[PATCH] fs/proc: fix uaf in proc_readdir_de()

[albin_yang@163.com]

From: Wei Yang <albinwyang@tencent.com>

Pde is erased from subdir rbtree through rb_erase(), but not set the node to EMPTY,
which may result in uaf access. We should use RB_CLEAR_NODE() set the erased node
to EMPTY, then pde_subdir_next() will return NULL to avoid uaf access.

We found an uaf issue while using stree-ng testing, need to run testcase getdent and
tun in the same time. The steps of the issue is as follows:
1) use getdent to traverse dir /proc/pid/net/dev_snmp6/, and current pde is tun3;
2) in the [time windows] unregister netdevice tun3 and tun2, and erase them from
   rbtree. erase tun3 first, and then erase tun2. the pde(tun2) will be released
   to slab;
3) continue to getdent process, then pde_subdir_next() will return pde(tun2) which
   is released, it will case uaf access.

CPU 0                                      |    CPU 1
----------------------------------------------------------------------------------------------
traverse dir /proc/pid/net/dev_snmp6/      |   unregister_netdevice(tun->dev)   //tun3 tun2
sys_getdents64()                           |
  iterate_dir()                            |
    proc_readdir()                         |
      proc_readdir_de()                    |     snmp6_unregister_dev()
        pde_get(de);                       |       proc_remove()
        read_unlock(&proc_subdir_lock);    |         remove_proc_subtree()
                                           |           write_lock(&proc_subdir_lock);
        [time window]                      |           rb_erase(&root->subdir_node, &parent->subdir);
                                           |           write_unlock(&proc_subdir_lock);
        read_lock(&proc_subdir_lock);      |
        next = pde_subdir_next(de);        |
        pde_put(de);                       |
        de = next;    //UAF                |

rbtree of dev_snmp6
                        |
                    pde(tun3)
                     /    \
                  NULL  pde(tun2)

Signed-off-by: Wei Yang <albinwyang@tencent.com>
---
 fs/proc/generic.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 176281112273..501889856461 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -698,6 +698,12 @@ void pde_put(struct proc_dir_entry *pde)
 	}
 }
 
+static void pde_erase(struct proc_dir_entry *pde, struct proc_dir_entry *parent)
+{
+	rb_erase(&pde->subdir_node, &parent->subdir);
+	RB_CLEAR_NODE(&pde->subdir_node);
+}
+
 /*
  * Remove a /proc entry and free it if it's not currently in use.
  */
@@ -720,7 +726,7 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
 			WARN(1, "removing permanent /proc entry '%s'", de->name);
 			de = NULL;
 		} else {
-			rb_erase(&de->subdir_node, &parent->subdir);
+			pde_erase(de, parent);
 			if (S_ISDIR(de->mode))
 				parent->nlink--;
 		}
@@ -764,7 +770,7 @@ int remove_proc_subtree(const char *name, struct proc_dir_entry *parent)
 			root->parent->name, root->name);
 		return -EINVAL;
 	}
-	rb_erase(&root->subdir_node, &parent->subdir);
+	pde_erase(root, parent);
 
 	de = root;
 	while (1) {
@@ -776,7 +782,7 @@ int remove_proc_subtree(const char *name, struct proc_dir_entry *parent)
 					next->parent->name, next->name);
 				return -EINVAL;
 			}
-			rb_erase(&next->subdir_node, &de->subdir);
+			pde_erase(next, de);
 			de = next;
 			continue;
 		}
-- 
2.43.7