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skbuff: skb_segment, Call zero copy functions before using skbuff frags
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Commit bf5c25d ("skbuff: in skb_segment, call zerocopy functions
once per nskb") added the call to zero copy functions in skb_segment().
The change introduced a bug in skb_segment() because skb_orphan_frags()
may possibly change the number of fragments or allocate new fragments
altogether leaving nrfrags and frag to point to the old values. This can
cause a panic with stacktrace like the one below.

[  193.894380] BUG: kernel NULL pointer dereference, address: 00000000000000bc
[  193.895273] CPU: 13 PID: 18164 Comm: vh-net-17428 Kdump: loaded Tainted: G           O      5.15.123+ #26
[  193.903919] RIP: 0010:skb_segment+0xb0e/0x12f0
[  194.021892] Call Trace:
[  194.027422]  <TASK>
[  194.072861]  tcp_gso_segment+0x107/0x540
[  194.082031]  inet_gso_segment+0x15c/0x3d0
[  194.090783]  skb_mac_gso_segment+0x9f/0x110
[  194.095016]  __skb_gso_segment+0xc1/0x190
[  194.103131]  netem_enqueue+0x290/0xb10 [sch_netem]
[  194.107071]  dev_qdisc_enqueue+0x16/0x70
[  194.110884]  __dev_queue_xmit+0x63b/0xb30
[  194.121670]  bond_start_xmit+0x159/0x380 [bonding]
[  194.128506]  dev_hard_start_xmit+0xc3/0x1e0
[  194.131787]  __dev_queue_xmit+0x8a0/0xb30
[  194.138225]  macvlan_start_xmit+0x4f/0x100 [macvlan]
[  194.141477]  dev_hard_start_xmit+0xc3/0x1e0
[  194.144622]  sch_direct_xmit+0xe3/0x280
[  194.147748]  __dev_queue_xmit+0x54a/0xb30
[  194.154131]  tap_get_user+0x2a8/0x9c0 [tap]
[  194.157358]  tap_sendmsg+0x52/0x8e0 [tap]
[  194.167049]  handle_tx_zerocopy+0x14e/0x4c0 [vhost_net]
[  194.173631]  handle_tx+0xcd/0xe0 [vhost_net]
[  194.176959]  vhost_worker+0x76/0xb0 [vhost]
[  194.183667]  kthread+0x118/0x140
[  194.190358]  ret_from_fork+0x1f/0x30
[  194.193670]  </TASK>

In this case calling skb_orphan_frags() updated nr_frags leaving nrfrags
local variable in skb_segment() stale. This resulted in the code hitting
i >= nrfrags prematurely and trying to move to next frag_skb using
list_skb pointer, which was NULL, and caused kernel panic. Move the call
to zero copy functions before using frags and nr_frags.

Fixes: bf5c25d ("skbuff: in skb_segment, call zerocopy functions once per nskb")
Signed-off-by: Mohamed Khalfella <[email protected]>
Reported-by: Amit Goyal <[email protected]>
Cc: [email protected]
Reviewed-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
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Mohamed Khalfella authored and davem330 committed Sep 1, 2023
1 parent f2e977f commit 2ea3528
Showing 1 changed file with 20 additions and 14 deletions.
34 changes: 20 additions & 14 deletions net/core/skbuff.c
Original file line number Diff line number Diff line change
Expand Up @@ -4423,21 +4423,20 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
struct sk_buff *segs = NULL;
struct sk_buff *tail = NULL;
struct sk_buff *list_skb = skb_shinfo(head_skb)->frag_list;
skb_frag_t *frag = skb_shinfo(head_skb)->frags;
unsigned int mss = skb_shinfo(head_skb)->gso_size;
unsigned int doffset = head_skb->data - skb_mac_header(head_skb);
struct sk_buff *frag_skb = head_skb;
unsigned int offset = doffset;
unsigned int tnl_hlen = skb_tnl_header_len(head_skb);
unsigned int partial_segs = 0;
unsigned int headroom;
unsigned int len = head_skb->len;
struct sk_buff *frag_skb;
skb_frag_t *frag;
__be16 proto;
bool csum, sg;
int nfrags = skb_shinfo(head_skb)->nr_frags;
int err = -ENOMEM;
int i = 0;
int pos;
int nfrags, pos;

if ((skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY) &&
mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) {
Expand Down Expand Up @@ -4514,6 +4513,13 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
headroom = skb_headroom(head_skb);
pos = skb_headlen(head_skb);

if (skb_orphan_frags(head_skb, GFP_ATOMIC))
return ERR_PTR(-ENOMEM);

nfrags = skb_shinfo(head_skb)->nr_frags;
frag = skb_shinfo(head_skb)->frags;
frag_skb = head_skb;

do {
struct sk_buff *nskb;
skb_frag_t *nskb_frag;
Expand All @@ -4534,6 +4540,10 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
(skb_headlen(list_skb) == len || sg)) {
BUG_ON(skb_headlen(list_skb) > len);

nskb = skb_clone(list_skb, GFP_ATOMIC);
if (unlikely(!nskb))
goto err;

i = 0;
nfrags = skb_shinfo(list_skb)->nr_frags;
frag = skb_shinfo(list_skb)->frags;
Expand All @@ -4552,12 +4562,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
frag++;
}

nskb = skb_clone(list_skb, GFP_ATOMIC);
list_skb = list_skb->next;

if (unlikely(!nskb))
goto err;

if (unlikely(pskb_trim(nskb, len))) {
kfree_skb(nskb);
goto err;
Expand Down Expand Up @@ -4633,12 +4639,16 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
skb_shinfo(nskb)->flags |= skb_shinfo(head_skb)->flags &
SKBFL_SHARED_FRAG;

if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
goto err;

while (pos < offset + len) {
if (i >= nfrags) {
if (skb_orphan_frags(list_skb, GFP_ATOMIC) ||
skb_zerocopy_clone(nskb, list_skb,
GFP_ATOMIC))
goto err;

i = 0;
nfrags = skb_shinfo(list_skb)->nr_frags;
frag = skb_shinfo(list_skb)->frags;
Expand All @@ -4652,10 +4662,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
i--;
frag--;
}
if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
skb_zerocopy_clone(nskb, frag_skb,
GFP_ATOMIC))
goto err;

list_skb = list_skb->next;
}
Expand Down

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