Re: [PATCH v3] arm64: Improve kprobes test for atomic sequence

[Masami Hiramatsu]

On Fri,  9 Sep 2016 15:26:09 -0400
David Long <dave.long@xxxxxxxxxx> wrote:

> From: "David A. Long" <dave.long@xxxxxxxxxx>
> 
> Kprobes searches backwards a finite number of instructions to determine if
> there is an attempt to probe a load/store exclusive sequence. It stops when
> it hits the maximum number of instructions or a load or store exclusive.
> However this means it can run up past the beginning of the function and
> start looking at literal constants. This has been shown to cause a false
> positive and blocks insertion of the probe. To fix this, further limit the
> backwards search to stop if it hits a symbol address from kallsyms. The
> presumption is that this is the entry point to this code (particularly for
> the common case of placing probes at the beginning of functions).
> 
> This also improves efficiency by not searching code that is not part of the
> function. There may be some possibility that the label might not denote the
> entry path to the probed instruction but the likelihood seems low and this
> is just another example of how the kprobes user really needs to be
> careful about what they are doing.

Of course user should be careful, but also, in such case, kernel can reject
to probe it.


> 
> Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/probes/decode-insn.c | 48 ++++++++++++++++------------------
>  1 file changed, 23 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
> index 37e47a9..a691112 100644
> --- a/arch/arm64/kernel/probes/decode-insn.c
> +++ b/arch/arm64/kernel/probes/decode-insn.c
> @@ -16,6 +16,7 @@
>  #include <linux/kernel.h>
>  #include <linux/kprobes.h>
>  #include <linux/module.h>
> +#include <linux/kallsyms.h>
>  #include <asm/kprobes.h>
>  #include <asm/insn.h>
>  #include <asm/sections.h>
> @@ -122,7 +123,7 @@ arm_probe_decode_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi)
>  static bool __kprobes
>  is_probed_address_atomic(kprobe_opcode_t *scan_start, kprobe_opcode_t *scan_end)
>  {
> -	while (scan_start > scan_end) {
> +	while (scan_start >= scan_end) {
>  		/*
>  		 * atomic region starts from exclusive load and ends with
>  		 * exclusive store.
> @@ -142,33 +143,30 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
>  {
>  	enum kprobe_insn decoded;
>  	kprobe_opcode_t insn = le32_to_cpu(*addr);
> -	kprobe_opcode_t *scan_start = addr - 1;
> -	kprobe_opcode_t *scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE;
> -#if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
> -	struct module *mod;
> -#endif
> -
> -	if (addr >= (kprobe_opcode_t *)_text &&
> -	    scan_end < (kprobe_opcode_t *)_text)
> -		scan_end = (kprobe_opcode_t *)_text;
> -#if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
> -	else {
> -		preempt_disable();
> -		mod = __module_address((unsigned long)addr);
> -		if (mod && within_module_init((unsigned long)addr, mod) &&
> -			!within_module_init((unsigned long)scan_end, mod))
> -			scan_end = (kprobe_opcode_t *)mod->init_layout.base;
> -		else if (mod && within_module_core((unsigned long)addr, mod) &&
> -			!within_module_core((unsigned long)scan_end, mod))
> -			scan_end = (kprobe_opcode_t *)mod->core_layout.base;
> -		preempt_enable();
> +	kprobe_opcode_t *scan_end = 0;

Please use NULL for pointer.

> +	unsigned long size = 0, offset = 0;
> +
> +	/*
> +	 * If there's a symbol defined in front of and near enough to
> +	 * the probe address assume it is the entry point to this
> +	 * code and use it to further limit how far back we search
> +	 * when determining if we're in an atomic sequence. If we could
> +	 * not find any symbol skip the atomic test altogether as we
> +	 * could otherwise end up searching irrelevant text/literals.
> +	 * KPROBES depends on KALLSYMS so this last case should never
> +	 * happen.
> +	 */
> +	if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset)) {
> +		if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t)))
> +			scan_end = addr - (offset / sizeof(kprobe_opcode_t));
> +		else
> +			scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE;

        } else
               return INSN_REJECTED;

  that is what I expected...

Thank you,

>  	}
> -#endif
>  	decoded = arm_probe_decode_insn(insn, asi);
>  
> -	if (decoded == INSN_REJECTED ||
> -			is_probed_address_atomic(scan_start, scan_end))
> -		return INSN_REJECTED;
> +	if (decoded != INSN_REJECTED && scan_end)
> +		if (is_probed_address_atomic(addr - 1, scan_end))
> +			return INSN_REJECTED;
>  
>  	return decoded;
>  }
> -- 
> 2.5.0
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>