From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>,
David Woodhouse <dwmw2@infradead.org>,
Andi Kleen <andi@firstfloor.org>,
the arch/x86 maintainers <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Paul Turner <pjt@google.com>, Andrew Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Tom Lendacky <thomas.lendacky@amd.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
Greg Kroah-Hartman <gregkh@linux-foundation.org>,
Dave Hansen <dave.hansen@intel.com>,
Jiri Kosina <jikos@kernel.org>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking
Date: Wed, 10 Jan 2018 14:55:46 -0600 [thread overview]
Message-ID: <20180110205546.qn5kjil7icibfnkk@treble> (raw)
In-Reply-To: <CA+55aFzMzBwtfO1e=eC6oCx9mMMXPoRko0FvKKJrriUGqyQtEg@mail.gmail.com>
On Wed, Jan 10, 2018 at 12:26:25PM -0800, Linus Torvalds wrote:
> On Wed, Jan 10, 2018 at 12:15 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > I think .altinstruction relocations *do* work if they're for the first
> > instruction, and it's a jump or a call.
>
> Yes - for the alternative that is in-line - not in the "altinstruction" section.
>
> Because then the alternative is in the right spot at link-time already.
>
> But the "altinstruction" section definitely should not have
> relocations.
I misspoke, it's really .altinstr_replacement which has the replacement
instructions. And it has a bunch of relocations:
Relocation section [ 8] '.rela.altinstr_replacement' for section [ 7] '.altinstr_replacement' at offset 0x14439710 contains 355 entries:
> I guess you could hack them up by hand by explicitly
> trying to take the difference between the non-altinstruction and the
> altinstruction into account, but it would be error-prone and fragile
> as hell.
apply_alternatives() already does that today. It actually seems pretty
solid, except for the whole "only works on the first instruction" thing.
> > I think Boris had a patch floating around to add an instruction decoder
> > to alternatives, so you can do a call/jmp anywhere.
>
> .. and no, we're not doing that. Christ.
>
> People, we need to try to be *robust* here. That's doubly (triply!)
> true of things like altinstructions where people - very much by design
> - won't even *test* the alternatives very much, because very much by
> design the altinstructions are only used on certain architectures or
> in certain situations.
>
> And we almost certainly don't actuially _need_ relocations. But we
> need to protect against the "oops, I didn't realize" issue, exactly
> because testing won't actually catch the odd cases.
If we need objtool to detect them, it's certainly possible. But maybe I
missed the previous discussion -- what's the, um, alternative to
relocations, when we have calls and jumps being patched in?
> Because we don't want to be in the situation where some random poor
> user hits it because they have an old CPU that no developer has, and
> then the relocation will basically do completely random things.
>
> Imagine just how crazy that would be to debug. You'd be basically
> executing insane code, and looking at the sources - or even the
> binaries - it would _look_ completely sane.
Well, I think we already made that deal with the devil when we added
alternatives/paravirt/smp_locks/jump_labels/kprobes/ftrace/bpf, etc.
--
Josh
next prev parent reply other threads:[~2018-01-10 20:55 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-10 0:31 [PATCH] x86/retpoline: Fix NOSPEC_JMP for tip Andi Kleen
2018-01-10 0:39 ` Thomas Gleixner
2018-01-10 0:39 ` Linus Torvalds
2018-01-10 0:40 ` Thomas Gleixner
2018-01-10 0:45 ` Tom Lendacky
2018-01-10 0:50 ` Thomas Gleixner
2018-01-10 1:30 ` Andi Kleen
2018-01-10 1:36 ` Andi Kleen
2018-01-10 1:50 ` Thomas Gleixner
2018-01-10 1:57 ` Andi Kleen
2018-01-10 9:05 ` David Woodhouse
2018-01-10 0:51 ` Andi Kleen
2018-01-10 1:05 ` Thomas Gleixner
2018-01-10 7:15 ` David Woodhouse
2018-01-10 10:05 ` David Woodhouse
2018-01-10 11:28 ` [PATCH] x86/alternatives: Fix optimize_nops() checking Borislav Petkov
2018-01-10 11:36 ` David Woodhouse
2018-01-10 11:45 ` Borislav Petkov
2018-01-10 11:49 ` David Woodhouse
2018-01-10 11:57 ` Borislav Petkov
2018-01-10 11:58 ` David Woodhouse
2018-01-10 17:33 ` [tip:x86/pti] " tip-bot for Borislav Petkov
2018-01-10 18:12 ` tip-bot for Borislav Petkov
2018-01-10 18:39 ` tip-bot for Borislav Petkov
2018-01-10 19:38 ` [PATCH] " Linus Torvalds
2018-01-10 19:55 ` Thomas Gleixner
2018-01-10 20:15 ` Josh Poimboeuf
2018-01-10 20:19 ` David Woodhouse
2018-01-10 20:26 ` Linus Torvalds
2018-01-10 20:33 ` Peter Zijlstra
2018-01-10 20:36 ` David Woodhouse
2018-01-10 20:49 ` Linus Torvalds
2018-01-10 20:55 ` Borislav Petkov
2018-01-10 21:05 ` Linus Torvalds
2018-01-10 21:08 ` David Woodhouse
2018-01-10 21:11 ` Linus Torvalds
2018-01-10 21:16 ` Josh Poimboeuf
2018-01-10 21:17 ` Linus Torvalds
2018-01-10 21:27 ` Josh Poimboeuf
2018-01-10 21:10 ` Borislav Petkov
2018-01-10 20:55 ` Josh Poimboeuf [this message]
2018-01-10 20:05 ` Borislav Petkov
2018-01-10 20:20 ` Linus Torvalds
2018-01-10 20:50 ` Borislav Petkov
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