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[Xen-devel] [PATCH -tip v4 3/4] x86: xen: insn: Decode Xen and KVM emulate-prefix signature

Decode Xen and KVM's emulate-prefix signature by x86 insn decoder.
It is called "prefix" but actually not x86 instruction prefix, so
this adds insn.emulate_prefix_size field instead of reusing
insn.prefixes.

If x86 decoder finds a special sequence of instructions of
XEN_EMULATE_PREFIX and 'ud2a; .ascii "kvm"', it just counts the
length, set insn.emulate_prefix_size and fold it with the next
instruction. In other words, the signature and the next instruction
is treated as a single instruction.

Reported-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 Changes in v4:
  - Use asm/emulate_prefix.h instead of xen/prefix.h
  - Fix to add emulate_prefix.h to the checkist of perf.
---
 arch/x86/include/asm/insn.h                 |    6 +++++
 arch/x86/lib/insn.c                         |   34 +++++++++++++++++++++++++++
 tools/arch/x86/include/asm/emulate_prefix.h |   14 +++++++++++
 tools/arch/x86/include/asm/insn.h           |    6 +++++
 tools/arch/x86/lib/insn.c                   |   34 +++++++++++++++++++++++++++
 tools/objtool/sync-check.sh                 |    3 ++
 tools/perf/check-headers.sh                 |    3 ++
 7 files changed, 98 insertions(+), 2 deletions(-)
 create mode 100644 tools/arch/x86/include/asm/emulate_prefix.h

diff --git a/arch/x86/include/asm/insn.h b/arch/x86/include/asm/insn.h
index 154f27be8bfc..5c1ae3eff9d4 100644
--- a/arch/x86/include/asm/insn.h
+++ b/arch/x86/include/asm/insn.h
@@ -45,6 +45,7 @@ struct insn {
                struct insn_field immediate2;   /* for 64bit imm or seg16 */
        };
 
+       int     emulate_prefix_size;
        insn_attr_t attr;
        unsigned char opnd_bytes;
        unsigned char addr_bytes;
@@ -128,6 +129,11 @@ static inline int insn_is_evex(struct insn *insn)
        return (insn->vex_prefix.nbytes == 4);
 }
 
+static inline int insn_has_emulate_prefix(struct insn *insn)
+{
+       return !!insn->emulate_prefix_size;
+}
+
 /* Ensure this instruction is decoded completely */
 static inline int insn_complete(struct insn *insn)
 {
diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
index 0b5862ba6a75..404279563891 100644
--- a/arch/x86/lib/insn.c
+++ b/arch/x86/lib/insn.c
@@ -13,6 +13,8 @@
 #include <asm/inat.h>
 #include <asm/insn.h>
 
+#include <asm/emulate_prefix.h>
+
 /* Verify next sizeof(t) bytes can be on the same instruction */
 #define validate_next(t, insn, n)      \
        ((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
@@ -58,6 +60,36 @@ void insn_init(struct insn *insn, const void *kaddr, int 
buf_len, int x86_64)
                insn->addr_bytes = 4;
 }
 
+static const insn_byte_t xen_prefix[] = { __XEN_EMULATE_PREFIX };
+static const insn_byte_t kvm_prefix[] = { __KVM_EMULATE_PREFIX };
+
+static int __insn_get_emulate_prefix(struct insn *insn,
+                                    const insn_byte_t *prefix, size_t len)
+{
+       size_t i;
+
+       for (i = 0; i < len; i++) {
+               if (peek_nbyte_next(insn_byte_t, insn, i) != prefix[i])
+                       goto err_out;
+       }
+
+       insn->emulate_prefix_size = len;
+       insn->next_byte += len;
+
+       return 1;
+
+err_out:
+       return 0;
+}
+
+static void insn_get_emulate_prefix(struct insn *insn)
+{
+       if (__insn_get_emulate_prefix(insn, xen_prefix, sizeof(xen_prefix)))
+               return;
+
+       __insn_get_emulate_prefix(insn, kvm_prefix, sizeof(kvm_prefix));
+}
+
 /**
  * insn_get_prefixes - scan x86 instruction prefix bytes
  * @insn:      &struct insn containing instruction
@@ -76,6 +108,8 @@ void insn_get_prefixes(struct insn *insn)
        if (prefixes->got)
                return;
 
