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

Affected: Linux

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

inet: fully convert sk->sk_rx_dst to RCU rules

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

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

And following sequences in tcp_v4_do_rcv()/tcp_v6_do_rcv() are not following standard RCU rules.

[a] dst_release(dst); [b] sk->sk_rx_dst = NULL;

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

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

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

[1] BUG: KASAN: use-after-free in dst_check include/net/dst.h:470 [inline] BUG: KASAN: use-after-free in tcp_v4_early_demux+0x95b/0x960 net/ipv4/tcp_ipv4.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> __dump_stack lib/dump_stack.c:88 [inline] dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106 print_address_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247 __kasan_report mm/kasan/report.c:433 [inline] kasan_report.cold+0x83/0xdf mm/kasan/r…

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