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Subject[PATCH 3.19 2/3] x86, mpx: Short-circuit the instruction decoder for unexpected opcodes
Date
This reduces the degree to which we're exposing the instruction decoder
to malicious user code and very little complexity cost.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
arch/x86/mm/mpx.c | 26 +++++++++++++++++---------
1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 082ab9c4ac1c..cefa615becb3 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -230,6 +230,23 @@ static int mpx_insn_decode(struct insn *insn,
*/
if (!nr_copied)
return -EFAULT;
+
+ /*
+ * We only _really_ need to decode bndcl/bndcn/bndcu
+ * Error out on anything else. Check this before decoding the
+ * instruction to reduce our exposure to intentionally bad code
+ * to some extent. Note that this shortcut cat incorrectly return
+ * -EINVAL instead of -EFAULT under some circumstances. This
+ * discrepency has no effect.
+ */
+ if (nr_copied < 2)
+ goto bad_opcode;
+ if (insn->opcode.bytes[0] != 0x0f)
+ goto bad_opcode;
+ if ((insn->opcode.bytes[1] != 0x1a) &&
+ (insn->opcode.bytes[1] != 0x1b))
+ goto bad_opcode;
+
insn_init(insn, buf, nr_copied, x86_64);
insn_get_length(insn);
/*
@@ -244,15 +261,6 @@ static int mpx_insn_decode(struct insn *insn,
return -EFAULT;

insn_get_opcode(insn);
- /*
- * We only _really_ need to decode bndcl/bndcn/bndcu
- * Error out on anything else.
- */
- if (insn->opcode.bytes[0] != 0x0f)
- goto bad_opcode;
- if ((insn->opcode.bytes[1] != 0x1a) &&
- (insn->opcode.bytes[1] != 0x1b))
- goto bad_opcode;

return 0;
bad_opcode:
--
2.1.0


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