[PATCH] ARM: kprobes: Avoid fortify_panic() when copying optprobe template

[Andrew Jeffery]

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@xxxxxxxx>
---
 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