[PATCH] x86/kprobes: Fix optprobe to detect padding int3 correctly

[Masami Hiramatsu]

Fix optprobe to detect padding int3 correctly.

Since commit 7705dc855797 ("x86/vmlinux: Use INT3 instead of NOP
for linker fill bytes") changed the padding bytes between functions
from nop to int3, when optprobe decodes a target function it finds
int3 and gives up the jump optimization.

Instead of giving up any int3 detection, this checks whether the
rest of bytes to the end of the function are int3 or not. If all
of those are int3, those come from the linker. In that case,
optprobe continues jump optimization.

Fixes: 7705dc855797 ("x86/vmlinux: Use INT3 instead of NOP for linker fill bytes")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Adam Zabrocki <pi3@xxxxxxxxxx>
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---
 arch/x86/kernel/kprobes/opt.c |   22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 041f0b50bc27..b5cf39f1a855 100644
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -272,6 +272,19 @@ static int insn_is_indirect_jump(struct insn *insn)
 	return ret;
 }
 
+static bool is_padding_int3(unsigned long addr, unsigned long eaddr)
+{
+	unsigned char ops;
+
+	for (; addr < eaddr; addr++) {
+		if (get_kernel_nofault(ops, (void *)addr) < 0 ||
+		    ops != INT3_INSN_OPCODE)
+			return false;
+	}
+
+	return true;
+}
+
 /* Decode whole function to ensure any instructions don't jump into target */
 static int can_optimize(unsigned long paddr)
 {
@@ -310,9 +323,14 @@ static int can_optimize(unsigned long paddr)
 			return 0;
 		kernel_insn_init(&insn, (void *)recovered_insn, MAX_INSN_SIZE);
 		insn_get_length(&insn);
-		/* Another subsystem puts a breakpoint */
+		/*
+		 * In the case of detecting unknown breakpoint, this could be
+		 * a padding int3 between functions. Let's check that all the
+		 * rest of the bytes are also int3.
+		 */
 		if (insn.opcode.bytes[0] == INT3_INSN_OPCODE)
-			return 0;
+			return is_padding_int3(addr, paddr - offset + size) ? 1 : 0;
+
 		/* Recover address */
 		insn.kaddr = (void *)addr;
 		insn.next_byte = (void *)(addr + insn.length);