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Date:   Sun, 17 May 2020 14:48:52 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Andrew Jeffery <andrew@aj.id.au>
Cc:     linux-arm-kernel@lists.infradead.org, linux@armlinux.org.uk,
        mhiramat@kernel.org, labbott@redhat.com,
        mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ARM: kprobes: Avoid fortify_panic() when copying
 optprobe template
Message-ID: <202005171447.00CFE0C@keescook>
References: <20200517153959.293224-1-andrew@aj.id.au>
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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