[PATCH -tip] [BUGFIX] kprobes/x86: Fix kprobes to skip prefixes correctly

From: Masami Hiramatsu
Date: Tue Jun 29 2010 - 01:54:15 EST


Fix resume_execution() and is_IF_modifier() to skip x86 instruction
prefixes correctly by using x86 instruction attribute.

Without this fix, resume_execution() can't handle instructions which
have non-REX prefixes (REX prefixes are skipped). This will cause
unexpected kernel panic by hitting bad address when a kprobe hits
on two-byte ret (e.g. "repz ret" generated for Athlon/K8 optimization),
because it just checks "repz" and can't recognize the "ret" instruction.

These prefixes can be found easily with x86 instruction attribute.
This patch introduces skip_prefixes() and uses it in resume_execution()
and is_IF_modifier() to skip prefixes.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
---

arch/x86/kernel/kprobes.c | 33 +++++++++++++++++----------------
1 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index 345a4b1..175f85c 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -126,16 +126,22 @@ static void __kprobes synthesize_reljump(void *from, void *to)
}

/*
- * Check for the REX prefix which can only exist on X86_64
- * X86_32 always returns 0
+ * Skip the prefixes of the instruction.
*/
-static int __kprobes is_REX_prefix(kprobe_opcode_t *insn)
+static kprobe_opcode_t *__kprobes skip_prefixes(kprobe_opcode_t *insn)
{
+ insn_attr_t attr;
+
+ attr = inat_get_opcode_attribute((insn_byte_t)*insn);
+ while (inat_is_legacy_prefix(attr)) {
+ insn++;
+ attr = inat_get_opcode_attribute((insn_byte_t)*insn);
+ }
#ifdef CONFIG_X86_64
- if ((*insn & 0xf0) == 0x40)
- return 1;
+ if (inat_is_rex_prefix(attr))
+ insn++;
#endif
- return 0;
+ return insn;
}

/*
@@ -272,6 +278,9 @@ static int __kprobes can_probe(unsigned long paddr)
*/
static int __kprobes is_IF_modifier(kprobe_opcode_t *insn)
{
+ /* Skip prefixes */
+ insn = skip_prefixes(insn);
+
switch (*insn) {
case 0xfa: /* cli */
case 0xfb: /* sti */
@@ -280,13 +289,6 @@ static int __kprobes is_IF_modifier(kprobe_opcode_t *insn)
return 1;
}

- /*
- * on X86_64, 0x40-0x4f are REX prefixes so we need to look
- * at the next byte instead.. but of course not recurse infinitely
- */
- if (is_REX_prefix(insn))
- return is_IF_modifier(++insn);
-
return 0;
}

@@ -803,9 +805,8 @@ static void __kprobes resume_execution(struct kprobe *p,
unsigned long orig_ip = (unsigned long)p->addr;
kprobe_opcode_t *insn = p->ainsn.insn;

- /*skip the REX prefix*/
- if (is_REX_prefix(insn))
- insn++;
+ /* Skip prefixes */
+ insn = skip_prefixes(insn);

regs->flags &= ~X86_EFLAGS_TF;
switch (*insn) {



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