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