linux-kernel - [PATCH 01/11] x86/fault: Fix AMD erratum #91 errata fixup for user code

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Date:   Sun, 31 Jan 2021 09:24:32 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Christoph Hellwig <hch@....de>
Subject: [PATCH 01/11] x86/fault: Fix AMD erratum #91 errata fixup for user code

The recent rework of probe_kernel_read() and its conversion to
get_kernel_nofault() inadvertently broke is_prefetch().  We were using
probe_kernel_read() as a sloppy "read user or kernel memory" helper, but it
doens'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.

While we're at it, disable the workaround on all CPUs except AMD Family
0xF.  By my reading of the Revision Guide for AMD Athlon™ 64 and AMD
Opteron™ Processors, only family 0xF is affected.

Fixes: eab0c6089b68 ("maccess: unify the probe kernel arch hooks")
Cc: stable@...r.kernel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Christoph Hellwig <hch@....de>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/mm/fault.c | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 106b22d1d189..50dfdc71761e 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
@@ -124,23 +120,38 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
 	if (error_code & X86_PF_INSTR)
 		return 0;
 
+	if (likely(boot_cpu_data.x86_vendor != X86_VENDOR_AMD
+		   || boot_cpu_data.x86 != 0xf))
+		return 0;
+
 	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