Re: [Qemu-devel] [PATCH v4 02/10] raw: Check byte range uniformly

[Eric Blake <eblake@redhat.com>]

On 05/11/2018 07:08 AM, Fam Zheng wrote:
> We don't verify the request range against s->size in the I/O callbacks
> except for raw_co_pwritev. This is wrong (especially for
> raw_co_pwrite_zeroes and raw_co_pdiscard), so fix them.

Did you bother identifying how long the bug has been present (but read 
below, because I'm not sure there was even a bug)?

CC: qemu-stable@nongnu.org

> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>   block/raw-format.c | 63 ++++++++++++++++++++++++++++++++----------------------
>   1 file changed, 38 insertions(+), 25 deletions(-)
> 
> diff --git a/block/raw-format.c b/block/raw-format.c
> index a378547c99..803083f1a1 100644
> --- a/block/raw-format.c
> +++ b/block/raw-format.c
> @@ -167,16 +167,36 @@ static void raw_reopen_abort(BDRVReopenState *state)
>       state->opaque = NULL;
>   }
>   
> +/* Check and adjust the offset, against 'offset' and 'size' options. */
> +static inline int raw_adjust_offset(BlockDriverState *bs, uint64_t *offset,
> +                                    uint64_t bytes)
> +{
> +    BDRVRawState *s = bs->opaque;
> +
> +    if (s->has_size && (*offset > s->size || bytes > (s->size - *offset))) {
> +        /* There's not enough space for the data. Don't write anything and just
> +         * fail to prevent leaking out of the size specified in options. */
> +        return -ENOSPC;
> +    }

Can this even trigger? The block layer should already be clamping 
requests according to the device's reported size, and we already report 
a smaller size according to s->size and s->offset.  This could probably 
be an assertion instead.

> +
> +    if (*offset > UINT64_MAX - s->offset) {
> +        return -EINVAL;

Should this be against INT64_MAX instead?  After all, we really do use 
off_t (a 63-bit quantity, since it is signed), rather than uint64_t, as 
our maximum (theoretical) image size.  But again, can it even trigger, 
or can it be an assertion?

> +    }
> +    *offset += s->offset;
> +
> +    return 0;
> +}
> +
>   static int coroutine_fn raw_co_preadv(BlockDriverState *bs, uint64_t offset,
>                                         uint64_t bytes, QEMUIOVector *qiov,
>                                         int flags)
>   {
> -    BDRVRawState *s = bs->opaque;
> +    int ret;
>   
> -    if (offset > UINT64_MAX - s->offset) {
> -        return -EINVAL;
> +    ret = raw_adjust_offset(bs, &offset, bytes);

If I'm right and we can assert instead of failing, then 
raw_adjust_offset() doesn't return failure.  If I'm wrong, then there is 
now a code path where we can return ENOSPC on a read, which is unusual 
and probably wrong.

> +    if (ret) {
> +        return ret;
>       }
> -    offset += s->offset;
>   
>       BLKDBG_EVENT(bs->file, BLKDBG_READ_AIO);
>       return bdrv_co_preadv(bs->file, offset, bytes, qiov, flags);
> @@ -186,23 +206,11 @@ static int coroutine_fn raw_co_pwritev(BlockDriverState *bs, uint64_t offset,
>                                          uint64_t bytes, QEMUIOVector *qiov,
>                                          int flags)
>   {
> -    BDRVRawState *s = bs->opaque;
>       void *buf = NULL;
>       BlockDriver *drv;
>       QEMUIOVector local_qiov;
>       int ret;
>   
> -    if (s->has_size && (offset > s->size || bytes > (s->size - offset))) {
> -        /* There's not enough space for the data. Don't write anything and just
> -         * fail to prevent leaking out of the size specified in options. */
> -        return -ENOSPC;
> -    }
> -
> -    if (offset > UINT64_MAX - s->offset) {
> -        ret = -EINVAL;
> -        goto fail;
> -    }

Okay, so you're just doing code refactoring; perhaps we could have done 
assertions here.

> -
>       if (bs->probed && offset < BLOCK_PROBE_BUF_SIZE && bytes) {
>           /* Handling partial writes would be a pain - so we just
>            * require that guests have 512-byte request alignment if
> @@ -237,7 +245,10 @@ static int coroutine_fn raw_co_pwritev(BlockDriverState *bs, uint64_t offset,
>           qiov = &local_qiov;
>       }
>   
> -    offset += s->offset;
> +    ret = raw_adjust_offset(bs, &offset, bytes);
> +    if (ret) {
> +        goto fail;
> +    }
>   
>       BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
>       ret = bdrv_co_pwritev(bs->file, offset, bytes, qiov, flags);
> @@ -267,22 +278,24 @@ static int coroutine_fn raw_co_pwrite_zeroes(BlockDriverState *bs,
>                                                int64_t offset, int bytes,
>                                                BdrvRequestFlags flags)
>   {
> -    BDRVRawState *s = bs->opaque;
> -    if (offset > UINT64_MAX - s->offset) {
> -        return -EINVAL;
> +    int ret;
> +
> +    ret = raw_adjust_offset(bs, (uint64_t *)&offset, bytes);
> +    if (ret) {
> +        return ret;
>       }
> -    offset += s->offset;

If I'm right and raw_adjust_offset() can't fail, then this didn't add 
any protection.  If I'm wrong and it is possible to get the block layer 
to send a request beyond our advertised size, then this is indeed a bug 
fix worthy of being on the stable branch.

>       return bdrv_co_pwrite_zeroes(bs->file, offset, bytes, flags);
>   }
>   
>   static int coroutine_fn raw_co_pdiscard(BlockDriverState *bs,
>                                           int64_t offset, int bytes)
>   {
> -    BDRVRawState *s = bs->opaque;
> -    if (offset > UINT64_MAX - s->offset) {
> -        return -EINVAL;
> +    int ret;
> +
> +    ret = raw_adjust_offset(bs, (uint64_t *)&offset, bytes);
> +    if (ret) {
> +        return ret;
>       }
> -    offset += s->offset;
>       return bdrv_co_pdiscard(bs->file->bs, offset, bytes);
>   }
>   
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org