[PATCH] proc: #68991 Fix GPF in /proc/$PID/map_files

[Artem Fetishev <artem_fetishev-uRwfk40T5oI@public.gmane.org>]

https://bugzilla.kernel.org/show_bug.cgi?id=68991

The expected logic of proc_map_files_get_link() is either to return 0 and
initialize 'path' or return an error and leave 'path' uninitialized. By the time
dname_to_vma_addr() returns 0 the corresponding vma may have already be gone. In
this case the path is not initialized but the return value is still 0. This
results in 'general protection fault' inside d_path().

Steps to reproduce:
CONFIG_CHECKPOINT_RESTORE=y

fd = open(...);
while (1) {
	mmap(fd, ...);
	munmap(fd, ...);
}

ls -la /proc/$PID/map_files

Signed-off-by: Artem Fetishev <artem_fetishev-uRwfk40T5oI@public.gmane.org>
Signed-off-by: Aleksandr Terekhov <aleksandr_terekhov-uRwfk40T5oI@public.gmane.org>
---
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 5150706..b976062 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1824,6 +1824,7 @@ static int proc_map_files_get_link(struct dentry *dentry, struct path *path)
 	if (rc)
 		goto out_mmput;
 
+	rc = -ENOENT;
 	down_read(&mm->mmap_sem);
 	vma = find_exact_vma(mm, vm_start, vm_end);
 	if (vma && vma->vm_file) {