On Mon, 12 Sep 2016 14:21:27 -0400
David Long <dave.l...@linaro.org> wrote:
> From: "David A. Long" <dave.l...@linaro.org>
>
> 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.
>
> Signed-off-by: David A. Long <dave.l...@linaro.org>
> ---
> 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..d1731bf 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 = NULL;
> + 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;
> }
Hmm, could you tell me what will happen if kallsyms_lookup_size_offset()
failed here?
Thanks,
> -#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 <mhira...@kernel.org>
