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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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>

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

> ---
>  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