[PATCH net v2] ipv6: raw: fix use-after-free in rawv6_send_hdrinc()

[Zhou Qiuyue <n0ps1edzz@gmail.com>]

rawv6_send_hdrinc() caches the route before handing the skb to
l3mdev_ip6_out(). The VRF output path may replace skb->dst and release
the original route, leaving the cached pointer stale.

The function later dereferences that pointer for statistics accounting
and as the output device passed to NF_HOOK(). Its RCU read-side critical
section starts after l3mdev_ip6_out(), so it cannot prevent the old route
from being reclaimed in between.

Reload the route from the current skb destination after
l3mdev_ip6_out() returns.

KASAN reports a slab-use-after-free in rawv6_sendmsg(), with the route
allocated by ip6_dst_alloc(), freed by dst_destroy() from rcu_core(), and
the related RCU callback queued from vrf_l3_out().

Fixes: 4c1feac58e06 ("net: vrf: Flip IPv6 output path from FIB lookup hook to out hook")
Cc: stable@vger.kernel.org
Signed-off-by: Zhou Qiuyue <n0ps1edzz@gmail.com>
---
Changes in v2:
- Rebased onto the current net tree at 7076f684d513.
- Regenerated the patch with git-format-patch so it applies cleanly.
- Corrected the Fixes tag to the commit that made the IPv6 VRF path reachable.
- Shortened the code comment and added the required recipients.

v1: https://lore.kernel.org/r/6adad51d.8fc0b0c9.1ec657.1868@mx.google.com

Thank you to Hangbin Liu for reviewing v1 and pointing out the submission
issues.

 net/ipv6/raw.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/raw.c b/net/ipv6/raw.c
index b965258cf..bdea82543 100644
--- a/net/ipv6/raw.c
+++ b/net/ipv6/raw.c
@@ -654,6 +654,9 @@ static int rawv6_send_hdrinc(struct sock *sk, struct msghdr *msg, int length,
 	if (unlikely(!skb))
 		return 0;
 
+	/* l3mdev_ip6_out() may have replaced the skb dst. */
+	rt = dst_rt6_info(skb_dst(skb));
+
 	/* Acquire rcu_read_lock() in case we need to use rt->rt6i_idev
 	 * in the error path. Since skb has been freed, the dst could
 	 * have been queued for deletion.

base-commit: 7076f684d5132e6a6145e1da2fa74771ecd052e5
-- 
2.50.1.windows.1