[Xen-changelog] [xen stable-4.6] x86/emul: Correct the decoding of mov to/from cr/dr

[patchbot@xxxxxxx]

commit 18949dc6e1a520b6430f128a6981318552ca7823
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Mar 14 14:06:03 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Mar 14 14:06:03 2017 +0100

    x86/emul: Correct the decoding of mov to/from cr/dr
    
    The mov to/from cr/dr behave as if they were encoded with Mod = 3.  When
    encoded with Mod != 3, no displacement or SIB bytes are fetched.
    
    Add a test with a deliberately malformed ModRM byte.  (Also add the
    automatically-generated simd.h to .gitignore.)
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: c2e316b2f220af06dab76b1219e61441c31f6ff9
    master date: 2017-03-07 17:29:16 +0000
---
 tools/tests/x86_emulator/test_x86_emulator.c | 34 ++++++++++++++++++++++++++++
 xen/arch/x86/x86_emulate/x86_emulate.c       | 10 ++++++--
 2 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/tools/tests/x86_emulator/test_x86_emulator.c 
b/tools/tests/x86_emulator/test_x86_emulator.c
index c4d0f35..89603be 100644
--- a/tools/tests/x86_emulator/test_x86_emulator.c
+++ b/tools/tests/x86_emulator/test_x86_emulator.c
@@ -129,6 +129,18 @@ static inline uint64_t xgetbv(uint32_t xcr)
     (ebx & (1U << 5)) != 0; \
 })
 
+static int read_segment(
+    enum x86_segment seg,
+    struct segment_register *reg,
+    struct x86_emulate_ctxt *ctxt)
+{
+    if ( !is_x86_user_segment(seg) )
+        return X86EMUL_UNHANDLEABLE;
+    memset(reg, 0, sizeof(*reg));
+    reg->attr.fields.p = 1;
+    return X86EMUL_OKAY;
+}
+
 static int read_cr(
     unsigned int reg,
     unsigned long *val,
@@ -176,6 +188,7 @@ static struct x86_emulate_ops emulops = {
     .write      = write,
     .cmpxchg    = cmpxchg,
     .cpuid      = cpuid,
+    .read_segment = read_segment,
     .read_cr    = read_cr,
     .get_fpu    = get_fpu,
 };
@@ -649,6 +662,27 @@ int main(int argc, char **argv)
     printf("okay\n");
 #endif
 
+    printf("%-40s", "Testing mov %%cr0,%%esi (bad ModRM)...");
+    /*
+     * Mod = 1, Reg = 0, R/M = 6 would normally encode a memory reference of
+     * disp8(%esi), but mov to/from cr/dr are special and behave as if they
+     * were encoded with Mod == 3.
+     */
+    instr[0] = 0x0f; instr[1] = 0x20, instr[2] = 0x46;
+    instr[3] = 0; /* Supposed disp8. */
+    regs.esi = 0;
+    regs.eip = (unsigned long)&instr[0];
+    rc = x86_emulate(&ctxt, &emulops);
+    /*
+     * We don't care precicely what gets read from %cr4 into %esi, just so
+     * long as ModRM is treated as a register operand and 0(%esi) isn't
+     * followed as a memory reference.
+     */
+    if ( (rc != X86EMUL_OKAY) ||
+         (regs.eip != (unsigned long)&instr[3]) )
+        goto fail;
+    printf("okay\n");
+
 #define decl_insn(which) extern const unsigned char which[], which##_len[]
 #define put_insn(which, insn) ".pushsection .test, \"ax\", @progbits\n" \
                               #which ": " insn "\n"                     \
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index cca9034..7ea7d29 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1757,7 +1757,13 @@ x86_emulate(
         modrm_reg = ((rex_prefix & 4) << 1) | ((modrm & 0x38) >> 3);
         modrm_rm  = modrm & 0x07;
 
-        if ( modrm_mod == 3 )
+        if ( modrm_mod == 3 ||
+             /*
+              * Mov to/from cr/dr ignore the encoding of Mod, and behave as
+              * if they were encoded as reg/reg instructions.  No further
+              * disp/SIB bytes are fetched.
+              */
+             (twobyte && (b & ~3) == 0x20) )
         {
             modrm_rm |= (rex_prefix & 1) << 3;
             ea.type = OP_REG;
@@ -4275,7 +4281,7 @@ x86_emulate(
     case 0x21: /* mov dr,reg */
     case 0x22: /* mov reg,cr */
     case 0x23: /* mov reg,dr */
-        generate_exception_if(ea.type != OP_REG, EXC_UD, -1);
+        ASSERT(ea.type == OP_REG); /* Early operand adjustment ensures this. */
         generate_exception_if(!mode_ring0(), EXC_GP, 0);
         modrm_reg |= lock_prefix << 3;
         if ( b & 2 )
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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