Re: [PATCH net-next v5] skb_expand_head() adjust skb->truesize incorrectly
From: Christoph Paasch
Date: Thu Sep 02 2021 - 11:53:55 EST
On Thu, Sep 2, 2021 at 4:12 AM Vasily Averin <vvs@xxxxxxxxxxxxx> wrote:
>
> Christoph Paasch reports [1] about incorrect skb->truesize
> after skb_expand_head() call in ip6_xmit.
> This may happen because of two reasons:
> - skb_set_owner_w() for newly cloned skb is called too early,
> before pskb_expand_head() where truesize is adjusted for (!skb-sk) case.
> - pskb_expand_head() does not adjust truesize in (skb->sk) case.
> In this case sk->sk_wmem_alloc should be adjusted too.
>
> [1] https://lkml.org/lkml/2021/8/20/1082
>
> Fixes: f1260ff15a71 ("skbuff: introduce skb_expand_head()")
> Reported-by: Christoph Paasch <christoph.paasch@xxxxxxxxx>
> Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
> ---
> v5: fixed else condition, thanks to Eric
> reworked update of expanded skb,
> added corresponding comments
> v4: decided to use is_skb_wmem() after pskb_expand_head() call
> fixed 'return (EXPRESSION);' in os_skb_wmem according to Eric Dumazet
> v3: removed __pskb_expand_head(),
> added is_skb_wmem() helper for skb with wmem-compatible destructors
> there are 2 ways to use it:
> - before pskb_expand_head(), to create skb clones
> - after successfull pskb_expand_head() to change owner on extended skb.
> v2: based on patch version from Eric Dumazet,
> added __pskb_expand_head() function, which can be forced
> to adjust skb->truesize and sk->sk_wmem_alloc.
> ---
> include/net/sock.h | 1 +
> net/core/skbuff.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++--------
> net/core/sock.c | 8 +++++++
> 3 files changed, 63 insertions(+), 9 deletions(-)
Still the same issues around refcount as I reported in my other email.
Did you try running the syzkaller reproducer on your setup?
Christoph
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 95b2577..173d58c 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1695,6 +1695,7 @@ struct sk_buff *sock_wmalloc(struct sock *sk, unsigned long size, int force,
> gfp_t priority);
> void __sock_wfree(struct sk_buff *skb);
> void sock_wfree(struct sk_buff *skb);
> +bool is_skb_wmem(const struct sk_buff *skb);
> struct sk_buff *sock_omalloc(struct sock *sk, unsigned long size,
> gfp_t priority);
> void skb_orphan_partial(struct sk_buff *skb);
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index f931176..29bb92e7 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1804,28 +1804,73 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
> struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
> {
> int delta = headroom - skb_headroom(skb);
> + int osize = skb_end_offset(skb);
> + struct sk_buff *oskb = NULL;
> + struct sock *sk = skb->sk;
>
> if (WARN_ONCE(delta <= 0,
> "%s is expecting an increase in the headroom", __func__))
> return skb;
>
> - /* pskb_expand_head() might crash, if skb is shared */
> + delta = SKB_DATA_ALIGN(delta);
> + /* pskb_expand_head() might crash, if skb is shared.
> + * Also we should clone skb if its destructor does
> + * not adjust skb->truesize and sk->sk_wmem_alloc
> + */
> if (skb_shared(skb)) {
> struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
>
> - if (likely(nskb)) {
> - if (skb->sk)
> - skb_set_owner_w(nskb, skb->sk);
> - consume_skb(skb);
> - } else {
> + if (unlikely(!nskb)) {
> kfree_skb(skb);
> + return NULL;
> }
> + oskb = skb;
> skb = nskb;
> }
> - if (skb &&
> - pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
> + if (pskb_expand_head(skb, delta, 0, GFP_ATOMIC)) {
> kfree_skb(skb);
> - skb = NULL;
> + kfree_skb(oskb);
> + return NULL;
> + }
> + if (oskb) {
> + if (sk)
> + skb_set_owner_w(skb, sk);
> + consume_skb(oskb);
> + } else if (sk && skb->destructor != sock_edemux) {
> + bool ref, set_owner;
> +
> + ref = false; set_owner = false;
> + delta = osize - skb_end_offset(skb);
> + /* skb_set_owner_w() calls current skb destructor.
> + * It can decrease sk_wmem_alloc to 0 and release sk,
> + * To prevnt it we increase sk_wmem_alloc earlier.
> + * Another kind of destructors can release last sk_refcnt,
> + * so it will be impossible to call sock_hold for !fullsock
> + * Take extra sk_refcnt to prevent it.
> + * Otherwise just increase truesize of expanded skb.
> + */
> + refcount_add(delta, &sk->sk_wmem_alloc);
> + if (!is_skb_wmem(skb)) {
> + set_owner = true;
> + if (!sk_fullsock(sk) && IS_ENABLED(CONFIG_INET)) {
> + /* skb_set_owner_w can set sock_edemux */
> + ref = refcount_inc_not_zero(&sk->sk_refcnt);
> + if (!ref) {
> + set_owner = false;
> + WARN_ON(refcount_sub_and_test(delta, &sk->sk_wmem_alloc));
> + }
> + }
> + }
> + if (set_owner)
> + skb_set_owner_w(skb, sk);
> +#ifdef CONFIG_INET
> + if (skb->destructor == sock_edemux) {
> + WARN_ON(refcount_sub_and_test(delta, &sk->sk_wmem_alloc));
> + if (ref)
> + WARN_ON(refcount_dec_and_test(&sk->sk_refcnt));
> + }
> +#endif
> + skb->truesize += delta;
> }
> return skb;
> }
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 950f1e7..6cbda43 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2227,6 +2227,14 @@ void skb_set_owner_w(struct sk_buff *skb, struct sock *sk)
> }
> EXPORT_SYMBOL(skb_set_owner_w);
>
> +bool is_skb_wmem(const struct sk_buff *skb)
> +{
> + return skb->destructor == sock_wfree ||
> + skb->destructor == __sock_wfree ||
> + (IS_ENABLED(CONFIG_INET) && skb->destructor == tcp_wfree);
> +}
> +EXPORT_SYMBOL(is_skb_wmem);
> +
> static bool can_skb_orphan_partial(const struct sk_buff *skb)
> {
> #ifdef CONFIG_TLS_DEVICE
> --
> 1.8.3.1
>