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); > > Btw, if the reproducer that Zhihao has posted in the bugzilla link: > > #!/bin/bash > > mkdir -p /root/tmp/merge /root/tmp/upper/dir /root/tmp/lower /root/tmp/work > touch /root/tmp/upper/dir/file > chown freg:freg -R /root/tmp/upper/dir > mount -t overlay none -oupperdir=/root/tmp/upper,lowerdir=/root/tmp/lower,workdir=/root/tmp/work /root/tmp/merge > ls /root/tmp/merge/dir/file & > echo 3 > /proc/sys/vm/drop_caches > > is reliable we should add it to xfstests... If only it was that easy to trigger this race. Look at the debug kernel patch named 'diff' in bugzilla... Thanks, Amir.
