Re: [PATCH v2 1/2] ovl: ovl_permission: Fix null pointer dereference at realinode in rcu-walk mode

From: Zhihao Cheng
Date: Tue May 16 2023 - 08:27:53 EST

在 2023/5/16 19:45, Amir Goldstein 写道:
On Tue, May 16, 2023 at 4:16 AM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:

在 2023/5/15 23:36, Amir Goldstein 写道:
On Mon, May 15, 2023 at 6:16 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:

On Mon, May 15, 2023 at 05:58:55PM +0300, Amir Goldstein wrote:
On Mon, May 15, 2023 at 4:58 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:

On Mon, May 15, 2023 at 09:36:28PM +0800, Zhihao Cheng wrote:
Following process:
P1 P2
path_lookupat
link_path_walk
inode_permission
ovl_permission
ovl_i_path_real(inode, &realpath)
path->dentry = ovl_i_dentry_upper(inode)
drop_cache
__dentry_kill(ovl_dentry)
iput(ovl_inode)
ovl_destroy_inode(ovl_inode)
dput(oi->__upperdentry)
dentry_kill(upperdentry)
dentry_unlink_inode
upperdentry->d_inode = NULL
realinode = d_inode(realpath.dentry) // return NULL
inode_permission(realinode)
inode->i_sb // NULL pointer dereference
, will trigger an null pointer dereference at realinode:
[ 335.664979] BUG: kernel NULL pointer dereference,
address: 0000000000000002
[ 335.668032] CPU: 0 PID: 2592 Comm: ls Not tainted 6.3.0
[ 335.669956] RIP: 0010:inode_permission+0x33/0x2c0
[ 335.678939] Call Trace:
[ 335.679165] <TASK>
[ 335.679371] ovl_permission+0xde/0x320
[ 335.679723] inode_permission+0x15e/0x2c0
[ 335.680090] link_path_walk+0x115/0x550
[ 335.680771] path_lookupat.isra.0+0xb2/0x200
[ 335.681170] filename_lookup+0xda/0x240
[ 335.681922] vfs_statx+0xa6/0x1f0
[ 335.682233] vfs_fstatat+0x7b/0xb0

Fetch a reproducer in [Link].

Add a new helper ovl_i_path_realinode() to get realpath and real inode
after non-nullptr checking, use the helper to replace the current realpath
getting logic.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217405
Fixes: 4b7791b2e958 ("ovl: handle idmappings in ovl_permission()")
Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
---
fs/overlayfs/inode.c | 31 ++++++++++++++++++++++++-------
1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 541cf3717fc2..cc3ef5a6666a 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -278,6 +278,26 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
return err;
}

+static inline int ovl_i_path_realinode(struct inode *inode,
+ struct path *realpath,
+ struct inode **realinode, int rcu)
+{
+ /* Careful in RCU walk mode */
+ ovl_i_path_real(inode, realpath);
+ if (!realpath->dentry) {
+ WARN_ON(!rcu);
+ return -ECHILD;
+ }
+
+ *realinode = d_inode(realpath->dentry);
+ if (!*realinode) {
+ WARN_ON(!rcu);
+ return -ECHILD;
+ }
+
+ return 0;
+}

If you want to return the inode wouldn't it possibly make more sense to
return the inode from the function directly? But not fuzzed. Maybe Amir
has a preference. As I said, I'm even fine with the original approach.

Sorry for not reviewing v1, I was traveling, even though it is hard to use
this excuse when I was traveling with Christian who did review v1 :)

Well, I did only do it this morning. :)



static inline struct inode *ovl_i_path_realinode(struct inode *inode,
struct path *realpath,
int rcu)
{
struct inode *realinode;

/* Careful in RCU walk mode */
ovl_i_path_real(inode, realpath);
if (!realpath->dentry) {
WARN_ON(!rcu);
return ERR_PTR(-ECHILD);
}

realinode = d_inode(realpath->dentry);
if (!realinode) {
WARN_ON(!rcu);
return ERR_PTR(-ECHILD);
}

return realinode;
}


I think this helper is over engineered ;-)

Yes. As mentioned before, I would've been happy even with v1 that didn't
have any helper.

The idea for a helper that returns inode is good,
but I see no reason to mix RCU walk in this helper
and don't even need a new helper (see untested patch below).

Looks good to me too.


Thanks,
Amir.

---
-void ovl_i_path_real(struct inode *inode, struct path *path)
+struct inode *ovl_i_path_real(struct inode *inode, struct path *path)
{
struct ovl_path *lowerpath = ovl_lowerpath(OVL_I_E(inode));

@@ -342,6 +342,8 @@ void ovl_i_path_real(struct inode *inode, struct path *path)
} else {
path->mnt = ovl_upper_mnt(OVL_FS(inode->i_sb));
}
+
+ return path->dentry ? d_inode(path->dentry) : NULL;
}

@@ -295,8 +295,8 @@ int ovl_permission(struct mnt_idmap *idmap,
int err;

/* Careful in RCU walk mode */
- ovl_i_path_real(inode, &realpath);
- if (!realpath.dentry) {
+ realinode = ovl_i_path_real(inode, &realpath);
+ if (!realpath.dentry || !realinode) {
WARN_ON(!(mask & MAY_NOT_BLOCK));
return -ECHILD;
}
@@ -309,7 +309,6 @@ int ovl_permission(struct mnt_idmap *idmap,

if (err)
return err;

- realinode = d_inode(realpath.dentry);
old_cred = ovl_override_creds(inode->i_sb);


Thanks for reviewings and helpful discussion from Amir and Christian.


You're welcome.

Note to self: there is be a trivial conflict with the change to
ovl_i_path_real()
return type and this patch from my ovl-lazy-lowerdata series:
https://lore.kernel.org/linux-unionfs/20230427130539.2798797-7-amir73il@xxxxxxxxx/

After Miklos takes your fixes, I will rebase.

Thanks for rebasing, so you will take v2 with modified helper ovl_i_path_real(return realinode)?


Thanks,
Amir.
.