proc: check vma->vm_file before dereferencing

7b540d0646ce ("proc_map_files_readdir(): don't bother with grabbing
files") switched proc_map_files_readdir() to use @f_mode directly instead of
grabbing @file reference, but same time the test for @vm_file presence was
lost leading to nil dereference.  The patch brings the test back.

The all proc_map_files feature is CONFIG_\10CHECKPOINT_RESTORE wrapped
(which is set to 'n' by default) so the bug doesn't affect regular
kernels.

The regression is 3.7-rc1 only as far as I can tell.

[gorcunov@openvz.org: provided changelog]
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 144a96732dd7d602df0d5f8a504e435cfdb07229..74fc5624be09792fff14109832cca4e43c0d7541 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1770,8 +1770,9 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
        if (!vma)
                goto out_no_vma;
 
-       result = proc_map_files_instantiate(dir, dentry, task,
-                       (void *)(unsigned long)vma->vm_file->f_mode);
+       if (vma->vm_file)
+               result = proc_map_files_instantiate(dir, dentry, task,
+                               (void *)(unsigned long)vma->vm_file->f_mode);
 
 out_no_vma:
        up_read(&mm->mmap_sem);