Patch "netlink: annotate lockless accesses to nlk->max_recvmsg_len" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    netlink: annotate lockless accesses to nlk->max_recvmsg_len

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     netlink-annotate-lockless-accesses-to-nlk-max_recvms.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 6b1da1c18ca1fda19a3531dada852085d10ca1ba
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Mon Apr 3 21:46:43 2023 +0000

    netlink: annotate lockless accesses to nlk->max_recvmsg_len
    
    [ Upstream commit a1865f2e7d10dde00d35a2122b38d2e469ae67ed ]
    
    syzbot reported a data-race in data-race in netlink_recvmsg() [1]
    
    Indeed, netlink_recvmsg() can be run concurrently,
    and netlink_dump() also needs protection.
    
    [1]
    BUG: KCSAN: data-race in netlink_recvmsg / netlink_recvmsg
    
    read to 0xffff888141840b38 of 8 bytes by task 23057 on cpu 0:
    netlink_recvmsg+0xea/0x730 net/netlink/af_netlink.c:1988
    sock_recvmsg_nosec net/socket.c:1017 [inline]
    sock_recvmsg net/socket.c:1038 [inline]
    __sys_recvfrom+0x1ee/0x2e0 net/socket.c:2194
    __do_sys_recvfrom net/socket.c:2212 [inline]
    __se_sys_recvfrom net/socket.c:2208 [inline]
    __x64_sys_recvfrom+0x78/0x90 net/socket.c:2208
    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
    entry_SYSCALL_64_after_hwframe+0x63/0xcd
    
    write to 0xffff888141840b38 of 8 bytes by task 23037 on cpu 1:
    netlink_recvmsg+0x114/0x730 net/netlink/af_netlink.c:1989
    sock_recvmsg_nosec net/socket.c:1017 [inline]
    sock_recvmsg net/socket.c:1038 [inline]
    ____sys_recvmsg+0x156/0x310 net/socket.c:2720
    ___sys_recvmsg net/socket.c:2762 [inline]
    do_recvmmsg+0x2e5/0x710 net/socket.c:2856
    __sys_recvmmsg net/socket.c:2935 [inline]
    __do_sys_recvmmsg net/socket.c:2958 [inline]
    __se_sys_recvmmsg net/socket.c:2951 [inline]
    __x64_sys_recvmmsg+0xe2/0x160 net/socket.c:2951
    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
    entry_SYSCALL_64_after_hwframe+0x63/0xcd
    
    value changed: 0x0000000000000000 -> 0x0000000000001000
    
    Reported by Kernel Concurrency Sanitizer on:
    CPU: 1 PID: 23037 Comm: syz-executor.2 Not tainted 6.3.0-rc4-syzkaller-00195-g5a57b48fdfcb #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/02/2023
    
    Fixes: 9063e21fb026 ("netlink: autosize skb lengthes")
    Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Reviewed-by: Simon Horman <simon.horman@xxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20230403214643.768555-1-edumazet@xxxxxxxxxx
    Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index e506712967918..99622c64081c4 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1941,7 +1941,7 @@ static int netlink_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 	struct scm_cookie scm;
 	struct sock *sk = sock->sk;
 	struct netlink_sock *nlk = nlk_sk(sk);
-	size_t copied;
+	size_t copied, max_recvmsg_len;
 	struct sk_buff *skb, *data_skb;
 	int err, ret;
 
@@ -1974,9 +1974,10 @@ static int netlink_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 #endif
 
 	/* Record the max length of recvmsg() calls for future allocations */
-	nlk->max_recvmsg_len = max(nlk->max_recvmsg_len, len);
-	nlk->max_recvmsg_len = min_t(size_t, nlk->max_recvmsg_len,
-				     SKB_WITH_OVERHEAD(32768));
+	max_recvmsg_len = max(READ_ONCE(nlk->max_recvmsg_len), len);
+	max_recvmsg_len = min_t(size_t, max_recvmsg_len,
+				SKB_WITH_OVERHEAD(32768));
+	WRITE_ONCE(nlk->max_recvmsg_len, max_recvmsg_len);
 
 	copied = data_skb->len;
 	if (len < copied) {
@@ -2225,6 +2226,7 @@ static int netlink_dump(struct sock *sk)
 	struct netlink_ext_ack extack = {};
 	struct netlink_callback *cb;
 	struct sk_buff *skb = NULL;
+	size_t max_recvmsg_len;
 	struct module *module;
 	int err = -ENOBUFS;
 	int alloc_min_size;
@@ -2247,8 +2249,9 @@ static int netlink_dump(struct sock *sk)
 	cb = &nlk->cb;
 	alloc_min_size = max_t(int, cb->min_dump_alloc, NLMSG_GOODSIZE);
 
-	if (alloc_min_size < nlk->max_recvmsg_len) {
-		alloc_size = nlk->max_recvmsg_len;
+	max_recvmsg_len = READ_ONCE(nlk->max_recvmsg_len);
+	if (alloc_min_size < max_recvmsg_len) {
+		alloc_size = max_recvmsg_len;
 		skb = alloc_skb(alloc_size,
 				(GFP_KERNEL & ~__GFP_DIRECT_RECLAIM) |
 				__GFP_NOWARN | __GFP_NORETRY);



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