[PATCH v2] hvf: Fix OOB write in RDTSCP instruction decode

[Cameron Esfahani <dirty@apple.com>]

A guest could craft a specific stream of instructions that will have QEMU
write 0xF9 to inappropriate locations in memory.  Add additional asserts
to check for this.  Generate a #UD if there are more than 14 prefix bytes.

Found by Julian Stecklina <julian.stecklina@cyberus-technology.de>

Signed-off-by: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 target/i386/hvf/x86_decode.c | 12 ++++++++++--
 target/i386/hvf/x86hvf.c     |  8 ++++++++
 target/i386/hvf/x86hvf.h     |  1 +
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/target/i386/hvf/x86_decode.c b/target/i386/hvf/x86_decode.c
index 062713b1a4..5d051252b4 100644
--- a/target/i386/hvf/x86_decode.c
+++ b/target/i386/hvf/x86_decode.c
@@ -24,8 +24,10 @@
 #include "vmx.h"
 #include "x86_mmu.h"
 #include "x86_descr.h"
+#include "x86hvf.h"
 
 #define OPCODE_ESCAPE   0xf
+#define X86_MAX_INSN_PREFIX_LENGTH (14)
 
 static void decode_invalid(CPUX86State *env, struct x86_decode *decode)
 {
@@ -541,7 +543,8 @@ static void decode_lidtgroup(CPUX86State *env, struct x86_decode *decode)
     };
     decode->cmd = group[decode->modrm.reg];
     if (0xf9 == decode->modrm.modrm) {
-        decode->opcode[decode->len++] = decode->modrm.modrm;
+        VM_PANIC_ON(decode->opcode_len >= sizeof(decode->opcode));
+        decode->opcode[decode->opcode_len++] = decode->modrm.modrm;
         decode->cmd = X86_DECODE_CMD_RDTSCP;
     }
 }
@@ -1847,7 +1850,8 @@ void calc_modrm_operand(CPUX86State *env, struct x86_decode *decode,
 
 static void decode_prefix(CPUX86State *env, struct x86_decode *decode)
 {
-    while (1) {
+    /* At most X86_MAX_INSN_PREFIX_LENGTH prefix bytes. */
+    for (int i = 0; i < X86_MAX_INSN_PREFIX_LENGTH; i++) {
         /*
          * REX prefix must come after legacy prefixes.
          * REX before legacy is ignored.
@@ -1892,6 +1896,8 @@ static void decode_prefix(CPUX86State *env, struct x86_decode *decode)
             return;
         }
     }
+    /* Too many prefixes!  Generate #UD. */
+    hvf_inject_ud(env);
 }
 
 void set_addressing_size(CPUX86State *env, struct x86_decode *decode)
@@ -2090,11 +2096,13 @@ static void decode_opcodes(CPUX86State *env, struct x86_decode *decode)
     uint8_t opcode;
 
     opcode = decode_byte(env, decode);
+    VM_PANIC_ON(decode->opcode_len >= sizeof(decode->opcode));
     decode->opcode[decode->opcode_len++] = opcode;
     if (opcode != OPCODE_ESCAPE) {
         decode_opcode_1(env, decode, opcode);
     } else {
         opcode = decode_byte(env, decode);
+        VM_PANIC_ON(decode->opcode_len >= sizeof(decode->opcode));
         decode->opcode[decode->opcode_len++] = opcode;
         decode_opcode_2(env, decode, opcode);
     }
diff --git a/target/i386/hvf/x86hvf.c b/target/i386/hvf/x86hvf.c
index 05ec1bddc4..a805c125d9 100644
--- a/target/i386/hvf/x86hvf.c
+++ b/target/i386/hvf/x86hvf.c
@@ -425,6 +425,14 @@ bool hvf_inject_interrupts(CPUState *cpu_state)
             & (CPU_INTERRUPT_INIT | CPU_INTERRUPT_TPR));
 }
 
+void hvf_inject_ud(CPUX86State *env)
+{
+    env->exception_nr = EXCP06_ILLOP;
+    env->exception_injected = 1;
+    env->has_error_code = false;
+    env->error_code = 0;
+}
+
 int hvf_process_events(CPUState *cpu_state)
 {
     X86CPU *cpu = X86_CPU(cpu_state);
diff --git a/target/i386/hvf/x86hvf.h b/target/i386/hvf/x86hvf.h
index 99ed8d608d..ef472a32f9 100644
--- a/target/i386/hvf/x86hvf.h
+++ b/target/i386/hvf/x86hvf.h
@@ -22,6 +22,7 @@
 
 int hvf_process_events(CPUState *);
 bool hvf_inject_interrupts(CPUState *);
+void hvf_inject_ud(CPUX86State *);
 void hvf_set_segment(struct CPUState *cpu, struct vmx_segment *vmx_seg,
                      SegmentCache *qseg, bool is_tr);
 void hvf_get_segment(SegmentCache *qseg, struct vmx_segment *vmx_seg);
-- 
2.32.0 (Apple Git-132)