[PATCH] arm64: Improve kprobes test for atomic sequence

[David Long]

From: "David A. Long" <dave.long at linaro.org>

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 add a test to see
if the typical:

	"stp x29, x30, [sp, #n]!"

instruction beginning a function gets hit. This also improves efficiency by
not testing code that is not part of the function. There is some
possibility that a function will not begin with this instruction, in which
case the fixed code will behave no worse than before.

There could also be the case that the stp instruction is found further in
the body of the function, which could theoretically allow probing of an
atomic squence. The likelihood of this seems low, and this would not be the
only aspect of kprobes where the user needs to be careful to avoid
problems.

Signed-off-by: David A. Long <dave.long at linaro.org>
---
 arch/arm64/kernel/probes/decode-insn.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
index 37e47a9..248e820 100644
--- a/arch/arm64/kernel/probes/decode-insn.c
+++ b/arch/arm64/kernel/probes/decode-insn.c
@@ -122,16 +122,28 @@ 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)
 {
+	const u32 stp_x29_x30_sp_pre = 0xa9807bfd;
+	const u32 stp_ignore_index_mask = 0xffc07fff;
+	u32 instruction = le32_to_cpu(*scan_start);
+
 	while (scan_start > scan_end) {
 		/*
-		 * atomic region starts from exclusive load and ends with
-		 * exclusive store.
+		 * Atomic region starts from exclusive load and ends with
+		 * exclusive store. If we hit a "stp x29, x30, [sp, #n]!"
+		 * assume it is the beginning of the function and end the
+		 * search. This helps avoid false positives from literal
+		 * constants that look like a load-exclusive, in addition
+		 * to being more efficient.
 		 */
-		if (aarch64_insn_is_store_ex(le32_to_cpu(*scan_start)))
+		if ((instruction & stp_ignore_index_mask) == stp_x29_x30_sp_pre)
 			return false;
-		else if (aarch64_insn_is_load_ex(le32_to_cpu(*scan_start)))
-			return true;
+
 		scan_start--;
+		instruction = le32_to_cpu(*scan_start);
+		if (aarch64_insn_is_store_ex(instruction))
+			return false;
+		else if (aarch64_insn_is_load_ex(instruction))
+			return true;
 	}
 
 	return false;
@@ -142,7 +154,6 @@ 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;
@@ -167,7 +178,7 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
 	decoded = arm_probe_decode_insn(insn, asi);
 
 	if (decoded == INSN_REJECTED ||
-			is_probed_address_atomic(scan_start, scan_end))
+			is_probed_address_atomic(addr, scan_end))
 		return INSN_REJECTED;
 
 	return decoded;
-- 
2.5.0