Commit 4657ee0f authored by Eric Dumazet's avatar Eric Dumazet Committed by Greg Kroah-Hartman

tcp: limit payload size of sacked skbs

commit 3b4929f65b0d8249f19a50245cd88ed1a2f78cff upstream.

Jonathan Looney reported that TCP can trigger the following crash
in tcp_shifted_skb() :

	BUG_ON(tcp_skb_pcount(skb) < pcount);

This can happen if the remote peer has advertized the smallest
MSS that linux TCP accepts : 48

An skb can hold 17 fragments, and each fragment can hold 32KB
on x86, or 64KB on PowerPC.

This means that the 16bit witdh of TCP_SKB_CB(skb)->tcp_gso_segs
can overflow.

Note that tcp_sendmsg() builds skbs with less than 64KB
of payload, so this problem needs SACK to be enabled.
SACK blocks allow TCP to coalesce multiple skbs in the retransmit
queue, thus filling the 17 fragments to maximal capacity.

CVE-2019-11477 -- u16 overflow of TCP_SKB_CB(skb)->tcp_gso_segs

Backport notes, provided by Joao Martins <>

v4.15 or since commit 737ff314 ("tcp: use sequence distance to
detect reordering") had switched from the packet-based FACK tracking and
switched to sequence-based.

v4.14 and older still have the old logic and hence on
tcp_skb_shift_data() needs to retain its original logic and have
@fack_count in sync. In other words, we keep the increment of pcount with
tcp_skb_pcount(skb) to later used that to update fack_count. To make it
more explicit we track the new skb that gets incremented to pcount in
@next_pcount, and we get to avoid the constant invocation of
tcp_skb_pcount(skb) all together.

Fixes: 832d11c5 ("tcp: Try to restore large SKBs while SACK processing")
Signed-off-by: default avatarEric Dumazet <>
Reported-by: default avatarJonathan Looney <>
Acked-by: default avatarNeal Cardwell <>
Reviewed-by: default avatarTyler Hicks <>
Cc: Yuchung Cheng <>
Cc: Bruce Curtis <>
Cc: Jonathan Lemon <>
Signed-off-by: default avatarDavid S. Miller <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent d7b7345c
......@@ -419,4 +419,7 @@ static inline void tcp_saved_syn_free(struct tcp_sock *tp)
tp->saved_syn = NULL;
int tcp_skb_shift(struct sk_buff *to, struct sk_buff *from, int pcount,
int shiftlen);
#endif /* _LINUX_TCP_H */
......@@ -54,6 +54,8 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
#define TCP_MIN_SND_MSS 48
* Never offer a window over 32767 without using window scaling. Some
......@@ -3144,6 +3144,7 @@ void __init tcp_init(void)
int max_rshare, max_wshare, cnt;
unsigned int i;
sock_skb_cb_check_size(sizeof(struct tcp_skb_cb));
percpu_counter_init(&tcp_sockets_allocated, 0, GFP_KERNEL);
......@@ -1275,7 +1275,7 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
TCP_SKB_CB(skb)->seq += shifted;
tcp_skb_pcount_add(prev, pcount);
BUG_ON(tcp_skb_pcount(skb) < pcount);
WARN_ON_ONCE(tcp_skb_pcount(skb) < pcount);
tcp_skb_pcount_add(skb, -pcount);
/* When we're adding to gso_segs == 1, gso_size will be zero,
......@@ -1337,6 +1337,21 @@ static int skb_can_shift(const struct sk_buff *skb)
return !skb_headlen(skb) && skb_is_nonlinear(skb);
int tcp_skb_shift(struct sk_buff *to, struct sk_buff *from,
int pcount, int shiftlen)
/* TCP min gso_size is 8 bytes (TCP_MIN_GSO_SIZE)
* Since TCP_SKB_CB(skb)->tcp_gso_segs is 16 bits, we need
* to make sure not storing more than 65535 * 8 bytes per skb,
* even if current MSS is bigger.
if (unlikely(to->len + shiftlen >= 65535 * TCP_MIN_GSO_SIZE))
return 0;
if (unlikely(tcp_skb_pcount(to) + pcount > 65535))
return 0;
return skb_shift(to, from, shiftlen);
/* Try collapsing SACK blocks spanning across multiple skbs to a single
* skb.
......@@ -1348,6 +1363,7 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *prev;
int mss;
int next_pcount;
int pcount = 0;
int len;
int in_sack;
......@@ -1442,7 +1458,7 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
if (!after(TCP_SKB_CB(skb)->seq + len, tp->snd_una))
goto fallback;
if (!skb_shift(prev, skb, len))
if (!tcp_skb_shift(prev, skb, pcount, len))
goto fallback;
if (!tcp_shifted_skb(sk, skb, state, pcount, len, mss, dup_sack))
goto out;
......@@ -1461,11 +1477,11 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
goto out;
len = skb->len;
if (skb_shift(prev, skb, len)) {
pcount += tcp_skb_pcount(skb);
tcp_shifted_skb(sk, skb, state, tcp_skb_pcount(skb), len, mss, 0);
next_pcount = tcp_skb_pcount(skb);
if (tcp_skb_shift(prev, skb, next_pcount, len)) {
pcount += next_pcount;
tcp_shifted_skb(sk, skb, state, next_pcount, len, mss, 0);
state->fack_count += pcount;
return prev;
......@@ -1327,8 +1327,8 @@ static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
mss_now -= icsk->icsk_ext_hdr_len;
/* Then reserve room for full set of TCP options and 8 bytes of data */
if (mss_now < 48)
mss_now = 48;
if (mss_now < TCP_MIN_SND_MSS)
mss_now = TCP_MIN_SND_MSS;
return mss_now;
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