+       insn_get_emulate_prefix(insn);
+
        nb = 0;
        lb = 0;
        b = peek_next(insn_byte_t, insn);
diff --git a/tools/arch/x86/include/asm/emulate_prefix.h 
b/tools/arch/x86/include/asm/emulate_prefix.h
new file mode 100644
index 000000000000..70f5b98a5286
--- /dev/null
+++ b/tools/arch/x86/include/asm/emulate_prefix.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_EMULATE_PREFIX_H
+#define _ASM_X86_EMULATE_PREFIX_H
+
+/*
+ * Virt escape sequences to trigger instruction emulation;
+ * ideally these would decode to 'whole' instruction and not destroy
+ * the instruction stream; sadly this is not true for the 'kvm' one :/
+ */
+
+#define __XEN_EMULATE_PREFIX  0x0f,0x0b,0x78,0x65,0x6e  /* ud2 ; .ascii "xen" 
*/
+#define __KVM_EMULATE_PREFIX  0x0f,0x0b,0x6b,0x76,0x6d /* ud2 ; .ascii "kvm" */
+
+#endif
diff --git a/tools/arch/x86/include/asm/insn.h 
b/tools/arch/x86/include/asm/insn.h
index 37a4c390750b..568854b14d0a 100644
--- a/tools/arch/x86/include/asm/insn.h
+++ b/tools/arch/x86/include/asm/insn.h
@@ -45,6 +45,7 @@ struct insn {
                struct insn_field immediate2;   /* for 64bit imm or seg16 */
        };
 
+       int     emulate_prefix_size;
        insn_attr_t attr;
        unsigned char opnd_bytes;
        unsigned char addr_bytes;
@@ -128,6 +129,11 @@ static inline int insn_is_evex(struct insn *insn)
        return (insn->vex_prefix.nbytes == 4);
 }
 
+static inline int insn_has_emulate_prefix(struct insn *insn)
+{
+       return !!insn->emulate_prefix_size;
+}
+
 /* Ensure this instruction is decoded completely */
 static inline int insn_complete(struct insn *insn)
 {
diff --git a/tools/arch/x86/lib/insn.c b/tools/arch/x86/lib/insn.c
index 79e048f1d902..0151dfc6da61 100644
--- a/tools/arch/x86/lib/insn.c
+++ b/tools/arch/x86/lib/insn.c
@@ -13,6 +13,8 @@
 #include "../include/asm/inat.h"
 #include "../include/asm/insn.h"
 
+#include "../include/asm/emulate_prefix.h"
+
 /* Verify next sizeof(t) bytes can be on the same instruction */
 #define validate_next(t, insn, n)      \
        ((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
@@ -58,6 +60,36 @@ void insn_init(struct insn *insn, const void *kaddr, int 
buf_len, int x86_64)
                insn->addr_bytes = 4;
 }
 
+static const insn_byte_t xen_prefix[] = { __XEN_EMULATE_PREFIX };
+static const insn_byte_t kvm_prefix[] = { __KVM_EMULATE_PREFIX };
+
+static int __insn_get_emulate_prefix(struct insn *insn,
+                                    const insn_byte_t *prefix, size_t len)
+{
+       size_t i;
+
+       for (i = 0; i < len; i++) {
+               if (peek_nbyte_next(insn_byte_t, insn, i) != prefix[i])
+                       goto err_out;
+       }
+
+       insn->emulate_prefix_size = len;
+       insn->next_byte += len;
+
+       return 1;
+
+err_out:
+       return 0;
+}
+
+static void insn_get_emulate_prefix(struct insn *insn)
+{
+       if (__insn_get_emulate_prefix(insn, xen_prefix, sizeof(xen_prefix)))
+               return;
+
+       __insn_get_emulate_prefix(insn, kvm_prefix, sizeof(kvm_prefix));
+}
+
 /**
  * insn_get_prefixes - scan x86 instruction prefix bytes
  * @insn:      &struct insn containing instruction
@@ -76,6 +108,8 @@ void insn_get_prefixes(struct insn *insn)
        if (prefixes->got)
                return;
 
