Messages in this thread Patch in this message | | | From | Andrew Jeffery <> | Subject | [PATCH] ARM: kprobes: Avoid fortify_panic() when copying optprobe template | Date | Mon, 18 May 2020 01:09:59 +0930 |
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Setting both CONFIG_KPROBES=y and CONFIG_FORTIFY_SOURCE=y on ARM leads to a panic in memcpy() when injecting a kprobe despite the fixes found in commit e46daee53bb5 ("ARM: 8806/1: kprobes: Fix false positive with FORTIFY_SOURCE") and commit 0ac569bf6a79 ("ARM: 8834/1: Fix: kprobes: optimized kprobes illegal instruction").
arch/arm/include/asm/kprobes.h effectively declares the target type of the optprobe_template_entry assembly label as a u32, which leads memcpy()'s __builtin_object_size() call to determine that the pointed-to object is of size four. In practical terms the symbol is used as a handle for the optimised probe assembly template that is at least 96 bytes in size. The symbol's use despite its type blows up the memcpy() in ARM's arch_prepare_optimized_kprobe() with a false-positive fortify_panic() when it should instead copy the optimised probe template into place.
As mentioned, a couple of attempts have been made to address the issue by casting a pointer to optprobe_template_entry before providing it to memcpy(), however gccs such as Ubuntu 20.04's arm-linux-gnueabi-gcc 9.3.0 (Ubuntu 9.3.0-10ubuntu1) see through these efforts.
Squash the false-positive by aliasing the template assembly with a new symbol 'arm_optprobe_template'; declare it as a function object and pass the function object as the argument to memcpy() such that __builtin_object_size() cannot immediately determine the object size.
Fixes: e46daee53bb5 ("ARM: 8806/1: kprobes: Fix false positive with FORTIFY_SOURCE") Fixes: 0ac569bf6a79 ("ARM: 8834/1: Fix: kprobes: optimized kprobes illegal instruction") Signed-off-by: Andrew Jeffery <andrew@aj.id.au> --- arch/arm/include/asm/kprobes.h | 7 +++++++ arch/arm/probes/kprobes/opt-arm.c | 4 +++- 2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/arm/include/asm/kprobes.h b/arch/arm/include/asm/kprobes.h index 213607a1f45c..94db8bf25f9c 100644 --- a/arch/arm/include/asm/kprobes.h +++ b/arch/arm/include/asm/kprobes.h @@ -43,6 +43,13 @@ int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr); int kprobe_exceptions_notify(struct notifier_block *self, unsigned long val, void *data); +/* + * The optprobe template buffer is not anything that should be called directly, + * however describe it as a function to give ourselves a handle to it that + * bypasses CONFIG_FORTIFY_SOURCE=y sanity checks in memcpy(). + */ +extern __visible void arm_optprobe_template(void); + /* optinsn template addresses */ extern __visible kprobe_opcode_t optprobe_template_entry; extern __visible kprobe_opcode_t optprobe_template_val; diff --git a/arch/arm/probes/kprobes/opt-arm.c b/arch/arm/probes/kprobes/opt-arm.c index 7a449df0b359..59133d59616a 100644 --- a/arch/arm/probes/kprobes/opt-arm.c +++ b/arch/arm/probes/kprobes/opt-arm.c @@ -31,6 +31,8 @@ * to the stack cost of the instruction. */ asm ( + ".global arm_optprobe_template\n" + "arm_optprobe_template:\n" ".global optprobe_template_entry\n" "optprobe_template_entry:\n" ".global optprobe_template_sub_sp\n" @@ -234,7 +236,7 @@ int arch_prepare_optimized_kprobe(struct optimized_kprobe *op, struct kprobe *or } /* Copy arch-dep-instance from template. */ - memcpy(code, (unsigned long *)&optprobe_template_entry, + memcpy(code, arm_optprobe_template, TMPL_END_IDX * sizeof(kprobe_opcode_t)); /* Adjust buffer according to instruction. */ -- 2.25.1
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