On Wed, Jul 27, 2022 at 01:34:13PM +0200, Christian Brauner wrote: > On Tue, Jul 26, 2022 at 04:27:56PM -0400, Jeff Layton wrote: > > On Tue, 2022-07-26 at 16:23 -0400, Jeff Layton wrote: > > > NFS server is exporting a sticky directory (mode 01777) with root > > > squashing enabled. Client has protect_regular enabled and then tries to > > > open a file as root in that directory. File is created (with ownership > > > set to nobody:nobody) but the open syscall returns an error. > > > > > > The problem is may_create_in_sticky, which rejects the open even though > > > the file has already been created/opened. Only call may_create_in_sticky > > > if the file hasn't already been opened or created. > > > > > > Cc: Christian Brauner <brauner@xxxxxxxxxx> > > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829 > > > Reported-by: Yongchen Yang <yoyang@xxxxxxxxxx> > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > > --- > > > fs/namei.c | 13 +++++++++---- > > > 1 file changed, 9 insertions(+), 4 deletions(-) > > > > > > diff --git a/fs/namei.c b/fs/namei.c > > > index 1f28d3f463c3..7480b6dc8d27 100644 > > > --- a/fs/namei.c > > > +++ b/fs/namei.c > > > @@ -3495,10 +3495,15 @@ static int do_open(struct nameidata *nd, > > > return -EEXIST; > > > if (d_is_dir(nd->path.dentry)) > > > return -EISDIR; > > > - error = may_create_in_sticky(mnt_userns, nd, > > > - d_backing_inode(nd->path.dentry)); > > > - if (unlikely(error)) > > > - return error; > > > + if (!(file->f_mode & (FMODE_OPENED | FMODE_CREATED))) { > > > + error = may_create_in_sticky(mnt_userns, nd, > > > + d_backing_inode(nd->path.dentry)); > > > + if (unlikely(error)) { > > > + printk("%s: f_mode=0x%x oflag=0x%x\n", > > > + __func__, file->f_mode, open_flag); > > > + return error; > > > + } > > > + } > > > } > > > if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry)) > > > return -ENOTDIR; > > > > I'm pretty sure this patch is the wrong approach, actually, since it > > doesn't fix the regular (non-atomic) open codepath. Any thoughts on what > > Hey Jeff, > > I haven't quite understood why that won't work for the regular open > codepaths. I'm probably missing something obvious. > > > the right fix might be? > > When an actual creation has taken place - and not just a lookup - then > may_create_in_sticky() assumes that the owner of the inode matches > current_fsuid(). That'd would also be problematic on fat or in fact on > any fs where the actual inode->i_{g,u}id are based on e.g. uid/gid mount > options and not on current_fsuid(), I think? > > So in order to improve this we would need to work around that assumption > in some way. Either by skipping may_create_in_sticky() if the file got > created or by adapting may_create_in_sticky(). > > I only wonder whether skipping may_create_in_sticky() altogether might > be a bit too lax. One possibility that came to my mind might be to relax > this assumption when the file has been created and the creator has > CAP_FOWNER. > > So (not compile tested or in any way) sm like: > > diff --git a/fs/namei.c b/fs/namei.c > index 1f28d3f463c3..239e9f423346 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -1221,7 +1221,8 @@ int may_linkat(struct user_namespace *mnt_userns, struct path *link) > * Returns 0 if the open is allowed, -ve on error. > */ > static int may_create_in_sticky(struct user_namespace *mnt_userns, > - struct nameidata *nd, struct inode *const inode) > + struct nameidata *nd, struct inode *const inode, > + bool created) > { > umode_t dir_mode = nd->dir_mode; > kuid_t dir_uid = nd->dir_uid; > @@ -1230,7 +1231,9 @@ static int may_create_in_sticky(struct user_namespace *mnt_userns, > (!sysctl_protected_regular && S_ISREG(inode->i_mode)) || > likely(!(dir_mode & S_ISVTX)) || > uid_eq(i_uid_into_mnt(mnt_userns, inode), dir_uid) || > - uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode))) > + uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)) || > + (created && > + capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FOWNER))) Sorry, this should be inode_owner_or_capable(mnt_userns, inode) > return 0; > > if (likely(dir_mode & 0002) || > @@ -3496,7 +3499,8 @@ static int do_open(struct nameidata *nd, > if (d_is_dir(nd->path.dentry)) > return -EISDIR; > error = may_create_in_sticky(mnt_userns, nd, > - d_backing_inode(nd->path.dentry)); > + d_backing_inode(nd->path.dentry), > + (file->f_mode & FMODE_CREATED)); > if (unlikely(error)) > return error; > }
