Re: [PATCH -tip v9 3/7] kprobes: checks probe address is instruction boudary on x86

[Ananth N Mavinakayanahalli <ananth@in.ibm.com>]

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@redhat.com>
> Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
> Cc: Jim Keniston <jkenisto@us.ibm.com>
> Cc: Ingo Molnar <mingo@elte.hu>

Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>

> ---
> 
>  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@redhat.com