Re: [PATCH v3 01/10] virtiofsd: Limit setxattr()'s creds-dropped region

[Max Reitz]

On 06.08.21 16:16, Vivek Goyal wrote:
> On Fri, Jul 30, 2021 at 05:01:25PM +0200, Max Reitz wrote:
> > We only need to drop/switch our credentials for the (f)setxattr() call
> > alone, not for the openat() or fchdir() around it.
> >
> > (Right now, this may not be that big of a problem, but with inodes being
> > identified by file handles instead of an O_PATH fd, we will need
> > open_by_handle_at() calls here, which is really fickle when it comes to
> > credentials being dropped.)
> >
> > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > ---
> >   tools/virtiofsd/passthrough_ll.c | 34 +++++++++++++++++++++++---------
> >   1 file changed, 25 insertions(+), 9 deletions(-)
> >
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index 38b2af8599..1f27eeabc5 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -3121,6 +3121,7 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, 
> > const char *in_name,
> >       bool switched_creds = false;
> >       bool cap_fsetid_dropped = false;
> >       struct lo_cred old = {};
> > +    bool open_inode;
> >   
> >       if (block_xattr(lo, in_name)) {
> >           fuse_reply_err(req, EOPNOTSUPP);
> > @@ -3155,7 +3156,24 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, 
> > const char *in_name,
> >       fuse_log(FUSE_LOG_DEBUG, "lo_setxattr(ino=%" PRIu64
> >                ", name=%s value=%s size=%zd)\n", ino, name, value, size);
> >   
> > +    /*
> > +     * We can only open regular files or directories.  If the inode is
> > +     * something else, we have to enter /proc/self/fd and use
> > +     * setxattr() on the link's filename there.
> > +     */
> > +    open_inode = S_ISREG(inode->filetype) || S_ISDIR(inode->filetype);
> >       sprintf(procname, "%i", inode->fd);
> > +    if (open_inode) {
> > +        fd = openat(lo->proc_self_fd, procname, O_RDONLY);
> > +        if (fd < 0) {
> > +            saverr = errno;
> > +            goto out;
> > +        }
> > +    } else {
> > +        /* fchdir should not fail here */
> > +        FCHDIR_NOFAIL(lo->proc_self_fd);
> > +    }
> > +
> >       /*
> >        * If we are setting posix access acl and if SGID needs to be
> >        * cleared, then switch to caller's gid and drop CAP_FSETID
> > @@ -3176,20 +3194,13 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, 
> > const char *in_name,
> >           }
> >           switched_creds = true;
> >       }
> > -    if (S_ISREG(inode->filetype) || S_ISDIR(inode->filetype)) {
> > -        fd = openat(lo->proc_self_fd, procname, O_RDONLY);
> > -        if (fd < 0) {
> > -            saverr = errno;
> > -            goto out;
> > -        }
> > +    if (open_inode) {
> > +        assert(fd >= 0);
> >           ret = fsetxattr(fd, name, value, size, flags);
> >           saverr = ret == -1 ? errno : 0;
> >       } else {
> > -        /* fchdir should not fail here */
> > -        FCHDIR_NOFAIL(lo->proc_self_fd);
> >           ret = setxattr(procname, name, value, size, flags);
> >           saverr = ret == -1 ? errno : 0;
> > -        FCHDIR_NOFAIL(lo->root.fd);
> >       }
> >       if (switched_creds) {
> >           if (cap_fsetid_dropped)
> > @@ -3198,6 +3209,11 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, 
> > const char *in_name,
> >               lo_restore_cred(&old, false);
> >       }
> >   
> > +    if (!open_inode) {
> > +        /* Change CWD back, fchdir should not fail here */
> > +        FCHDIR_NOFAIL(lo->root.fd);
> > +    }
> > +
> This FCHDIR_NOFAIL() will also need to be called if lo_drop_cap_change_cred()
> fails.
>
>          ret = lo_drop_cap_change_cred(req, &old, false, "FSETID",
>                                        &cap_fsetid_dropped);
>          if (ret) {
>              saverr = ret;
>              goto out;
>          }

Oh, right, thanks!

Max