by Vasily Averin

[permalink] [raw]

Subject: [PATCH net v6] skb_expand_head() adjust skb->truesize incorrectly

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()")
Fixes: 2d85a1b31dde ("ipv6: ip6_finish_output2: set sk into newly allocated nskb")
Reported-by: Christoph Paasch <[email protected]>
Signed-off-by: Vasily Averin <[email protected]>
---
v6: fixed delta,
improved comments
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 | 60 ++++++++++++++++++++++++++++++++++++++++++++++--------
net/core/sock.c | 8 ++++++++
3 files changed, 60 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..e2a2aa31 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1804,28 +1804,70 @@ 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. */
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 = skb_end_offset(skb) - osize;
+ /* skb_set_owner_w() calls current skb destructor.
+ * It can reduce sk_wmem_alloc to 0 and release sk,
+ * To prevnt this, we increase sk_wmem_alloc in advance.
+ * Some destructors might release the last sk_refcnt,
+ * so it won't be possible to call sock_hold for !fullsock
+ * We take an extra sk_refcnt to prevent this.
+ * In any case we 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

2021-09-06 18:11:43

by Vasily Averin

[permalink] [raw]

Subject: Re: [PATCH net v6] skb_expand_head() adjust skb->truesize incorrectly

I've finally reproduced original issue by using reproducer from Christoph Paasch,
and was able locally validate this patch.

Thank you,
Vasily Averin

On 9/6/21 9:01 PM, 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()")
> Fixes: 2d85a1b31dde ("ipv6: ip6_finish_output2: set sk into newly allocated nskb")
> Reported-by: Christoph Paasch <[email protected]>
> Signed-off-by: Vasily Averin <[email protected]>
> ---
> v6: fixed delta,
> improved comments
> 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 | 60 ++++++++++++++++++++++++++++++++++++++++++++++--------
> net/core/sock.c | 8 ++++++++
> 3 files changed, 60 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..e2a2aa31 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1804,28 +1804,70 @@ 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. */
> 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 = skb_end_offset(skb) - osize;
> + /* skb_set_owner_w() calls current skb destructor.
> + * It can reduce sk_wmem_alloc to 0 and release sk,
> + * To prevnt this, we increase sk_wmem_alloc in advance.
> + * Some destructors might release the last sk_refcnt,
> + * so it won't be possible to call sock_hold for !fullsock
> + * We take an extra sk_refcnt to prevent this.
> + * In any case we 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
>