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

On Sat, 25 Oct 2025 10:42:33 +0800, albin_yang@163.com wrote:
> 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.
> 
> [...]

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] fs/proc: fix uaf in proc_readdir_de()
      https://git.kernel.org/vfs/vfs/c/60a7b9983b80