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Message-ID: <CAOQ4uxjo3rzdrjmbXr=SgJbrBf_EA-HpXH25LORo_vPY=q0jWQ@mail.gmail.com>
Date: Mon, 15 May 2023 17:58:55 +0300
From: Amir Goldstein <amir73il@...il.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Zhihao Cheng <chengzhihao1@...wei.com>, miklos@...redi.hu,
linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] ovl: ovl_permission: Fix null pointer dereference
at realinode in rcu-walk mode
On Mon, May 15, 2023 at 4:58 PM Christian Brauner <brauner@...nel.org> 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@...wei.com>
> > Suggested-by: Christian Brauner <brauner@...nel.org>
> > ---
> > 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 :)
>
> 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 ;-)
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).
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);
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