Re: Use-after-free in ppoll

[Rainer Weikusat <rweikusat@mobileactivedefense.com>]

Dmitry Vyukov <dvyukov@google.com> writes:
> Hello,
>
> On commit f2d10565b9bdbb722bd43e6e1a759eeddb9645c8 (Nov 20).
>
> The following program triggers use-after-free:
>
> // autogenerated by syzkaller (http://github.com/google/syzkaller)
> #include <syscall.h>
> #include <string.h>
> #include <stdint.h>
> #include <pthread.h>
>
> void *thread(void *p)
> {
>         syscall(SYS_write, (long)p, 0x2000278ful, 0x1ul, 0, 0, 0);
>         return 0;
> }

[...]


>         long r1 = syscall(SYS_socketpair, 0x1ul, 0x3ul, 0x0ul,

[...]

>         long r5 = syscall(SYS_close, r2, 0, 0, 0, 0, 0);
>         pthread_t th;
>         pthread_create(&th, 0, thread, (void*)(long)r3);

[...]

>         long r21 = syscall(SYS_ppoll, 0x20000ffful, 0x3ul, 0x20000ffcul, 0x20000ffdul, 0x8ul, 0);
>         return 0;
> }

That's one of the already known sequences for triggering this issue: The
close will clear the peer pointer of the closed socket, hence, the 2nd
sock_poll_wait will be called by unix_dgram_poll. The write will
execute unix_dgram_sendmsg which detects that the peer is dead and
disconnects from it, causing the corresponding structures to be freed
despite they're still used.

NB: I didn't execute this but I spend a fair amount of time with the
af_unix.c code during the last couple of weeks and consider myself
"reasonably familiar" with it and that's IMO what should happen here.