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

[Miklos Szeredi]

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?

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.

Thanks,
Miklos



>      err = lo_do_open(lo, inode, -1, fi);
>      lo_inode_put(lo, &inode);
>      if (err) {
> --
> 2.31.1
>