On Wed, Jul 27, 2022 at 08:30:48AM -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. Bypass the checks in > may_create_in_sticky if the task has CAP_FOWNER in the given namespace. > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829 > Reported-by: Yongchen Yang <yoyang@xxxxxxxxxx> > Suggested-by: Christian Brauner <brauner@xxxxxxxxxx> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > --- > fs/namei.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/fs/namei.c b/fs/namei.c > index 1f28d3f463c3..170c2396ba29 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -1230,7 +1230,8 @@ 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)) || > + ns_capable(mnt_userns, CAP_FOWNER)) > return 0; Hm, no. You really want inode_owner_or_capable() here.. You need to verify that you have a mapping for the inode->i_{g,u}id in question and that you're having CAP_FOWNER in the caller's userns. I'm pretty sure we should also restrict this to the case were the caller actually created the file otherwise we introduce a potential issue where the caller is susceptible to data spoofing. For example, the file was created by another user racing the caller's O_CREAT.
