New subject: [tip: x86/mm] x86/fault: Fix AMD erratum #91 errata fixup for user code

10 Feb 2021

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 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, we don't need the
open-coded address bounds check.
Fixes: eab0c6089b68 ("maccess: unify the probe kernel arch hooks")
Cc: stable@vger.kernel.org
Cc: Dave Hansen dave.hansen@linux.intel.com
Cc: Peter Zijlstra peterz@infradead.org
Cc: Christoph Hellwig hch@lst.de
Cc: Alexei Starovoitov ast@kernel.org
Cc: Daniel Borkmann daniel@iogearbox.net
Cc: Masami Hiramatsu mhiramat@kernel.org
Signed-off-by: Andy Lutomirski luto@kernel.org
---
 arch/x86/mm/fault.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index f1f1b5a0956a..441c3e9b8971 100644
--- a/arch/x86/mm/fault.c
+++ b/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;
 }
-- 
2.29.2


    

[PATCH v2 01/14] x86/fault: Fix AMD erratum #91 errata fixup for user code