[PATCH 6.1 581/609] ipv6: prevent in6_dev_get() from resurrecting inet6_dev

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kyle Zeng <[email protected]>

commit 0e243671bc7b8eaf00f83dd2f4367436dc0cff98 upstream.

in6_dev_get() reads dev->ip6_ptr under RCU and then unconditionally
increments its refcount. Device teardown can clear the pointer and drop
the last reference between these operations. The increment then
resurrects an object whose RCU free has already been queued, so callers
can use it after it is freed.

Use refcount_inc_not_zero() and return NULL when the object has already
reached zero. RCU keeps the memory accessible through the attempted
reference acquisition, and a successful increment pins the object for
the caller.

An independent run on the exact unpatched 6f5156d7a31a (v7.2-rc3)
kernel reproduced the invalid reference acquisition as UID 1000:

  refcount_t: addition on 0; use-after-free.
  ip6_mc_source+0xef4/0x17e0

It was followed by the corresponding reference underflow in
ip6_mc_source(). The supplied trace from the same unpatched revision
additionally shows the access after the RCU read-side section ends:

  BUG: KASAN: slab-use-after-free in mutex_lock+0x76/0xe0
  Write of size 8 at addr ffff888015b50240 by task poc/1219

Bug found and triaged by OpenAI Security Research and
validated by Trail of Bits.

Fixes: 8814c4b53381 ("[IPV6] ADDRCONF: Convert addrconf_lock to RCU.")
Cc: [email protected]
Signed-off-by: Kyle Zeng <[email protected]>
Co-developed-by: David Lee <[email protected]>
Signed-off-by: David Lee <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 include/net/addrconf.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/net/addrconf.h
+++ b/include/net/addrconf.h
@@ -370,8 +370,8 @@ static inline struct inet6_dev *in6_dev_
 
 	rcu_read_lock();
 	idev = rcu_dereference(dev->ip6_ptr);
-	if (idev)
-		refcount_inc(&idev->refcnt);
+	if (idev && !refcount_inc_not_zero(&idev->refcnt))
+		idev = NULL;
 	rcu_read_unlock();
 	return idev;
 }