inetpeer: add a missing read_seqretry() in inet_getpeer()

When performing a lockless lookup over the inet_peer rbtree,
if a matching node is found, inet_getpeer() returns it immediately
without validating the seqlock sequence.

This missing check introduces a race condition:

Trigger Path: When a host receives an incoming fragmented IPv4 packet,
ip4_frag_init() (in net/ipv4/ip_fragment.c) calls inet_getpeer_v4()
to track the peer.

The Race: If the packet is from a new source IP, CPU A acquires the
write_seqlock, allocates a new inet_peer node (p), sets its IP address
(daddr), and links it to the rbtree (rb_link_node).

Uninitialized Access: Due to the lack of memory barriers between
rb_link_node and the initialization of the rest of the struct
(like refcount_set(&p->refcnt, 1)), CPU A can make the node visible
to readers before its refcnt is initialized.
This is especially true on weakly-ordered architectures like ARM64
where the CPU can reorder the memory stores.

Lockless Reader: Concurrently, CPU B processes a second fragmented packet
from the same source IP. CPU B does a lockless lookup, finds the newly
inserted node, and returns it immediately.

Use-After-Free (UAF): CPU B reads p->refcnt as uninitialized garbage
(left over from previous kmalloc-128/192 allocations).
If the garbage is > 0, refcount_inc_not_zero(&p->refcnt) succeeds.
CPU A then executes refcount_set(&p->refcnt, 1), overwriting CPU B's increment.
When CPU B finishes with the fragment queue, it calls inet_putpeer(),
which drops the refcount to 0 and frees the node via RCU.
The node is now freed but remains linked in the rbtree,
resulting in a Use-After-Free in the rbtree.

Fixes: b145425f26 ("inetpeer: remove AVL implementation in favor of RB tree")
Reported-by: Damiano Melotti <melotti@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260505133233.3039575-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/inetpeer.c

@@ -179,7 +179,8 @@ struct inet_peer *inet_getpeer(struct inet_peer_base *base,
 	seq = read_seqbegin(&base->lock);
 	p = lookup(daddr, base, seq, NULL, &gc_cnt, &parent, &pp);
 
-	if (p)
+	/* Make sure tree was not modified during our lookup. */
+	if (p && !read_seqretry(&base->lock, seq))
 		return p;
 
 	/* retry an exact lookup, taking the lock before.