Re: [PATCH v2] arm64: Improve kprobes test for atomic sequence

[Masami Hiramatsu]

On Wed, 7 Sep 2016 10:12:14 -0400
David Long <dave.long@xxxxxxxxxx> wrote:

> On 09/07/2016 01:52 AM, Masami Hiramatsu wrote:
> > On Tue,  6 Sep 2016 13:54:59 -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.
> >
> > Hi Dave,
> >
> > kallsyms_lookup_size_offset() part looks good to me. By the way,
> > is there any reason we'll check the _text and module's base address
> > boundary?
> > I think those are already searced by kallsyms_lookup_size_offset(),
> > so you don't need to check those. If the address is not found by
> > kallsyms_lookup_size_offset(), that address maybe out-of-text.
> >
> 
> CONFIG KPROBES does currently select CONFIG_KALLSYMS, but is it wise for 
> this code to depend on that?  Perhaps the text boundary checking should 
> be moved under an else clause for the case of 
> kallsyms_lookup_size_offset() failing?

Would you have any case where the address is in kernel_text but kallsyms_lookup
failed to find symbol? IMHO, even if there is, we should reject probing on
such "nowhere" address. 

Thank you,

> 
> > Thank you,
> >>
> >> Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>
> >> ---
> >>   arch/arm64/kernel/probes/decode-insn.c | 48 +++++++++++++++++++++++-----------
> >>   1 file changed, 33 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
> >> index 37e47a9..356ee52 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.
> >> @@ -144,26 +145,43 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
> >>   	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;
> >> +	unsigned long size = 0, offset = 0;
> >>   #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 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 (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));
> >> +
> >> +	if (scan_end <= scan_start) {
> >> +		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();
> >> -	}
> >> +		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();
> >> +		}
> >>   #endif
> >> +	}
> >>   	decoded = arm_probe_decode_insn(insn, asi);
> >>
> >>   	if (decoded == INSN_REJECTED ||
> >> --
> >> 2.5.0
> >>
> 
> Thanks,
> -dl
> 
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>