uprobes/x86: Teach arch_uprobe_post_xol() to restart if possible

SIGILL after the failed arch_uprobe_post_xol() should only be used as
a last resort, we should try to restart the probed insn if possible.

Currently only adjust_ret_addr() can fail, and this can only happen if
another thread unmapped our stack after we executed "call" out-of-line.
Most probably the application if buggy, but even in this case it can
have a handler for SIGSEGV/etc. And in theory it can be even correct
and do something non-trivial with its memory.

Of course we can't restart unconditionally, so arch_uprobe_post_xol()
does this only if ->post_xol() returns -ERESTART even if currently this
is the only possible error.

default_post_xol_op(UPROBE_FIX_CALL) can always restart, but as Jim
pointed out it should not forget to pop off the return address pushed
by this insn executed out-of-line.

Note: this is not "perfect", we do not want the extra handler_chain()
after restart, but I think this is the best solution we can realistically
do without too much uglifications.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: Jim Keniston <jkenisto@us.ibm.com>

1 files changed, 16 insertions, 4 deletions

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index e72903eacd43..cdd6909affcb 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -443,16 +443,22 @@ static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs
 {
         struct uprobe_task *utask = current->utask;
         long correction = (long)(utask->vaddr - utask->xol_vaddr);
-        int ret = 0;
 
         handle_riprel_post_xol(auprobe, regs, &correction);
         if (auprobe->fixups & UPROBE_FIX_IP)
                 regs->ip += correction;
 
-        if (auprobe->fixups & UPROBE_FIX_CALL)
-                ret = adjust_ret_addr(regs->sp, correction);
+        if (auprobe->fixups & UPROBE_FIX_CALL) {
+                if (adjust_ret_addr(regs->sp, correction)) {
+                        if (is_ia32_task())
+                                regs->sp += 4;
+                        else
+                                regs->sp += 8;
+                        return -ERESTART;
+                }
+        }
 
-        return ret;
+        return 0;
 }
 
 static struct uprobe_xol_ops default_xol_ops = {
@@ -599,6 +605,12 @@ int arch_uprobe_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
                 int err = auprobe->ops->post_xol(auprobe, regs);
                 if (err) {
                         arch_uprobe_abort_xol(auprobe, regs);
+                        /*
+                         * Restart the probed insn. ->post_xol() must ensure
+                         * this is really possible if it returns -ERESTART.
+                         */
+                        if (err == -ERESTART)
+                                return 0;
                         return err;
                 }
         }