netfilter: xtables: fix reentrancy

commit f3c5c1bfd4308 (make ip_tables reentrant) introduced a race in
handling the stackptr restore, at the end of ipt_do_table()

We should do it before the call to xt_info_rdunlock_bh(), or we allow
cpu preemption and another cpu overwrites stackptr of original one.

A second fix is to change the underflow test to check the origptr value
instead of 0 to detect underflow, or else we allow a jump from different
hooks.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
index b09ed0d080f9..ffcea0d1678e 100644
--- a/net/ipv4/netfilter/ip_tables.c
+++ b/net/ipv4/netfilter/ip_tables.c
@@ -387,7 +387,7 @@ ipt_do_table(struct sk_buff *skb,
                                         verdict = (unsigned)(-v) - 1;
                                         break;
                                 }
-                                if (*stackptr == 0) {
+                                if (*stackptr <= origptr) {
                                         e = get_entry(table_base,
                                             private->underflow[hook]);
                                         pr_debug("Underflow (this is normal) "
@@ -427,10 +427,10 @@ ipt_do_table(struct sk_buff *skb,
                         /* Verdict */
                         break;
         } while (!acpar.hotdrop);
-        xt_info_rdunlock_bh();
         pr_debug("Exiting %s; resetting sp from %u to %u\n",
                  __func__, *stackptr, origptr);
         *stackptr = origptr;
+        xt_info_rdunlock_bh();
 #ifdef DEBUG_ALLOW_ALL
         return NF_ACCEPT;
 #else