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Message-ID: <ce783b33-c81f-4760-1f9e-90b7d8c51fd7@gmail.com>
Date: Wed, 1 Sep 2021 12:17:31 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Vasily Averin <vvs@...tuozzo.com>,
Christoph Paasch <christoph.paasch@...il.com>,
"David S. Miller" <davem@...emloft.net>
Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
David Ahern <dsahern@...nel.org>,
Jakub Kicinski <kuba@...nel.org>,
netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org,
kernel@...nvz.org, Alexey Kuznetsov <kuznet@....inr.ac.ru>,
Julian Wiedmann <jwi@...ux.ibm.com>
Subject: Re: [PATCH net-next v4] skb_expand_head() adjust skb->truesize
incorrectly
On 9/1/21 1:11 AM, Vasily Averin 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@...il.com>
> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
> ---
> 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 | 35 ++++++++++++++++++++++++++---------
> net/core/sock.c | 8 ++++++++
> 3 files changed, 35 insertions(+), 9 deletions(-)
>
> 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..09991cb 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1804,28 +1804,45 @@ 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)
if (is_skb_wmem(oskb))
Again, it is not valid to call skb_set_owner_w(skb, sk) on all kind of sockets.
> + skb_set_owner_w(skb, sk);
> + consume_skb(oskb);
> + } else if (sk) {
&& (skb->destructor != sock_edemux)
(Because in this case , pskb_expand_head() already adjusted skb->truesize)
> + delta = osize - skb_end_offset(skb);
> + if (!is_skb_wmem(skb))
> + skb_set_owner_w(skb, sk);
This is dangerous, even if a socket is there, its sk->sk_wmem_alloc could be zero.
We can not add skb->truesize to a refcount_t that already reached 0 (sk_free())
If is_skb_wmem() is false, you probably should do nothing, and leave
current destructor as it is.
(skb->truesize can be adjusted without issue)
> + skb->truesize += delta;
> + if (sk_fullsock(sk))
if (is_skb_wmem(skb))
> + refcount_add(delta, &sk->sk_wmem_alloc);
> }
> 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
>
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