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Message-ID: <b1d3c63ef5a7e8f98966552b4509381aae25afb6.camel@kernel.org>
Date: Wed, 27 Jul 2022 08:04:34 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
Yongchen Yang <yoyang@...hat.com>
Subject: Re: [RFC PATCH] vfs: don't check may_create_in_sticky if the file
is already open/created
On Wed, 2022-07-27 at 13:34 +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@...nel.org>
> > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829
> > > Reported-by: Yongchen Yang <yoyang@...hat.com>
> > > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > > ---
> > > 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.
>
In the normal open codepaths, FMODE_OPENED | FMODE_CREATED are still
clear. If we're not doing an atomic_open (i.e. the dentry doesn't exist
yet or is negative), then nothing really happens until you get to the
vfs_open call.
> > 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.
>
That may be the best option. I'll tinker around with that and see if I
can get it to work. Thanks for the suggestion.
> 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)))
> 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;
> }
I think that still won't fix it in the normal open codepath.
FMODE_CREATED won't be set, so this will just end up failing again.
--
Jeff Layton <jlayton@...nel.org>
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