On Mon, Jun 01, 2009 at 08:37:31PM -0400, Masami Hiramatsu wrote:
> 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@xxxxxxxxxx>
> Cc: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
> Cc: Jim Keniston <jkenisto@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxx>
Acked-by: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
> ---
>
> 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 7b5169d..41d524f 100644
> --- a/arch/x86/kernel/kprobes.c
> +++ b/arch/x86/kernel/kprobes.c
> @@ -48,12 +48,14 @@
> #include <linux/preempt.h>
> #include <linux/module.h>
> #include <linux/kdebug.h>
> +#include <linux/kallsyms.h>
>
> #include <asm/cacheflush.h>
> #include <asm/desc.h>
> #include <asm/pgtable.h>
> #include <asm/uaccess.h>
> #include <asm/alternative.h>
> +#include <asm/insn.h>
>
> void jprobe_return_end(void);
>
> @@ -244,6 +246,71 @@ 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.
> */
> @@ -359,6 +426,8 @@ 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)
>
>
> --
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
>
> e-mail: mhiramat@xxxxxxxxxx
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