[PATCH net] ipv6: mcast: Fix use-after-free when processing MLD queries

[Ido Schimmel <idosch@nvidia.com>]

When processing an MLD query, a pointer to the multicast group address
is retrieved when initially parsing the packet. This pointer is later
dereferenced without being reloaded despite the fact that the skb header
might have been reallocated following the pskb_may_pull() calls, leading
to a use-after-free [1].

Fix by copying the multicast group address when the packet is initially
parsed.

[1]
BUG: KASAN: slab-use-after-free in __mld_query_work (net/ipv6/mcast.c:1512)
Read of size 8 at addr ffff8881154b8e90 by task kworker/4:1/118

Workqueue: mld mld_query_work
Call Trace:
<TASK>
dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
print_address_description.constprop.0 (mm/kasan/report.c:378)
print_report (mm/kasan/report.c:482)
kasan_report (mm/kasan/report.c:595)
__mld_query_work (net/ipv6/mcast.c:1512)
mld_query_work (net/ipv6/mcast.c:1563)
process_one_work (kernel/workqueue.c:3314)
worker_thread (kernel/workqueue.c:3397 kernel/workqueue.c:3478)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)
</TASK>

[...]

Freed by task 118:
kasan_save_stack (mm/kasan/common.c:57)
kasan_save_track (mm/kasan/common.c:78)
kasan_save_free_info (mm/kasan/generic.c:584)
__kasan_slab_free (mm/kasan/common.c:253 mm/kasan/common.c:285)
kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6251 mm/slub.c:6566)
pskb_expand_head (net/core/skbuff.c:2335)
__pskb_pull_tail (net/core/skbuff.c:2878 (discriminator 4))
__mld_query_work (net/ipv6/mcast.c:1495 (discriminator 1))
mld_query_work (net/ipv6/mcast.c:1563)
process_one_work (kernel/workqueue.c:3314)
worker_thread (kernel/workqueue.c:3397 kernel/workqueue.c:3478)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

Fixes: 97300b5fdfe2 ("[MCAST] IPv6: Check packet size when process Multicast")
Reported-by: Leo Lin <leo@depthfirst.com>
Reviewed-by: David Ahern <dahern@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
---
 net/ipv6/mcast.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 3330adcf26db..d9b855d5191b 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -1424,9 +1424,9 @@ void igmp6_event_query(struct sk_buff *skb)
 static void __mld_query_work(struct sk_buff *skb)
 {
 	struct mld2_query *mlh2 = NULL;
-	const struct in6_addr *group;
 	unsigned long max_delay;
 	struct inet6_dev *idev;
+	struct in6_addr group;
 	struct ifmcaddr6 *ma;
 	struct mld_msg *mld;
 	int group_type;
@@ -1458,8 +1458,8 @@ static void __mld_query_work(struct sk_buff *skb)
 		goto kfree_skb;
 
 	mld = (struct mld_msg *)icmp6_hdr(skb);
-	group = &mld->mld_mca;
-	group_type = ipv6_addr_type(group);
+	group = mld->mld_mca;
+	group_type = ipv6_addr_type(&group);
 
 	if (group_type != IPV6_ADDR_ANY &&
 	    !(group_type&IPV6_ADDR_MULTICAST))
@@ -1509,7 +1509,7 @@ static void __mld_query_work(struct sk_buff *skb)
 		}
 	} else {
 		for_each_mc_mclock(idev, ma) {
-			if (!ipv6_addr_equal(group, &ma->mca_addr))
+			if (!ipv6_addr_equal(&group, &ma->mca_addr))
 				continue;
 			if (ma->mca_flags & MAF_TIMER_RUNNING) {
 				/* gsquery <- gsquery && mark */
-- 
2.54.0