net/sched: act_pedit: really ensure the skb is writable

Currently pedit tries to ensure that the accessed skb offset
is writeble via skb_unclone(). The action potentially allows
touching any skb bytes, so it may end-up modifying shared data.

The above causes some sporadic MPTCP self-test failures.

Address the issue keeping track of a rough over-estimate highest skb
offset accessed by the action and ensure such offset is really
writable.

Note that this may cause performance regressions in some scenario,
but hopefully pedit is not critical path.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Acked-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Tested-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/tc_act/tc_pedit.h

@@ -14,6 +14,7 @@ struct tcf_pedit {
  struct tc_action common;
  unsigned char tcfp_nkeys;
  unsigned char tcfp_flags;
+ u32 tcfp_off_max_hint;
  struct tc_pedit_key *tcfp_keys;
  struct tcf_pedit_key_ex *tcfp_keys_ex;
 };

net/sched/act_pedit.c

@@ -149,7 +149,7 @@ static int tcf_pedit_init(struct net *net, struct nlattr *nla,
  struct nlattr *pattr;
  struct tcf_pedit *p;
  int ret = 0, err;
- int ksize;
+ int i, ksize;
  u32 index;
 
  if (!nla) {
@@ -228,6 +228,20 @@ static int tcf_pedit_init(struct net *net, struct nlattr *nla,
  p->tcfp_nkeys = parm->nkeys;
  }
  memcpy(p->tcfp_keys, parm->keys, ksize);
+ p->tcfp_off_max_hint = 0;
+ for (i = 0; i < p->tcfp_nkeys; ++i) {
+ u32 cur = p->tcfp_keys[i].off;
+
+ /* The AT option can read a single byte, we can bound the actual
+ * value with uchar max. Each key touches 4 bytes starting from
+ * the computed offset
+ */
+ if (p->tcfp_keys[i].offmask) {
+ cur += 255 >> p->tcfp_keys[i].shift;
+ cur = max(p->tcfp_keys[i].at, cur);
+ }
+ p->tcfp_off_max_hint = max(p->tcfp_off_max_hint, cur + 4);
+ }
 
  p->tcfp_flags = parm->flags;
  goto_ch = tcf_action_set_ctrlact(*a, parm->action, goto_ch);
@@ -308,9 +322,14 @@ static int tcf_pedit_act(struct sk_buff *skb, const struct tc_action *a,
  struct tcf_result *res)
 {
  struct tcf_pedit *p = to_pedit(a);
+ u32 max_offset;
  int i;
 
- if (skb_unclone(skb, GFP_ATOMIC))
+ max_offset = (skb_transport_header_was_set(skb) ?
+ skb_transport_offset(skb) :
+ skb_network_offset(skb)) +
+ p->tcfp_off_max_hint;
+ if (skb_ensure_writable(skb, min(skb->len, max_offset)))
  return p->tcf_action;
 
  spin_lock(&p->tcf_lock);