x86/unwind: Add hardcoded ORC entry for NULL

x86/unwind: Add hardcoded ORC entry for NULL

commit ac5ceccce5501e43d217c596e4ee859f2a3fef79 upstream.

When the ORC unwinder is invoked for an oops caused by IP==0,
it currently has no idea what to do because there is no debug information
for the stack frame of NULL.

But if RIP is NULL, it is very likely that the last successfully executed
instruction was an indirect CALL/JMP, and it is possible to unwind out in
the same way as for the first instruction of a normal function. Hardcode
a corresponding ORC entry.

With an artificially-added NULL call in prctl_set_seccomp(), before this
patch, the trace is:

Call Trace:
 ? __x64_sys_prctl+0x402/0x680
 ? __ia32_sys_prctl+0x6e0/0x6e0
 ? __do_page_fault+0x457/0x620
 ? do_syscall_64+0x6d/0x160
 ? entry_SYSCALL_64_after_hwframe+0x44/0xa9

After this patch, the trace looks like this:

Call Trace:
 __x64_sys_prctl+0x402/0x680
 ? __ia32_sys_prctl+0x6e0/0x6e0
 ? __do_page_fault+0x457/0x620
 do_syscall_64+0x6d/0x160
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

prctl_set_seccomp() still doesn't show up in the trace because for some
reason, tail call optimization is only disabled in builds that use the
frame pointer unwinder.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: syzbot <syzbot+ca95b2b7aef9e7cbd6ab@syzkaller.appspotmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: linux-kbuild@vger.kernel.org
Link: https://lkml.kernel.org/r/20190301031201.7416-2-jannh@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>