Re: [PATCH] tmpfs: verify {g,u}id mount options correctly

Christian Brauner
2023-08-02T12:06:36Z

On Tue, Aug 01, 2023 at 11:47:41AM -0500, Seth Forshee wrote:
> On Tue, Aug 01, 2023 at 06:17:04PM +0200, Christian Brauner wrote:
> > A while ago we received the following report:
> >
> > "The other outstanding issue I noticed comes from the fact that
> > fsconfig syscalls may occur in a different userns than that which
> > called fsopen. That means that resolving the uid/gid via
> > current_user_ns() can save a kuid that isn't mapped in the associated
> > namespace when the filesystem is finally mounted. This means that it
> > is possible for an unprivileged user to create files owned by any
> > group in a tmpfs mount (since we can set the SUID bit on the tmpfs
> > directory), or a tmpfs that is owned by any user, including the root
> > group/user."
> >
> > The contract for {g,u}id mount options and {g,u}id values in general set
> > from userspace has always been that they are translated according to the
> > caller's idmapping. In so far, tmpfs has been doing the correct thing.
> > But since tmpfs is mountable in unprivileged contexts it is also
> > necessary to verify that the resulting {k,g}uid is representable in the
> > namespace of the superblock to avoid such bugs as above.
> >
> > The new mount api's cross-namespace delegation abilities are already
> > widely used. After having talked to a bunch of userspace this is the
> > most faithful solution with minimal regression risks. I know of one
> > users - systemd - that makes use of the new mount api in this way and
> > they don't set unresolable {g,u}ids. So the regression risk is minimal.
> >
> > Link: https://lore.kernel.org/lkml/CALxfFW4BXhEwxR0Q5LSkg-8Vb4r2MONKCcUCVioehXQKr35eHg@mail.gmail.com
> > Fixes: f32356261d44 ("vfs: Convert ramfs, shmem, tmpfs, devtmpfs, rootfs to use the new mount API")
> > Reported-by: Seth Jenkins <sethjenkins@google.com>
> > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > ---
> >
> > ---
> > mm/shmem.c | 28 ++++++++++++++++++++++++----
> > 1 file changed, 24 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 2f2e0e618072..1c0b2dafafe5 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -3636,6 +3636,8 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
> > unsigned long long size;
> > char *rest;
> > int opt;
> > + kuid_t kuid;
> > + kgid_t kgid;
> >
> > opt = fs_parse(fc, shmem_fs_parameters, param, &result);
> > if (opt < 0)
> > @@ -3671,14 +3673,32 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
> > ctx->mode = result.uint_32 & 07777;
> > break;
> > case Opt_uid:
> > - ctx->uid = make_kuid(current_user_ns(), result.uint_32);
> > - if (!uid_valid(ctx->uid))
> > + kuid = make_kuid(current_user_ns(), result.uint_32);
> > + if (!uid_valid(kuid))
> > goto bad_value;
> > +
> > + /*
> > + * The requested uid must be representable in the
> > + * filesystem's idmapping.
> > + */
> > + if (!kuid_has_mapping(fc->user_ns, kuid))
> > + goto bad_value;
> > +
> > + ctx->uid = kuid;
>
> This seems like the most sensible way to handle ids in mount options.
> Wouldn't some other filesystems (e.g. fuse) benefit from the same sort
> of handling though? Rather than having filesystems handle these checks
> themselves, what about adding k{uid,gid}_t members to the
> fs_parse_result union with fsparam_is_{uid,gid}() helpers which peform
> these checks?

Yes, I like that proposal. Let's see if that works.