linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
	kbuild test robot <fengguang.wu@intel.com>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f
Date: Wed, 7 Feb 2018 11:43:37 -0800	[thread overview]
Message-ID: <CA+55aFzSZzdVZXatze=xjGSwpOPO7kvbWnz7JiZ3sTx_2Pohjw@mail.gmail.com> (raw)
In-Reply-To: <20180207192816.GA19536@pd.tnic>

On Wed, Feb 7, 2018 at 11:28 AM, Borislav Petkov <bp@alien8.de> wrote:
> On Wed, Feb 07, 2018 at 08:14:51PM +0100, Peter Zijlstra wrote:
>> Then someone went and wrecked it.
>
> Yeah, note says UD0 didn't eat a ModRM byte on old CPUs. But then that
> changed too. Fun stuff changing insn encoding underway.
>
> So if we opt for adding a ModRM byte, could a 0x90 NOP work so that it
> doesn't shit itself on those old CPUs?

We could just also decide that the only thing that the modrm bytes of
UD0 actually *affect* is how the CPU might act for a page-crossing
instruction.

Because I think that's the only semantic difference: if it's a
page-crosser, the instruction could take a page fault before raising
the #UD.

Is there any other decode issue we might want to look out for?

Anyway, then we'd make the rule be:

 - we promise to always pad up the following bytes (our extra warning
information etc) so that we never have a missing page afterwards (this
is presumably practically speaking already the case). It might still
be a page-crossing instruction, but we won't take a page fault in
kernel space (due to it being at the end of some text sectoin or
whatever that changes the NX bit or due to DEBUG_PAGEALLOC having
unmapped the next page).

 - we special-case the decoder so that we don't get this warning

and then we just ignore the issue entirely.

Hmm?

              Linus

  reply	other threads:[~2018-02-07 19:43 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-07 11:44 [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f kbuild test robot
2018-02-07 18:13 ` Linus Torvalds
2018-02-07 18:35   ` Borislav Petkov
2018-02-07 18:49     ` Peter Zijlstra
2018-02-07 19:03       ` Linus Torvalds
2018-02-07 19:14         ` Peter Zijlstra
2018-02-07 19:28           ` Borislav Petkov
2018-02-07 19:43             ` Linus Torvalds [this message]
2018-02-07 20:24               ` Borislav Petkov
2018-02-08  9:13               ` Peter Zijlstra
2018-02-08  9:35                 ` Peter Zijlstra
2018-02-08  9:46                 ` Borislav Petkov
2018-02-08  9:47                 ` David Laight
2018-02-08 10:13                   ` Peter Zijlstra
2018-02-08 17:27                 ` Linus Torvalds
2018-02-08 18:03                   ` Peter Zijlstra
2018-02-08 18:15                     ` Linus Torvalds
2018-02-08 19:44                       ` Peter Zijlstra
2018-02-08 20:02                         ` Linus Torvalds
2018-02-08 20:31                         ` Borislav Petkov
2018-02-08 23:09                           ` [PATCH 0/2] objtool fixes on top of Peter's WARN UD2 patch Josh Poimboeuf
2018-02-08 23:09                             ` [PATCH 1/2] objtool: Fix seg fault in ignore_unreachable_insn() Josh Poimboeuf
2018-02-13 11:29                               ` [tip:x86/pti] objtool: Fix segfault " tip-bot for Josh Poimboeuf
2018-02-15  0:26                               ` tip-bot for Josh Poimboeuf
2018-02-08 23:09                             ` [PATCH 2/2] x86: Annotate WARN-related UD2 as reachable Josh Poimboeuf
2018-02-13 11:30                               ` [tip:x86/pti] x86/debug, objtool: Annotate WARN()-related " tip-bot for Josh Poimboeuf
2018-02-15  0:26                               ` tip-bot for Josh Poimboeuf
2018-02-09  8:13                             ` [PATCH 0/2] objtool fixes on top of Peter's WARN UD2 patch Peter Zijlstra
2018-02-09  8:12                           ` [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f Peter Zijlstra
2018-02-13 11:30                         ` [tip:x86/pti] x86/debug: Use UD2 for WARN() tip-bot for Peter Zijlstra
2018-02-15  0:27                         ` tip-bot for Peter Zijlstra
2018-02-07 18:38   ` [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f Randy Dunlap
2018-02-07 19:01     ` Linus Torvalds
2018-02-07 19:06       ` Peter Zijlstra
2018-02-07 19:10         ` Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CA+55aFzSZzdVZXatze=xjGSwpOPO7kvbWnz7JiZ3sTx_2Pohjw@mail.gmail.com' \
    --to=torvalds@linux-foundation.org \
    --cc=bp@alien8.de \
    --cc=fengguang.wu@intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@kernel.org \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).