On Wed, Jul 27, 2022 at 08:55:35AM -0400, Jeff Layton wrote: > On Wed, 2022-07-27 at 14:37 +0200, Christian Brauner wrote: > > 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. > > > > Ok, I should be able to make that change and test it out. > > > 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. > > That won't be sufficient to fix the testcase, I think. If a file already > exists in the sticky dir and is owned by nobody:nobody, do we really > want to prevent root from opening it? I wouldn't think so. Afaict, the whole stick behind the protected_regular thing in may_create_in_sticky() thing is that you prevent scenarios where you can be tricked into opening a file that you didn't intend to with O_CREAT. That's specifically also a protection for root. So say root specifies O_CREAT but someone beats root to it and creates the file dumping malicious data in there. The uid_eq() requirement is supposed to prevent such attacks and it's a sysctl that userspace opted into. We'd be relaxing that restriction quite a bit if we not just allow newly created but also pre-existing file to be opened even with the CAP_FOWNER requirement. So the dd call should really fail if O_CREAT is passed but the file is pre-existing, imho. It's a different story if dd created that file and has CAP_FOWNER imho.
