Re: [Qemu-devel] [PATCH] qed: fix use-after-free during l2 cache commit

[Anthony Liguori <aliguori@us.ibm.com>]

On 09/30/2011 05:39 AM, Stefan Hajnoczi wrote:
> QED's metadata caching strategy allows two parallel requests to race for
> metadata lookup.  The first one to complete will populate the metadata
> cache and the second one will drop the data it just read in favor of the
> cached data.
>
> There is a use-after-free in qed_read_l2_table_cb() and
> qed_commit_l2_update() where l2_table->offset was used after the
> l2_table may have been freed due to a metadata lookup race.  Fix this by
> keeping the l2_offset in a local variable and not reaching into the
> possibly freed l2_table.
>
> Reported-by: Amit Shah<amit.shah@redhat.com>
> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>

Applied.  Thanks.

Regards,

Anthony Liguori

> ---
> Hi Amit,
> Thanks for reporting the assertion failure you saw at http://fpaste.org/CDuv/.
> Does this patch fix the problem?
>
> If not, please send details on your setup and how to reproduce the issue.
>
> Thanks,
> Stefan
>
>   block/qed-table.c |    6 +++---
>   block/qed.c       |    4 ++--
>   2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/block/qed-table.c b/block/qed-table.c
> index d96afa8..f31f9ff 100644
> --- a/block/qed-table.c
> +++ b/block/qed-table.c
> @@ -222,21 +222,21 @@ static void qed_read_l2_table_cb(void *opaque, int ret)
>       QEDRequest *request = read_l2_table_cb->request;
>       BDRVQEDState *s = read_l2_table_cb->s;
>       CachedL2Table *l2_table = request->l2_table;
> +    uint64_t l2_offset = read_l2_table_cb->l2_offset;
>
>       if (ret) {
>           /* can't trust loaded L2 table anymore */
>           qed_unref_l2_cache_entry(l2_table);
>           request->l2_table = NULL;
>       } else {
> -        l2_table->offset = read_l2_table_cb->l2_offset;
> +        l2_table->offset = l2_offset;
>
>           qed_commit_l2_cache_entry(&s->l2_cache, l2_table);
>
>           /* This is guaranteed to succeed because we just committed the entry
>            * to the cache.
>            */
> -        request->l2_table = qed_find_l2_cache_entry(&s->l2_cache,
> -                                                    l2_table->offset);
> +        request->l2_table = qed_find_l2_cache_entry(&s->l2_cache, l2_offset);
>           assert(request->l2_table != NULL);
>       }
>
> diff --git a/block/qed.c b/block/qed.c
> index 624e261..e87dc4d 100644
> --- a/block/qed.c
> +++ b/block/qed.c
> @@ -911,14 +911,14 @@ static void qed_commit_l2_update(void *opaque, int ret)
>       QEDAIOCB *acb = opaque;
>       BDRVQEDState *s = acb_to_s(acb);
>       CachedL2Table *l2_table = acb->request.l2_table;
> +    uint64_t l2_offset = l2_table->offset;
>
>       qed_commit_l2_cache_entry(&s->l2_cache, l2_table);
>
>       /* This is guaranteed to succeed because we just committed the entry to the
>        * cache.
>        */
> -    acb->request.l2_table = qed_find_l2_cache_entry(&s->l2_cache,
> -                                                    l2_table->offset);
> +    acb->request.l2_table = qed_find_l2_cache_entry(&s->l2_cache, l2_offset);
>       assert(acb->request.l2_table != NULL);
>
>       qed_aio_next_io(opaque, ret);