Re: [PATCH v4 2/3] fs: name_to_handle_at() support for "explicit connectable" file handles

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On Fri, Oct 11, 2024 at 4:00 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> On Fri, 2024-10-11 at 11:00 +0200, Amir Goldstein wrote:
> > nfsd encodes "connectable" file handles for the subtree_check feature,
> > which can be resolved to an open file with a connected path.
> > So far, userspace nfs server could not make use of this functionality.
> >
> > Introduce a new flag AT_HANDLE_CONNECTABLE to name_to_handle_at(2).
> > When used, the encoded file handle is "explicitly connectable".
> >
> > The "explicitly connectable" file handle sets bits in the high 16bit of
> > the handle_type field, so open_by_handle_at(2) will know that it needs
> > to open a file with a connected path.
> >
> > old kernels will now recognize the handle_type with high bits set,
> > so "explicitly connectable" file handles cannot be decoded by
> > open_by_handle_at(2) on old kernels.
> >
> > The flag AT_HANDLE_CONNECTABLE is not allowed together with either
> > AT_HANDLE_FID or AT_EMPTY_PATH.
> >
> > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> > ---
> >  fs/fhandle.c               | 48 ++++++++++++++++++++++++++++++++++----
> >  include/linux/exportfs.h   |  2 ++
> >  include/uapi/linux/fcntl.h |  1 +
> >  3 files changed, 46 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/fhandle.c b/fs/fhandle.c
> > index 218511f38cbb..8339a1041025 100644
> > --- a/fs/fhandle.c
> > +++ b/fs/fhandle.c
> > @@ -31,6 +31,14 @@ static long do_sys_name_to_handle(const struct path *path,
> >       if (!exportfs_can_encode_fh(path->dentry->d_sb->s_export_op, fh_flags))
> >               return -EOPNOTSUPP;
> >
> > +     /*
> > +      * A request to encode a connectable handle for a disconnected dentry
> > +      * is unexpected since AT_EMPTY_PATH is not allowed.
> > +      */
> > +     if (fh_flags & EXPORT_FH_CONNECTABLE &&
> > +         WARN_ON(path->dentry->d_flags & DCACHE_DISCONNECTED))
>
> Is this even possible? The dentry in this case will have been reached
> by pathwalk. Oh, but I guess the dfd could point to a disconnected
> dentry and then you pass in AT_EMPTY_PATH.

But see comment above "...is unexpected since AT_EMPTY_PATH is not allowed."

and see below

+        * AT_EMPTY_PATH could be used along with a dfd that refers to a
+        * disconnected non-directory, which cannot be used to encode a
+        * connectable file handle, because its parent is unknown.
+        */
+       if (flag & AT_HANDLE_CONNECTABLE &&
+           flag & (AT_HANDLE_FID | AT_EMPTY_PATH))
                return -EINVAL;

The code/API should not allow this also for a malicious user,
unless I missed something, hence, the assertion.

>
> I'm not sure we want to warn in that case though, since this is a
> situation that an unprivileged user could be able to arrange. Maybe we
> should just return a more distinct error code in this case?
>
> Since the scenario involves a dfd that is disconnected, how about:
>
>     #define EBADFD          77      /* File descriptor in bad state */
>

To me it does not look like a good fit, but let's see what others think.
In the end, it is a rare condition that should never happen
(hence assert), so I don't think the error value matters that much?

> > +             return -EINVAL;
> > +
> >       if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle)))
> >               return -EFAULT;
> >
> > @@ -45,7 +53,7 @@ static long do_sys_name_to_handle(const struct path *path,
> >       /* convert handle size to multiple of sizeof(u32) */
> >       handle_dwords = f_handle.handle_bytes >> 2;
> >
> > -     /* we ask for a non connectable maybe decodeable file handle */
> > +     /* Encode a possibly decodeable/connectable file handle */
> >       retval = exportfs_encode_fh(path->dentry,
> >                                   (struct fid *)handle->f_handle,
> >                                   &handle_dwords, fh_flags);
> > @@ -67,8 +75,23 @@ static long do_sys_name_to_handle(const struct path *path,
> >                * non variable part of the file_handle
> >                */
> >               handle_bytes = 0;
> > -     } else
> > +     } else {
> > +             /*
> > +              * When asked to encode a connectable file handle, encode this
> > +              * property in the file handle itself, so that we later know
> > +              * how to decode it.
> > +              * For sanity, also encode in the file handle if the encoded
> > +              * object is a directory and verify this during decode, because
> > +              * decoding directory file handles is quite different than
> > +              * decoding connectable non-directory file handles.
> > +              */
> > +             if (fh_flags & EXPORT_FH_CONNECTABLE) {
> > +                     handle->handle_type |= FILEID_IS_CONNECTABLE;
> > +                     if (d_is_dir(path->dentry))
> > +                             fh_flags |= FILEID_IS_DIR;
> > +             }
> >               retval = 0;
> > +     }
> >       /* copy the mount id */
> >       if (unique_mntid) {
> >               if (put_user(real_mount(path->mnt)->mnt_id_unique,
> > @@ -109,15 +132,30 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
> >  {
> >       struct path path;
> >       int lookup_flags;
> > -     int fh_flags;
> > +     int fh_flags = 0;
> >       int err;
> >
> >       if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID |
> > -                  AT_HANDLE_MNT_ID_UNIQUE))
> > +                  AT_HANDLE_MNT_ID_UNIQUE | AT_HANDLE_CONNECTABLE))
> > +             return -EINVAL;
> > +
> > +     /*
> > +      * AT_HANDLE_FID means there is no intention to decode file handle
> > +      * AT_HANDLE_CONNECTABLE means there is an intention to decode a
> > +      * connected fd (with known path), so these flags are conflicting.
> > +      * AT_EMPTY_PATH could be used along with a dfd that refers to a
> > +      * disconnected non-directory, which cannot be used to encode a
> > +      * connectable file handle, because its parent is unknown.
> > +      */
> > +     if (flag & AT_HANDLE_CONNECTABLE &&
>
> nit: might need parenthesis around the above & check.
>
> > +         flag & (AT_HANDLE_FID | AT_EMPTY_PATH))

I don't think it is needed, but for readability I don't mind adding them.
I am having a hard time remembering the operation precedence  myself,
but this one is clear to me so I don't bother with ().

Thanks,
Amir.





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