| Message ID | 20230607145954.19324-4-fw@strlen.de (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | net/sched: act_ipt bug fixes | expand |
On Wed, Jun 07, 2023 at 04:59:54PM +0200, Florian Westphal wrote: > xtables relies on skb being owned by ip stack, i.e. with ipv4 > check in place skb->cb is supposed to be IPCB. > > I don't see an immediate problem (REJECT target cannot be used anymore > now that PRE/POSTROUTING hook validation has been fixed), but better be > safe than sorry. > > A much better patch would be to either mark act_ipt as > "depends on BROKEN" or remove it altogether. I plan to do this > for -next in the near future. > > This tc extension is broken in the sense that tc lacks an > equivalent of NF_STOLEN verdict. > > With NF_STOLEN, target function takes complete ownership of skb, caller > cannot dereference it anymore. > > ACT_STOLEN cannot be used for this: it has a different meaning, caller > is allowed to dereference the skb. > > At this time NF_STOLEN won't be returned by any targets as far as I can > see, but this may change in the future. > > It might be possible to work around this via explcit list of allowed nit: explcit -> explicit > target extensions, that only return DROP or ACCEPT verdicts, but this > seems to be error prone. > > Existing selftest only validates xt_LOG and act_ipt is restricted > to ipv4 so I don't think this action is used widely. > > Signed-off-by: Florian Westphal <fw@strlen.de> Reviewed-by: Simon Horman <simon.horman@corigine.com>
diff --git a/net/sched/act_ipt.c b/net/sched/act_ipt.c index 2f0b39cc4e37..ec04bcfa0f4b 100644 --- a/net/sched/act_ipt.c +++ b/net/sched/act_ipt.c @@ -21,6 +21,7 @@ #include <linux/tc_act/tc_ipt.h> #include <net/tc_act/tc_ipt.h> #include <net/tc_wrapper.h> +#include <net/ip.h> #include <linux/netfilter_ipv4/ip_tables.h> @@ -254,6 +255,7 @@ TC_INDIRECT_SCOPE int tcf_ipt_act(struct sk_buff *skb, const struct tc_action *a, struct tcf_result *res) { + char saved_cb[sizeof_field(struct sk_buff, cb)]; int ret = 0, result = 0; struct tcf_ipt *ipt = to_ipt(a); struct xt_action_param par; @@ -280,6 +282,8 @@ TC_INDIRECT_SCOPE int tcf_ipt_act(struct sk_buff *skb, state.out = skb->dev; } + memcpy(saved_cb, skb->cb, sizeof(saved_cb)); + spin_lock(&ipt->tcf_lock); tcf_lastuse_update(&ipt->tcf_tm); @@ -292,6 +296,9 @@ TC_INDIRECT_SCOPE int tcf_ipt_act(struct sk_buff *skb, par.state = &state; par.target = ipt->tcfi_t->u.kernel.target; par.targinfo = ipt->tcfi_t->data; + + memset(IPCB(skb), 0, sizeof(struct inet_skb_parm)); + ret = par.target->target(skb, &par); switch (ret) { @@ -312,6 +319,9 @@ TC_INDIRECT_SCOPE int tcf_ipt_act(struct sk_buff *skb, break; } spin_unlock(&ipt->tcf_lock); + + memcpy(skb->cb, saved_cb, sizeof(skb->cb)); + return result; }
xtables relies on skb being owned by ip stack, i.e. with ipv4 check in place skb->cb is supposed to be IPCB. I don't see an immediate problem (REJECT target cannot be used anymore now that PRE/POSTROUTING hook validation has been fixed), but better be safe than sorry. A much better patch would be to either mark act_ipt as "depends on BROKEN" or remove it altogether. I plan to do this for -next in the near future. This tc extension is broken in the sense that tc lacks an equivalent of NF_STOLEN verdict. With NF_STOLEN, target function takes complete ownership of skb, caller cannot dereference it anymore. ACT_STOLEN cannot be used for this: it has a different meaning, caller is allowed to dereference the skb. At this time NF_STOLEN won't be returned by any targets as far as I can see, but this may change in the future. It might be possible to work around this via explcit list of allowed target extensions, that only return DROP or ACCEPT verdicts, but this seems to be error prone. Existing selftest only validates xt_LOG and act_ipt is restricted to ipv4 so I don't think this action is used widely. Signed-off-by: Florian Westphal <fw@strlen.de> --- net/sched/act_ipt.c | 10 ++++++++++ 1 file changed, 10 insertions(+)