ptrace: PTRACE_GETFDPIC: fix the unsafe usage of child->mm

Now that Mike Frysinger unified the FDPIC ptrace code, we can fix the
unsafe usage of child->mm in ptrace_request(PTRACE_GETFDPIC).

We have the reference to task_struct, and ptrace_check_attach() verified
the tracee is stopped.  But nothing can protect from SIGKILL after that,
we must not assume child->mm != NULL.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Mike Frysinger <vapier.adi@gmail.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Greg Ungerer <gerg@snapgear.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 4b4f72084d1b..74a3d693c196 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -596,18 +596,24 @@ int ptrace_request(struct task_struct *child, long request,
 
 #ifdef CONFIG_BINFMT_ELF_FDPIC
         case PTRACE_GETFDPIC: {
+                struct mm_struct *mm = get_task_mm(child);
                 unsigned long tmp = 0;
 
+                ret = -ESRCH;
+                if (!mm)
+                        break;
+
                 switch (addr) {
                 case PTRACE_GETFDPIC_EXEC:
-                        tmp = child->mm->context.exec_fdpic_loadmap;
+                        tmp = mm->context.exec_fdpic_loadmap;
                         break;
                 case PTRACE_GETFDPIC_INTERP:
-                        tmp = child->mm->context.interp_fdpic_loadmap;
+                        tmp = mm->context.interp_fdpic_loadmap;
                         break;
                 default:
                         break;
                 }
+                mmput(mm);
 
                 ret = put_user(tmp, (unsigned long __user *) data);
                 break;