[PATCH v2] arm64: Improve kprobes test for atomic sequence

[David Long]

From: "David A. Long" <dave.long@xxxxxxxxxx>

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, further limit the
backwards search to stop if it hits a symbol address from kallsyms. The
presumption is that this is the entry point to this code (particularly for
the common case of placing probes at the beginning of functions).

This also improves efficiency by not searching code that is not part of the
function. There may be some possibility that the label might not denote the
entry path to the probed instruction but the likelihood seems low and this
is just another example of how the kprobes user really needs to be
careful about what they are doing.

Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>
---
 arch/arm64/kernel/probes/decode-insn.c | 48 +++++++++++++++++++++++-----------
 1 file changed, 33 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
index 37e47a9..356ee52 100644
--- a/arch/arm64/kernel/probes/decode-insn.c
+++ b/arch/arm64/kernel/probes/decode-insn.c
@@ -16,6 +16,7 @@
 #include <linux/kernel.h>
 #include <linux/kprobes.h>
 #include <linux/module.h>
+#include <linux/kallsyms.h>
 #include <asm/kprobes.h>
 #include <asm/insn.h>
 #include <asm/sections.h>
@@ -122,7 +123,7 @@ 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)
 {
-	while (scan_start > scan_end) {
+	while (scan_start >= scan_end) {
 		/*
 		 * atomic region starts from exclusive load and ends with
 		 * exclusive store.
@@ -144,26 +145,43 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
 	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;
+	unsigned long size = 0, offset = 0;
 #if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
 	struct module *mod;
 #endif
 
-	if (addr >= (kprobe_opcode_t *)_text &&
-	    scan_end < (kprobe_opcode_t *)_text)
-		scan_end = (kprobe_opcode_t *)_text;
+	/*
+	 * If there's a symbol defined in front of and near enough to
+	 * the probe address assume it is the entry point to this
+	 * code and use it to further limit how far back we search
+	 * when determining if we're in an atomic sequence.
+	 */
+	if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset))
+		if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t)))
+			scan_end = addr - (offset / sizeof(kprobe_opcode_t));
+
+	if (scan_end <= scan_start) {
+		if (addr >= (kprobe_opcode_t *)_text &&
+		    scan_end < (kprobe_opcode_t *)_text)
+			scan_end = (kprobe_opcode_t *)_text;
 #if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
-	else {
-		preempt_disable();
-		mod = __module_address((unsigned long)addr);
-		if (mod && within_module_init((unsigned long)addr, mod) &&
-			!within_module_init((unsigned long)scan_end, mod))
-			scan_end = (kprobe_opcode_t *)mod->init_layout.base;
-		else if (mod && within_module_core((unsigned long)addr, mod) &&
-			!within_module_core((unsigned long)scan_end, mod))
-			scan_end = (kprobe_opcode_t *)mod->core_layout.base;
-		preempt_enable();
-	}
+		else {
+			preempt_disable();
+			mod = __module_address((unsigned long)addr);
+			if (mod &&
+			    within_module_init((unsigned long)addr, mod) &&
+			    !within_module_init((unsigned long)scan_end, mod))
+				scan_end =
+				    (kprobe_opcode_t *)mod->init_layout.base;
+			else if (mod &&
+			    within_module_core((unsigned long)addr, mod) &&
+			    !within_module_core((unsigned long)scan_end, mod))
+				scan_end =
+				    (kprobe_opcode_t *)mod->core_layout.base;
+			preempt_enable();
+		}
 #endif
+	}
 	decoded = arm_probe_decode_insn(insn, asi);
 
 	if (decoded == INSN_REJECTED ||
-- 
2.5.0