On Fri, 18 Jun 2021 10:58:33 +0200 Miklos Szeredi <mik...@szeredi.hu> wrote:
> On Thu, 17 Jun 2021 at 16:15, Greg Kurz <gr...@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 <gr...@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... 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. Cheers, -- Greg > Thanks, > Miklos > > > > > err = lo_do_open(lo, inode, -1, fi); > > lo_inode_put(lo, &inode); > > if (err) { > > -- > > 2.31.1 > >