[Virtio-fs] [PATCH] virtiofsd: Don't allow file creation with FUSE_OPEN

[Stefan Hajnoczi]

On Thu, Jun 17, 2021 at 04:15:18PM +0200, Greg Kurz 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 at kaod.org>
> ---
>  tools/virtiofsd/passthrough_ll.c | 6 ++++++
>  1 file changed, 6 insertions(+)

Thank you!

Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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