CVE-2024-41041: udp: Set SOCK_RCU_FREE earlier in udp_lib_get_port().

In the Linux kernel, the following vulnerability has been resolved:

udp: Set SOCKRCUFREE earlier in udplibgetport().

syzkaller triggered the warning [0] in udpv4earlydemux().

In udpv[46]earlydemux() and sklookup(), we do not touch the refcount of the looked-up sk and use sockpfree() as skb->destructor, so we check SOCKRCUFREE to ensure that the sk is safe to access during the RCU grace period.

Currently, SOCKRCUFREE is flagged for a bound socket after being put into the hash table. Moreover, the SOCKRCUFREE check is done too early in udpv[46]earlydemux() and sklookup(), so there could be a small race window:

CPU1 CPU2 ---- ---- udpv4earlydemux() udplibgetport() | |- hlistaddheadrcu() |- sk = udp4libdemuxlookup() | |- DEBUGNETWARNONONCE(skisrefcounted(sk)); - socksetflag(sk, SOCKRCUFREE)

We had the same bug in TCP and fixed it in commit 871019b22d1b ("net: set SOCKRCUFREE before inserting socket into hashtable").

Let's apply the same fix for UDP.

[0]: WARNING: CPU: 0 PID: 11198 at net/ipv4/udp.c:2599 udpv4earlydemux+0x481/0xb70 net/ipv4/udp.c:2599 Modules linked in: CPU: 0 PID: 11198 Comm: syz-executor.1 Not tainted 6.9.0-g93bda33046e7 #13 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014 RIP: 0010:udpv4earlydemux+0x481/0xb70 net/ipv4/udp.c:2599 Code: c5 7a 15 fe bb 01 00 00 00 44 89 e9 31 ff d3 e3 81 e3 bf ef ff ff 89 de e8 2c 74 15 fe 85 db 0f 85 02 06 00 00 e8 9f 7a 15 fe <0f> 0b e8 98 7a 15 fe 49 8d 7e 60 e8 4f 39 2f fe 49 c7 46 60 20 52 RSP: 0018:ffffc9000ce3fa58 EFLAGS: 00010293 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff8318c92c RDX: ffff888036ccde00 RSI: ffffffff8318c2f1 RDI: 0000000000000001 RBP: ffff88805a2dd6e0 R08: 0000000000000001 R09: 0000000000000000 R10: 0000000000000000 R11: 0001ffffffffffff R12: ffff88805a2dd680 R13: 0000000000000007 R14: ffff88800923f900 R15: ffff88805456004e FS: 00007fc449127640(0000) GS:ffff88807dc00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fc449126e38 CR3: 000000003de4b002 CR4: 0000000000770ef0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600 PKRU: 55555554 Call Trace: <TASK> iprcvfinishcore.constprop.0+0xbdd/0xd20 net/ipv4/ipinput.c:349 iprcvfinish+0xda/0x150 net/ipv4/ipinput.c:447 NFHOOK include/linux/netfilter.h:314 [inline] NFHOOK include/linux/netfilter.h:308 [inline] iprcv+0x16c/0x180 net/ipv4/ipinput.c:569 netifreceiveskbonecore+0xb3/0xe0 net/core/dev.c:5624 netifreceiveskb+0x21/0xd0 net/core/dev.c:5738 netifreceiveskbinternal net/core/dev.c:5824 [inline] netifreceiveskb+0x271/0x300 net/core/dev.c:5884 tunrxbatched drivers/net/tun.c:1549 [inline] tungetuser+0x24db/0x2c50 drivers/net/tun.c:2002 tunchrwriteiter+0x107/0x1a0 drivers/net/tun.c:2048 newsyncwrite fs/readwrite.c:497 [inline] vfswrite+0x76f/0x8d0 fs/readwrite.c:590 ksyswrite+0xbf/0x190 fs/readwrite.c:643 dosyswrite fs/readwrite.c:655 [inline] sesyswrite fs/readwrite.c:652 [inline] x64syswrite+0x41/0x50 fs/readwrite.c:652 x64syscall+0xe66/0x1990 arch/x86/include/generated/asm/syscalls64.h:2 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0x4b/0x110 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x4b/0x53 RIP: 0033:0x7fc44a68bc1f Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 e9 cf f5 ff 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 3c d0 f5 ff 48 RSP: 002b:00007fc449126c90 EFLAGS: 00000293 ORIGRAX: 0000000000000001 RAX: ffffffffffffffda RBX: 00000000004bc050 RCX: 00007fc44a68bc1f R ---truncated---