kprobes/x86: Return correct length in __copy_instruction()

On x86-64, __copy_instruction() always returns 0 (error) if the
instruction uses %rip-relative addressing. This is because
kernel_insn_init() is called the second time for 'insn' instance
in such cases and sets all its fields to 0.

Because of this, trying to place a kprobe on such instruction
will fail, register_kprobe() will return -EINVAL.

This patch fixes the problem.

Signed-off-by: Eugene Shatokhin <eugene.shatokhin@rosalab.ru>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Link: http://lkml.kernel.org/r/20150317100918.28349.94654.stgit@localhost.localdomain
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 4e3d5a9621fe..03189d86357d 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -354,6 +354,7 @@ int __copy_instruction(u8 *dest, u8 *src)
 {
         struct insn insn;
         kprobe_opcode_t buf[MAX_INSN_SIZE];
+        int length;
         unsigned long recovered_insn =
                 recover_probed_instruction(buf, (unsigned long)src);
 
@@ -361,16 +362,18 @@ int __copy_instruction(u8 *dest, u8 *src)
                 return 0;
         kernel_insn_init(&insn, (void *)recovered_insn, MAX_INSN_SIZE);
         insn_get_length(&insn);
+        length = insn.length;
+
         /* Another subsystem puts a breakpoint, failed to recover */
         if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
                 return 0;
-        memcpy(dest, insn.kaddr, insn.length);
+        memcpy(dest, insn.kaddr, length);
 
 #ifdef CONFIG_X86_64
         if (insn_rip_relative(&insn)) {
                 s64 newdisp;
                 u8 *disp;
-                kernel_insn_init(&insn, dest, insn.length);
+                kernel_insn_init(&insn, dest, length);
                 insn_get_displacement(&insn);
                 /*
                  * The copied instruction uses the %rip-relative addressing
@@ -394,7 +397,7 @@ int __copy_instruction(u8 *dest, u8 *src)
                 *(s32 *) disp = (s32) newdisp;
         }
 #endif
-        return insn.length;
+        return length;
 }
 
 static int arch_copy_kprobe(struct kprobe *p)