Re: [PATCH] virtiofsd: Don't allow file creation with FUSE_OPEN

[Miklos Szeredi <miklos@szeredi.hu>]

On Fri, 18 Jun 2021 at 11:21, Greg Kurz <groug@kaod.org> wrote:
>
> On Fri, 18 Jun 2021 10:58:33 +0200
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > On Thu, 17 Jun 2021 at 16:15, Greg Kurz <groug@kaod.org> wrote:
> > >
> > > A well behaved FUSE client uses FUSE_CREATE to create files. It isn't
> > > supposed to pass O_CREAT along a FUSE_OPEN request, as documented in
> > > the "fuse_lowlevel.h" header :
> > >
> > >     /**
> > >      * Open a file
> > >      *
> > >      * Open flags are available in fi->flags. The following rules
> > >      * apply.
> > >      *
> > >      *  - Creation (O_CREAT, O_EXCL, O_NOCTTY) flags will be
> > >      *    filtered out / handled by the kernel.
> > >
> > > But if it does anyway, virtiofsd crashes with:
> > >
> > > *** invalid openat64 call: O_CREAT or O_TMPFILE without mode ***: terminated
> > >
> > > This is because virtiofsd ends up passing this flag to openat() without
> > > passing a mode_t 4th argument which is mandatory with O_CREAT, and glibc
> > > aborts.
> > >
> > > The offending path is:
> > >
> > > lo_open()
> > >     lo_do_open()
> > >         lo_inode_open()
> > >
> > > Other callers of lo_inode_open() only pass O_RDWR and lo_create()
> > > passes a valid fd to lo_do_open() which thus doesn't even call
> > > lo_inode_open() in this case.
> > >
> > > Specifying O_CREAT with FUSE_OPEN is a protocol violation. Check this
> > > in lo_open() and return an error to the client : EINVAL since this is
> > > already what glibc returns with other illegal flag combinations.
> > >
> > > The FUSE filesystem doesn't currently support O_TMPFILE, but the very
> > > same would happen if O_TMPFILE was passed in a FUSE_OPEN request. Check
> > > that as well.
> > >
> > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > ---
> > >  tools/virtiofsd/passthrough_ll.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > > index 49c21fd85570..14f62133131c 100644
> > > --- a/tools/virtiofsd/passthrough_ll.c
> > > +++ b/tools/virtiofsd/passthrough_ll.c
> > > @@ -2145,6 +2145,12 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
> > >          return;
> > >      }
> > >
> > > +    /* File creation is handled by lo_create() */
> > > +    if (fi->flags & (O_CREAT | O_TMPFILE)) {
> > > +        fuse_reply_err(req, EINVAL);
> > > +        return;
> > > +    }
> > > +
> >
> > Okay.  Question comes to mind whether the check should be even more
> > strict, possibly allowing just a specific set of flags, and erroring
> > out on everything else?
> >
>
> I've focused on O_CREAT and O_TMPFILE because they cause an explicit abort()
> in glibc when the code is compiled with -D_FORTIFY_SOURCE=2, but yes,
> maybe it could make sense to check more of them.
>
> > AFAICS linux kernel should never pass anything to FUSE_OPEN outside of this set:
> >
> > O_RDONLY
> > O_WRONLY
> > O_RDWR
> > O_APPEND
> > O_NDELAY
> > O_NONBLOCK
> > __O_SYNC
> > O_DSYNC
> > FASYNC
> > O_DIRECT
> > O_LARGEFILE
> > O_NOFOLLOW
> > O_NOATIME
> >
> > A separate question is whether virtiofsd should also be silently
> > ignoring some of the above flags.
> >
>
> Dunno on the top of my head...

Let's discuss this separately as this is mostly unrelated.  Added an
item to the virtiofs-todo etherpad.

>
> BTW, as suggested by Dave, I've submitted a similar patch to upstream
> libfuse:
>
> https://github.com/libfuse/libfuse/pull/615
>
> And I got interesting suggestions:
> 1) do it in core FUSE, i.e. fuse_lowlevel.c, since this isn't specific to
>    passthrough_ll AFAICT
> 2) print out an error
> 3) exit
>
> 1 makes a lot of sense. I guess 2 is fine this cannot be used by a
> buggy guest to flood some log file on the host. 3 doesn't seems
> to be an acceptable solution, and it wouldn't change much the
> outcome compared to what we have now.
>
> So I will go for 1 and 2.

Okay, good.

Thanks,
Miklos