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

Kees Cook
2020-05-17T21:48:55Z

On Mon, May 18, 2020 at 01:09:59AM +0930, Andrew Jeffery wrote:
> 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.

Ah, dang. :P

How about converting them all to unsized arrays, which would also allow
the code to drop the "&" everywhere, I think. This is untested:

diff --git a/arch/arm/include/asm/kprobes.h b/arch/arm/include/asm/kprobes.h
index 213607a1f45c..e26a278d301a 100644
--- a/arch/arm/include/asm/kprobes.h
+++ b/arch/arm/include/asm/kprobes.h
@@ -44,20 +44,20 @@ int kprobe_exceptions_notify(struct notifier_block *self,
unsigned long val, void *data);

/* optinsn template addresses */
-extern __visible kprobe_opcode_t optprobe_template_entry;
-extern __visible kprobe_opcode_t optprobe_template_val;
-extern __visible kprobe_opcode_t optprobe_template_call;
-extern __visible kprobe_opcode_t optprobe_template_end;
-extern __visible kprobe_opcode_t optprobe_template_sub_sp;
-extern __visible kprobe_opcode_t optprobe_template_add_sp;
-extern __visible kprobe_opcode_t optprobe_template_restore_begin;
-extern __visible kprobe_opcode_t optprobe_template_restore_orig_insn;
-extern __visible kprobe_opcode_t optprobe_template_restore_end;
+extern __visible kprobe_opcode_t optprobe_template_entry[];
+extern __visible kprobe_opcode_t optprobe_template_val[];
+extern __visible kprobe_opcode_t optprobe_template_call[];
+extern __visible kprobe_opcode_t optprobe_template_end[];
+extern __visible kprobe_opcode_t optprobe_template_sub_sp[];
+extern __visible kprobe_opcode_t optprobe_template_add_sp[];
+extern __visible kprobe_opcode_t optprobe_template_restore_begin[];
+extern __visible kprobe_opcode_t optprobe_template_restore_orig_insn[];
+extern __visible kprobe_opcode_t optprobe_template_restore_end[];

#define MAX_OPTIMIZED_LENGTH 4
#define MAX_OPTINSN_SIZE \
- ((unsigned long)&optprobe_template_end - \
- (unsigned long)&optprobe_template_entry)
+ ((unsigned long)optprobe_template_end - \
+ (unsigned long)optprobe_template_entry)
#define RELATIVEJUMP_SIZE 4

struct arch_optimized_insn {
diff --git a/arch/arm/probes/kprobes/opt-arm.c b/arch/arm/probes/kprobes/opt-arm.c
index 7a449df0b359..c78180172120 100644
--- a/arch/arm/probes/kprobes/opt-arm.c
+++ b/arch/arm/probes/kprobes/opt-arm.c
@@ -85,21 +85,21 @@ asm (
"optprobe_template_end:\n");

#define TMPL_VAL_IDX \
- ((unsigned long *)&optprobe_template_val - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_val - (unsigned long *)optprobe_template_entry)
#define TMPL_CALL_IDX \
- ((unsigned long *)&optprobe_template_call - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_call - (unsigned long *)optprobe_template_entry)
#define TMPL_END_IDX \
- ((unsigned long *)&optprobe_template_end - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_end - (unsigned long *)optprobe_template_entry)
#define TMPL_ADD_SP \
- ((unsigned long *)&optprobe_template_add_sp - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_add_sp - (unsigned long *)optprobe_template_entry)
#define TMPL_SUB_SP \
- ((unsigned long *)&optprobe_template_sub_sp - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_sub_sp - (unsigned long *)optprobe_template_entry)
#define TMPL_RESTORE_BEGIN \
- ((unsigned long *)&optprobe_template_restore_begin - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_restore_begin - (unsigned long *)optprobe_template_entry)
#define TMPL_RESTORE_ORIGN_INSN \
- ((unsigned long *)&optprobe_template_restore_orig_insn - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_restore_orig_insn - (unsigned long *)optprobe_template_entry)
#define TMPL_RESTORE_END \
- ((unsigned long *)&optprobe_template_restore_end - (unsigned long *)&optprobe_template_entry)
+ ((unsigned long *)optprobe_template_restore_end - (unsigned long *)optprobe_template_entry)

/*
* ARM can always optimize an instruction when using ARM ISA, except
@@ -234,7 +234,7 @@ 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, (unsigned long *)optprobe_template_entry,
TMPL_END_IDX * sizeof(kprobe_opcode_t));

/* Adjust buffer according to instruction. */

--
Kees Cook