Re: [PATCH 3.2 42/70] net: possible use after free in dst_release

Francesco Ruggeri
2016-01-18T03:49:35Z

On Sun, Jan 17, 2016 at 7:18 PM, Ben Hutchings <ben@decadent.org.uk> wrote:
> 3.2.76-rc1 review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Francesco Ruggeri <fruggeri@aristanetworks.com>
>
> commit 07a5d38453599052aff0877b16bb9c1585f08609 upstream.
>
> dst_release should not access dst->flags after decrementing
> __refcnt to 0. The dst_entry may be in dst_busy_list and
> dst_gc_task may dst_destroy it before dst_release gets a chance
> to access dst->flags.
>
> Fixes: d69bbf88c8d0 ("net: fix a race in dst_release()")
> Fixes: 27b75c95f10d ("net: avoid RCU for NOCACHE dst")
> Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
> Acked-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> [bwh: Backported to 3.2: adjust context]
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> ---
> net/core/dst.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> --- a/net/core/dst.c
> +++ b/net/core/dst.c
> @@ -269,10 +269,11 @@ void dst_release(struct dst_entry *dst)
> {
> if (dst) {
> int newrefcnt;
> + unsigned short nocache = dst->flags & DST_NOCACHE;

Only one minor thing. Before 3.6 dst->flags was an int.

Thanks,
Francesco

>
> newrefcnt = atomic_dec_return(&dst->__refcnt);
> WARN_ON(newrefcnt < 0);
> - if (!newrefcnt && unlikely(dst->flags & DST_NOCACHE)) {
> + if (!newrefcnt && unlikely(nocache)) {
> dst = dst_destroy(dst);
> if (dst)
> __dst_free(dst);
>