[PATCH -tip -v10 3/7] kprobes: checks probe address is instruction boudary on x86

Masami Hiramatsu
2009-07-01T01:25:33Z

Ensure safeness of inserting kprobes by checking whether the specified
address is at the first byte of a instruction on x86.
This is done by decoding probed function from its head to the probe point.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Jim Keniston <jkenisto@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
---

arch/x86/kernel/kprobes.c | 69 +++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 69 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index b5b1848..5341842 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -48,6 +48,7 @@
#include <linux/preempt.h>
#include <linux/module.h>
#include <linux/kdebug.h>
+#include <linux/kallsyms.h>

#include <asm/cacheflush.h>
#include <asm/desc.h>
@@ -55,6 +56,7 @@
#include <asm/uaccess.h>
#include <asm/alternative.h>
#include <asm/debugreg.h>
+#include <asm/insn.h>

void jprobe_return_end(void);

@@ -245,6 +247,71 @@ retry:
}
}

+/* Recover the probed instruction at addr for further analysis. */
+static int recover_probed_instruction(kprobe_opcode_t *buf, unsigned long addr)
+{
+ struct kprobe *kp;
+ kp = get_kprobe((void *)addr);
+ if (!kp)
+ return -EINVAL;
+
+ /*
+ * Basically, kp->ainsn.insn has an original instruction.
+ * However, RIP-relative instruction can not do single-stepping
+ * at different place, fix_riprel() tweaks the displacement of
+ * that instruction. In that case, we can't recover the instruction
+ * from the kp->ainsn.insn.
+ *
+ * On the other hand, kp->opcode has a copy of the first byte of
+ * the probed instruction, which is overwritten by int3. And
+ * the instruction at kp->addr is not modified by kprobes except
+ * for the first byte, we can recover the original instruction
+ * from it and kp->opcode.
+ */
+ memcpy(buf, kp->addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
+ buf[0] = kp->opcode;
+ return 0;
+}
+
+/* Dummy buffers for kallsyms_lookup */
+static char __dummy_buf[KSYM_NAME_LEN];
+
+/* Check if paddr is at an instruction boundary */
+static int __kprobes can_probe(unsigned long paddr)
+{
+ int ret;
+ unsigned long addr, offset = 0;
+ struct insn insn;
+ kprobe_opcode_t buf[MAX_INSN_SIZE];
+
+ if (!kallsyms_lookup(paddr, NULL, &offset, NULL, __dummy_buf))
+ return 0;
+
+ /* Decode instructions */
+ addr = paddr - offset;
+ while (addr < paddr) {
+ kernel_insn_init(&insn, (void *)addr);
+ insn_get_opcode(&insn);
+
+ /* Check if the instruction has been modified. */
+ if (OPCODE1(&insn) == BREAKPOINT_INSTRUCTION) {
+ ret = recover_probed_instruction(buf, addr);
+ if (ret)
+ /*
+ * Another debugging subsystem might insert
+ * this breakpoint. In that case, we can't
+ * recover it.
+ */
+ return 0;
+ kernel_insn_init(&insn, buf);
+ }
+ insn_get_length(&insn);
+ addr += insn.length;
+ }
+
+ return (addr == paddr);
+}
+
/*
* Returns non-zero if opcode modifies the interrupt flag.
*/
@@ -360,6 +427,8 @@ static void __kprobes arch_copy_kprobe(struct kprobe *p)

int __kprobes arch_prepare_kprobe(struct kprobe *p)
{
+ if (!can_probe((unsigned long)p->addr))
+ return -EILSEQ;
/* insn: must be on special executable page on x86. */
p->ainsn.insn = get_insn_slot();
if (!p->ainsn.insn)