Re: BUG: unable to handle kernel paging request from pty_write [was: Linux 4.4.2]

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Jiri Slaby <jslaby@suse.cz>, Greg KH <gregkh@linuxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	stable <stable@vger.kernel.org>,
	lwn@lwn.net, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: BUG: unable to handle kernel paging request from pty_write [was: Linux 4.4.2]
Date: Thu, 25 Feb 2016 12:51:37 -0800	[thread overview]
Message-ID: <CA+55aFx306jsTaUkm_c4nJtEo=A3vdDFYqqpLJj1zKUh=wLxog@mail.gmail.com> (raw)
In-Reply-To: <56CF64C9.8050705@hurleysoftware.com>

On Thu, Feb 25, 2016 at 12:32 PM, Peter Hurley <peter@hurleysoftware.com> wrote:
>> But yes, the call trace looks accurate and makes sense, we haveL
>>
>>   tty_flip_buffer_push ->
>>     (queue_work is inline) ->
>>     queue_work_on ->
>>       __queue_work ->
>>         insert_work ->
>>           (wake_up_worker is inlined)
>>           wake_up_process ->
>
>               try_to_wake_up ->
>
>>             *insane non-code address*

The thing is, we don't actually have that try_to_wake_up() on the
stack in the oops report. There are other thigns on the stack, but the
first stack entry that is dumped that is a text address is that
"ffffffff810a5585" which is wake_up_process.

That's why I said it might be stack corruption: we might be returning
from try_to_wake_up(), but with a corrupt stack entry, and returning
to garbage.

If it was one of the calls _in_ try_to_wake_up() that called to insane
code, I would have expected to see try_to_wake_up on the stack.

That's particularly true on modern machines, where things like the
percpu area is nopefully marked NX, so that we shouldn't be executing
random instructions. Which is the fault that actually triggers
("kernel tried to execute NX-protected page"), so the "we corrupted
the stack by running random code at the original target of the jump"
scenario sounds much less likely.

So the whole oops looks odd. If it really was one of the calls from
try_to_wake_up(), why isn't that return address on the stack?

Since this is under qemu, I'm wondering if this is a qemu bug, where
the NX fault processing of a call instruction happens before the stack
is pushed, but when the instruction pointer already points to the new
address.

Another alternative *might* be that gcc has turned an indirect
tail-call call into a "jmp *", but I certainly don't see that when I
compile the file myself. I've seen it in the past in some (very
unusual) cases, so it's possible - gcc definitely knows about
tail-call jmp conversion (even if it makes debugging sometimes a
pain).

Jiri, can you check your try_to_wake_up() disassembly for some
indirect "jmp" instructions?

                        Linus