[v2] virtiofsd: Don't allow file creation with FUSE_OPEN

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 the client happens to do it anyway, the server ends up passing
this flag to open() without the mandatory mode_t 4th argument. Since
open() is a variadic function, glibc will happily pass whatever it
finds on the stack to the syscall. If this file is compiled with
-D_FORTIFY_SOURCE=2, glibc will even detect that and abort:

*** invalid openat64 call: O_CREAT or O_TMPFILE without mode ***: terminated

Specifying O_CREAT with FUSE_OPEN is a protocol violation. Check this
in do_open(), print out a message and return an error to the client,
EINVAL like we already do when fuse_mbuf_iter_advance() fails.

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>
---

v2:
 - do the check in core FUSE code instead of passthrough_ll (libfuse folks)


 tools/virtiofsd/fuse_lowlevel.c | 6 ++++++
 1 file changed, 6 insertions(+)

* 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 the client happens to do it anyway, the server ends up passing
> this flag to open() without the mandatory mode_t 4th argument. Since
> open() is a variadic function, glibc will happily pass whatever it
> finds on the stack to the syscall. If this file is compiled with
> -D_FORTIFY_SOURCE=2, glibc will even detect that and abort:
> 
> *** invalid openat64 call: O_CREAT or O_TMPFILE without mode ***: terminated
> 
> Specifying O_CREAT with FUSE_OPEN is a protocol violation. Check this
> in do_open(), print out a message and return an error to the client,
> EINVAL like we already do when fuse_mbuf_iter_advance() fails.
> 
> 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>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
> 
> v2:
>  - do the check in core FUSE code instead of passthrough_ll (libfuse folks)
> 
> 
>  tools/virtiofsd/fuse_lowlevel.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/tools/virtiofsd/fuse_lowlevel.c b/tools/virtiofsd/fuse_lowlevel.c
> index 7fe2cef1eb3b..3d725bcba2ca 100644
> --- a/tools/virtiofsd/fuse_lowlevel.c
> +++ b/tools/virtiofsd/fuse_lowlevel.c
> @@ -1084,6 +1084,12 @@ static void do_open(fuse_req_t req, fuse_ino_t nodeid,
>          return;
>      }
>  
> +    /* File creation is handled by do_create() or do_mknod() */
> +    if (arg->flags & (O_CREAT | O_TMPFILE)) {
> +        fuse_reply_err(req, EINVAL);
> +        return;
> +    }
> +
>      memset(&fi, 0, sizeof(fi));
>      fi.flags = arg->flags;
>      fi.kill_priv = arg->open_flags & FUSE_OPEN_KILL_SUIDGID;
> -- 
> 2.31.1
>

* 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 the client happens to do it anyway, the server ends up passing
> this flag to open() without the mandatory mode_t 4th argument. Since
> open() is a variadic function, glibc will happily pass whatever it
> finds on the stack to the syscall. If this file is compiled with
> -D_FORTIFY_SOURCE=2, glibc will even detect that and abort:
> 
> *** invalid openat64 call: O_CREAT or O_TMPFILE without mode ***: terminated
> 
> Specifying O_CREAT with FUSE_OPEN is a protocol violation. Check this
> in do_open(), print out a message and return an error to the client,
> EINVAL like we already do when fuse_mbuf_iter_advance() fails.
> 
> 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>

Queued

> ---
> 
> v2:
>  - do the check in core FUSE code instead of passthrough_ll (libfuse folks)
> 
> 
>  tools/virtiofsd/fuse_lowlevel.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/tools/virtiofsd/fuse_lowlevel.c b/tools/virtiofsd/fuse_lowlevel.c
> index 7fe2cef1eb3b..3d725bcba2ca 100644
> --- a/tools/virtiofsd/fuse_lowlevel.c
> +++ b/tools/virtiofsd/fuse_lowlevel.c
> @@ -1084,6 +1084,12 @@ static void do_open(fuse_req_t req, fuse_ino_t nodeid,
>          return;
>      }
>  
> +    /* File creation is handled by do_create() or do_mknod() */
> +    if (arg->flags & (O_CREAT | O_TMPFILE)) {
> +        fuse_reply_err(req, EINVAL);
> +        return;
> +    }
> +
>      memset(&fi, 0, sizeof(fi));
>      fi.flags = arg->flags;
>      fi.kill_priv = arg->open_flags & FUSE_OPEN_KILL_SUIDGID;
> -- 
> 2.31.1
> 
>