+       insn_get_emulate_prefix(insn);
+
        nb = 0;
        lb = 0;
        b = peek_next(insn_byte_t, insn);
diff --git a/tools/objtool/sync-check.sh b/tools/objtool/sync-check.sh
index 0a832e265a50..9bd04bbed01e 100755
--- a/tools/objtool/sync-check.sh
+++ b/tools/objtool/sync-check.sh
@@ -4,6 +4,7 @@
 FILES='
 arch/x86/include/asm/inat_types.h
 arch/x86/include/asm/orc_types.h
+arch/x86/include/asm/emulate_prefix.h
 arch/x86/lib/x86-opcode-map.txt
 arch/x86/tools/gen-insn-attr-x86.awk
 '
@@ -46,6 +47,6 @@ done
 check arch/x86/include/asm/inat.h     '-I "^#include 
[\"<]\(asm/\)*inat_types.h[\">]"'
 check arch/x86/include/asm/insn.h     '-I "^#include 
[\"<]\(asm/\)*inat.h[\">]"'
 check arch/x86/lib/inat.c             '-I "^#include 
[\"<]\(../include/\)*asm/insn.h[\">]"'
-check arch/x86/lib/insn.c             '-I "^#include 
[\"<]\(../include/\)*asm/in\(at\|sn\).h[\">]"'
+check arch/x86/lib/insn.c             '-I "^#include 
[\"<]\(../include/\)*asm/in\(at\|sn\).h[\">]" -I "^#include 
[\"<]\(../include/\)*asm/emulate_prefix.h[\">]"'
 
 cd -
diff --git a/tools/perf/check-headers.sh b/tools/perf/check-headers.sh
index e2e0f06c97d0..988b1879ec98 100755
--- a/tools/perf/check-headers.sh
+++ b/tools/perf/check-headers.sh
@@ -27,6 +27,7 @@ arch/x86/include/asm/disabled-features.h
 arch/x86/include/asm/required-features.h
 arch/x86/include/asm/cpufeatures.h
 arch/x86/include/asm/inat_types.h
+arch/x86/include/asm/emulate_prefix.h
 arch/x86/include/uapi/asm/prctl.h
 arch/x86/lib/x86-opcode-map.txt
 arch/x86/tools/gen-insn-attr-x86.awk
@@ -115,7 +116,7 @@ check lib/ctype.c                 '-I "^EXPORT_SYMBOL" -I 
"^#include <linux/export.h>" -B
 check arch/x86/include/asm/inat.h     '-I "^#include 
[\"<]\(asm/\)*inat_types.h[\">]"'
 check arch/x86/include/asm/insn.h     '-I "^#include 
[\"<]\(asm/\)*inat.h[\">]"'
 check arch/x86/lib/inat.c            '-I "^#include 
[\"<]\(../include/\)*asm/insn.h[\">]"'
-check arch/x86/lib/insn.c            '-I "^#include 
[\"<]\(../include/\)*asm/in\(at\|sn\).h[\">]"'
+check arch/x86/lib/insn.c             '-I "^#include 
[\"<]\(../include/\)*asm/in\(at\|sn\).h[\">]" -I "^#include 
[\"<]\(../include/\)*asm/emulate_prefix.h[\">]"'
 
 # diff non-symmetric files
 check_2 tools/perf/arch/x86/entry/syscalls/syscall_64.tbl 
arch/x86/entry/syscalls/syscall_64.tbl


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