CVE-2021-47103: inet: fully convert sk->sk_rx_dst to RCU rules

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

inet: fully convert sk->skrxdst to RCU rules

syzbot reported various issues around early demux, one being included in this changelog [1]

sk->skrxdst is using RCU protection without clearly documenting it.

And following sequences in tcpv4dorcv()/tcpv6dorcv() are not following standard RCU rules.

[a] dstrelease(dst); [b] sk->skrxdst = NULL;

They look wrong because a delete operation of RCU protected pointer is supposed to clear the pointer before the callrcu()/synchronizercu() guarding actual memory freeing.

In some cases indeed, dst could be freed before [b] is done.

We could cheat by clearing skrxdst before calling dstrelease(), but this seems the right time to stick to standard RCU annotations and debugging facilities.

[1] BUG: KASAN: use-after-free in dstcheck include/net/dst.h:470 [inline] BUG: KASAN: use-after-free in tcpv4earlydemux+0x95b/0x960 net/ipv4/tcpipv4.c:1792 Read of size 2 at addr ffff88807f1cb73a by task syz-executor.5/9204

CPU: 0 PID: 9204 Comm: syz-executor.5 Not tainted 5.16.0-rc5-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: <TASK> dumpstack lib/dumpstack.c:88 [inline] dumpstacklvl+0xcd/0x134 lib/dumpstack.c:106 printaddressdescription.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247 kasanreport mm/kasan/report.c:433 [inline] kasanreport.cold+0x83/0xdf mm/kasan/report.c:450 dstcheck include/net/dst.h:470 [inline] tcpv4earlydemux+0x95b/0x960 net/ipv4/tcpipv4.c:1792 iprcvfinishcore.constprop.0+0x15de/0x1e80 net/ipv4/ipinput.c:340 iplistrcvfinish.constprop.0+0x1b2/0x6e0 net/ipv4/ipinput.c:583 ipsublistrcv net/ipv4/ipinput.c:609 [inline] iplistrcv+0x34e/0x490 net/ipv4/ipinput.c:644 netifreceiveskblistptype net/core/dev.c:5508 [inline] netifreceiveskblistcore+0x549/0x8e0 net/core/dev.c:5556 netifreceiveskblist net/core/dev.c:5608 [inline] netifreceiveskblistinternal+0x75e/0xd80 net/core/dev.c:5699 gronormallist net/core/dev.c:5853 [inline] gronormallist net/core/dev.c:5849 [inline] napicompletedone+0x1f1/0x880 net/core/dev.c:6590 virtqueuenapicomplete drivers/net/virtionet.c:339 [inline] virtnetpoll+0xca2/0x11b0 drivers/net/virtionet.c:1557 napipoll+0xaf/0x440 net/core/dev.c:7023 napipoll net/core/dev.c:7090 [inline] netrxaction+0x801/0xb40 net/core/dev.c:7177 dosoftirq+0x29b/0x9c2 kernel/softirq.c:558 invokesoftirq kernel/softirq.c:432 [inline] irqexitrcu+0x123/0x180 kernel/softirq.c:637 irqexitrcu+0x5/0x20 kernel/softirq.c:649 commoninterrupt+0x52/0xc0 arch/x86/kernel/irq.c:240 asmcommoninterrupt+0x1e/0x40 arch/x86/include/asm/idtentry.h:629 RIP: 0033:0x7f5e972bfd57 Code: 39 d1 73 14 0f 1f 80 00 00 00 00 48 8b 50 f8 48 83 e8 08 48 39 ca 77 f3 48 39 c3 73 3e 48 89 13 48 8b 50 f8 48 89 38 49 8b 0e <48> 8b 3e 48 83 c3 08 48 83 c6 08 eb bc 48 39 d1 72 9e 48 39 d0 73 RSP: 002b:00007fff8a413210 EFLAGS: 00000283 RAX: 00007f5e97108990 RBX: 00007f5e97108338 RCX: ffffffff81d3aa45 RDX: ffffffff81d3aa45 RSI: 00007f5e97108340 RDI: ffffffff81d3aa45 RBP: 00007f5e97107eb8 R08: 00007f5e97108d88 R09: 0000000093c2e8d9 R10: 0000000000000000 R11: 0000000000000000 R12: 00007f5e97107eb0 R13: 00007f5e97108338 R14: 00007f5e97107ea8 R15: 0000000000000019 </TASK>

Allocated by task 13: kasansavestack+0x1e/0x50 mm/kasan/common.c:38 kasansettrack mm/kasan/common.c:46 [inline] setallocinfo mm/kasan/common.c:434 [inline] kasanslaballoc+0x90/0xc0 mm/kasan/common.c:467 kasanslaballoc include/linux/kasan.h:259 [inline] slabpostallochook mm/slab.h:519 [inline] slaballocnode mm/slub.c:3234 [inline] slaballoc mm/slub.c:3242 [inline] kmemcachealloc+0x202/0x3a0 mm/slub.c:3247 dstalloc+0x146/0x1f0 net/core/dst.c:92 rtdstalloc+0x73/0x430 net/ipv4/route.c:1613 iprouteinputslow+0x1817/0x3a20 net/ipv4/route.c:234 ---truncated---