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

["linfeng (M)" <linfeng23@huawei.com>]

* Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
> Subject: Re: [v4] migration: fix the memory overwriting risk in add_to_iovec
> 
> * 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?
Sorry for that rookie mistake. I have tested it again with gdb-fault injection. It can fix my original problem.
Thanks for helping me complete my first qemu patch submission. Really helped a lot.
> 
> 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