CVE-2024-36905

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

tcp: defer shutdown(SENDSHUTDOWN) for TCPSYN_RECV sockets

TCPSYNRECV state is really special, it is only used by cross-syn connections, mostly used by fuzzers.

In the following crash [1], syzbot managed to trigger a divide by zero in tcprcvspace_adjust()

A socket makes the following state transitions, without ever calling tcpinittransfer(), meaning tcpinitbuffer_space() is also not called.

     TCP_CLOSE

connect() TCPSYNSENT TCPSYNRECV shutdown() -> tcpshutdown(sk, SENDSHUTDOWN) TCPFINWAIT1

To fix this issue, change tcpshutdown() to not perform a TCPSYNRECV -> TCPFIN_WAIT1 transition, which makes no sense anyway.

When tcprcvstateprocess() later changes socket state from TCPSYNRECV to TCPESTABLISH, then look at sk->skshutdown to finally enter TCPFIN_WAIT1 state, and send a FIN packet from a sane socket state.

This means tcpsendfin() can now be called from BH context, and must use GFP_ATOMIC allocations.

[1] divide error: 0000 [#1] PREEMPT SMP KASAN NOPTI CPU: 1 PID: 5084 Comm: syz-executor358 Not tainted 6.9.0-rc6-syzkaller-00022-g98369dccd2f8 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024 RIP: 0010:tcprcvspaceadjust+0x2df/0x890 net/ipv4/tcpinput.c:767 Code: e3 04 4c 01 eb 48 8b 44 24 38 0f b6 04 10 84 c0 49 89 d5 0f 85 a5 03 00 00 41 8b 8e c8 09 00 00 89 e8 29 c8 48 0f af c3 31 d2 <48> f7 f1 48 8d 1c 43 49 8d 96 76 08 00 00 48 89 d0 48 c1 e8 03 48 RSP: 0018:ffffc900031ef3f0 EFLAGS: 00010246 RAX: 0c677a10441f8f42 RBX: 000000004fb95e7e RCX: 0000000000000000 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 RBP: 0000000027d4b11f R08: ffffffff89e535a4 R09: 1ffffffff25e6ab7 R10: dffffc0000000000 R11: ffffffff8135e920 R12: ffff88802a9f8d30 R13: dffffc0000000000 R14: ffff88802a9f8d00 R15: 1ffff1100553f2da FS: 00005555775c0380(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f1155bf2304 CR3: 000000002b9f2000 CR4: 0000000000350ef0 Call Trace: <TASK> tcprecvmsglocked+0x106d/0x25a0 net/ipv4/tcp.c:2513 tcprecvmsg+0x25d/0x920 net/ipv4/tcp.c:2578 inet6recvmsg+0x16a/0x730 net/ipv6/afinet6.c:680 sockrecvmsgnosec net/socket.c:1046 [inline] sockrecvmsg+0x109/0x280 net/socket.c:1068 _sysrecvmsg+0x1db/0x470 net/socket.c:2803 sysrecvmsg net/socket.c:2845 [inline] dorecvmmsg+0x474/0xae0 net/socket.c:2939 _sysrecvmmsg net/socket.c:3018 [inline] _dosysrecvmmsg net/socket.c:3041 [inline] _sesysrecvmmsg net/socket.c:3034 [inline] _x64sysrecvmmsg+0x199/0x250 net/socket.c:3034 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xf5/0x240 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x7faeb6363db9 Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 c1 17 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ffcc1997168 EFLAGS: 00000246 ORIG_RAX: 000000000000012b RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007faeb6363db9 RDX: 0000000000000001 RSI: 0000000020000bc0 RDI: 0000000000000005 RBP: 0000000000000000 R08: 0000000000000000 R09: 000000000000001c R10: 0000000000000122 R11: 0000000000000246 R12: 0000000000000000 R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000001