[RFC PATCH v2 2/2] [RESEND] x86: Fix insn decoder for longerinstruction

[Masami Hiramatsu]

Fix x86 insn decoder for hardening against invalid length
instructions. This adds length checkings for each byte-read
site and if it exceeds MAX_INSN_SIZE, returns immediately.
This can happen when decoding unreliable binary.

 For example, without this patch,

$ echo  66 66 66 66 66 66 66 66 66 66 66 66 66 66 66 66 |\
 ./insn_sanity -i -
Error: Found an access violation:
Instruction = {
        .prefixes = {
                .value = 1711276134, bytes[] = {66, 0, 0, 66},
                .got = 1, .nbytes = 16},
        .rex_prefix = {
                .value = 0, bytes[] = {0, 0, 0, 0},
                .got = 1, .nbytes = 0},
        .vex_prefix = {
                .value = 0, bytes[] = {0, 0, 0, 0},
                .got = 1, .nbytes = 0},
        .opcode = {
                .value = 144, bytes[] = {90, 0, 0, 0},
                .got = 1, .nbytes = 1},
...
        .attr = 0, .opnd_bytes = 2, .addr_bytes = 4,
        .length = 17, .x86_64 = 0, .kaddr = 0x7fffedc67ae0}
You can reproduce this with below command(s);
 $ echo  66 66 66 66 66 66 66 66 66 66 66 66 66 66 66 66 | ./insn_sanity -i -
Failure: decoded and checked 1 given instructions with 1 errors (seed:0x0)


 With this patch;


 $ echo  66 66 66 66 66 66 66 66 66 66 66 66 66 66 66 66 |\
  ./insn_sanity -i -
 Success: decoded and checked 1 given instructions with 0
 errors (seed:0x0)


Caller can check whether it happened by insn_complete().

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
---

 arch/x86/lib/insn.c |   48 +++++++++++++++++++++++++++++++++++++++++++-----
 1 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
index 9f33b98..374562e 100644
--- a/arch/x86/lib/insn.c
+++ b/arch/x86/lib/insn.c
@@ -22,14 +22,23 @@
 #include <asm/inat.h>
 #include <asm/insn.h>
 
-#define get_next(t, insn)	\
-	({t r; r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
+/* Verify next sizeof(t) bytes can be on the same instruction */
+#define validate_next(t, insn, n)	\
+	((insn)->next_byte + sizeof(t) + n - (insn)->kaddr <= MAX_INSN_SIZE)
+
+#define __get_next(t, insn)	\
+	({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
+
+#define __peek_nbyte_next(t, insn, n)	\
+	({ t r = *(t*)((insn)->next_byte + n); r; })
 
-#define peek_next(t, insn)	\
-	({t r; r = *(t*)insn->next_byte; r; })
+#define get_next(t, insn)	\
+	({ if (unlikely(!validate_next(t, insn, 0))) goto err_out; __get_next(t, insn); })
 
 #define peek_nbyte_next(t, insn, n)	\
-	({t r; r = *(t*)((insn)->next_byte + n); r; })
+	({ if (unlikely(!validate_next(t, insn, n))) goto err_out; __peek_nbyte_next(t, insn, n); })
+
+#define peek_next(t, insn)	peek_nbyte_next(t, insn, 0)
 
 /**
  * insn_init() - initialize struct insn
@@ -158,6 +167,8 @@ vex_end:
 	insn->vex_prefix.got = 1;
 
 	prefixes->got = 1;
+
+err_out:
 	return;
 }
 
@@ -208,6 +219,9 @@ void insn_get_opcode(struct insn *insn)
 		insn->attr = 0;	/* This instruction is bad */
 end:
 	opcode->got = 1;
+
+err_out:
+	return;
 }
 
 /**
@@ -241,6 +255,9 @@ void insn_get_modrm(struct insn *insn)
 	if (insn->x86_64 && inat_is_force64(insn->attr))
 		insn->opnd_bytes = 8;
 	modrm->got = 1;
+
+err_out:
+	return;
 }
 
 
@@ -290,6 +307,9 @@ void insn_get_sib(struct insn *insn)
 		}
 	}
 	insn->sib.got = 1;
+
+err_out:
+	return;
 }
 
 
@@ -351,6 +371,9 @@ void insn_get_displacement(struct insn *insn)
 	}
 out:
 	insn->displacement.got = 1;
+
+err_out:
+	return;
 }
 
 /* Decode moffset16/32/64 */
@@ -373,6 +396,9 @@ static void __get_moffset(struct insn *insn)
 		break;
 	}
 	insn->moffset1.got = insn->moffset2.got = 1;
+
+err_out:
+	return;
 }
 
 /* Decode imm v32(Iz) */
@@ -389,6 +415,9 @@ static void __get_immv32(struct insn *insn)
 		insn->immediate.nbytes = 4;
 		break;
 	}
+
+err_out:
+	return;
 }
 
 /* Decode imm v64(Iv/Ov) */
@@ -411,6 +440,9 @@ static void __get_immv(struct insn *insn)
 		break;
 	}
 	insn->immediate1.got = insn->immediate2.got = 1;
+
+err_out:
+	return;
 }
 
 /* Decode ptr16:16/32(Ap) */
@@ -432,6 +464,9 @@ static void __get_immptr(struct insn *insn)
 	insn->immediate2.value = get_next(unsigned short, insn);
 	insn->immediate2.nbytes = 2;
 	insn->immediate1.got = insn->immediate2.got = 1;
+
+err_out:
+	return;
 }
 
 /**
@@ -496,6 +531,9 @@ void insn_get_immediate(struct insn *insn)
 	}
 done:
 	insn->immediate.got = 1;
+
+err_out:
+	return;
 }
 
 /**

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