef.1612924255.git.luto@kernel.org

arch/x86/mm/fault.c

@@ -54,7 +54,7 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr)
  * 32-bit mode:
  *
  *   Sometimes AMD Athlon/Opteron CPUs report invalid exceptions on prefetch.
- *   Check that here and ignore it.
+ *   Check that here and ignore it.  This is AMD erratum #91.
  *
  * 64-bit mode:
  *
@@ -83,11 +83,7 @@ check_prefetch_opcode(struct pt_regs *regs, unsigned char *instr,
 #ifdef CONFIG_X86_64
 	case 0x40:
 		/*
-		 * In AMD64 long mode 0x40..0x4F are valid REX prefixes
-		 * Need to figure out under what instruction mode the
-		 * instruction was issued. Could check the LDT for lm,
-		 * but for now it's good enough to assume that long
-		 * mode only uses well known segments or kernel.
+		 * In 64-bit mode 0x40..0x4F are valid REX prefixes
 		 */
 		return (!user_mode(regs) || user_64bit_mode(regs));
 #endif
@@ -127,20 +123,31 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
 	instr = (void *)convert_ip_to_linear(current, regs);
 	max_instr = instr + 15;
 
-	if (user_mode(regs) && instr >= (unsigned char *)TASK_SIZE_MAX)
-		return 0;
+	/*
+	 * This code has historically always bailed out if IP points to a
+	 * not-present page (e.g. due to a race).  No one has ever
+	 * complained about this.
+	 */
+	pagefault_disable();
 
 	while (instr < max_instr) {
 		unsigned char opcode;
 
-		if (get_kernel_nofault(opcode, instr))
-			break;
+		if (user_mode(regs)) {
+			if (get_user(opcode, instr))
+				break;
+		} else {
+			if (get_kernel_nofault(opcode, instr))
+				break;
+		}
 
 		instr++;
 
 		if (!check_prefetch_opcode(regs, instr, opcode, &prefetch))
 			break;
 	}
+
+	pagefault_enable();
 	return prefetch;
 }