Re: [v4] migration: fix the memory overwriting risk in add_to_iovec

[Dr. David Alan Gilbert]

* Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
> * Lin Feng (linfeng23@huawei.com) wrote:
> > From: Feng Lin <linfeng23@huawei.com>
> > 
> > When testing migration, a Segmentation fault qemu core is generated.
> > 0  error_free (err=0x1)
> > 1  0x00007f8b862df647 in qemu_fclose (f=f@entry=0x55e06c247640)
> > 2  0x00007f8b8516d59a in migrate_fd_cleanup (s=s@entry=0x55e06c0e1ef0)
> > 3  0x00007f8b8516d66c in migrate_fd_cleanup_bh (opaque=0x55e06c0e1ef0)
> > 4  0x00007f8b8626a47f in aio_bh_poll (ctx=ctx@entry=0x55e06b5a16d0)
> > 5  0x00007f8b8626e71f in aio_dispatch (ctx=0x55e06b5a16d0)
> > 6  0x00007f8b8626a33d in aio_ctx_dispatch (source=<optimized out>, 
> > callback=<optimized out>, user_data=<optimized out>)
> > 7  0x00007f8b866bdba4 in g_main_context_dispatch ()
> > 8  0x00007f8b8626cde9 in glib_pollfds_poll ()
> > 9  0x00007f8b8626ce62 in os_host_main_loop_wait (timeout=<optimized out>)
> > 10 0x00007f8b8626cffd in main_loop_wait (nonblocking=nonblocking@entry=0)
> > 11 0x00007f8b862ef01f in main_loop ()
> > Using gdb print the struct QEMUFile f = {
> >   ...,
> >   iovcnt = 65, last_error = 21984,
> >   last_error_obj = 0x1, shutdown = true
> > }
> > Well iovcnt is overflow, because the max size of MAX_IOV_SIZE is 64.
> > struct QEMUFile {
> >     ...;
> >     struct iovec iov[MAX_IOV_SIZE];
> >     unsigned int iovcnt;
> >     int last_error;
> >     Error *last_error_obj;
> >     bool shutdown;
> > };
> > iovcnt and last_error is overwrited by add_to_iovec().
> > Right now, add_to_iovec() increase iovcnt before check the limit.
> > And it seems that add_to_iovec() assumes that iovcnt will set to zero
> > in qemu_fflush(). But qemu_fflush() will directly return when f->shutdown
> > is true.
> > 
> > The situation may occur when libvirtd restart during migration, after
> > f->shutdown is set, before calling qemu_file_set_error() in
> > qemu_file_shutdown().
> > 
> > So the safiest way is checking the iovcnt before increasing it.
> > 
> > Signed-off-by: Feng Lin <linfeng23@huawei.com>
> 
> Queued

Hmm this didn't actually build because that function is actually
misnamed 'qemu_file_is_writable' (no e!);  I've fixed that, but can you
just reconfirm that you've tested this fixes your original problem?

Dave

> > ---
> >  migration/qemu-file.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > index d6e03dbc0e..6879615197 100644
> > --- a/migration/qemu-file.c
> > +++ b/migration/qemu-file.c
> > @@ -416,6 +416,11 @@ static int add_to_iovec(QEMUFile *f, const uint8_t 
> > *buf, size_t size,
> >      {
> >          f->iov[f->iovcnt - 1].iov_len += size;
> >      } else {
> > +        if (f->iovcnt >= MAX_IOV_SIZE) {
> > +            /* Should only happen if a previous fflush failed */
> > +            assert(f->shutdown || !qemu_file_is_writeable(f));
> > +            return 1;
> > +        }
> >          if (may_free) {
> >              set_bit(f->iovcnt, f->may_free);
> >          }
> > -- 
> > 2.23.0
> > 
> > 
> -- 
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK