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

[David Long]

On 09/10/2016 01:48 AM, Masami Hiramatsu wrote:
> 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.

I'm not exactly sure what you mean.  I'm just saying when everything 
goes right we still cannot promise perfection in detecting a probe 
within an atomic sequence.  This patch will reject a probe that is after 
a ldx and has no intervening kallsyms label (and assuming it's within 
the defined maximum count of subsequent instructions).

> > 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.

A change has been made for v4.

> > +	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

Thanks,
-dl