From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
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>,
Rudolf Marek <r.marek@assembler.cz>
Subject: Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f
Date: Wed, 7 Feb 2018 21:24:35 +0100 [thread overview]
Message-ID: <20180207202435.GB19536@pd.tnic> (raw)
In-Reply-To: <CA+55aFzSZzdVZXatze=xjGSwpOPO7kvbWnz7JiZ3sTx_2Pohjw@mail.gmail.com>
On Wed, Feb 07, 2018 at 11:43:37AM -0800, Linus Torvalds wrote:
> 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?
Yes, AMD doesn't do UD0 with a ModRM:
"UD0 0F FF Raise an invalid opcode exception"
And I don't know about the other x86 vendors; what fun they've had with
the UD*.
I guess we should make sure the following bytes are a valid insn.
Btw Rudolf had experimented with this, CCed, and he showed that on Intel
you get a SIGSEGV for a page crosser with UD1 and SIGILL on AMD.
I guess you can get the same with UD0 apparently.
> 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).
Yeah, I guess we can control the bytes there.
> - we special-case the decoder so that we don't get this warning
That's easy - the decoder should simply say to upgrade objdump.
> and then we just ignore the issue entirely.
>
> Hmm?
Sounds like a plan but with those things the devil's in the detail.
Nothing sounds too nasty now, though.
--
Regards/Gruss,
Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
next prev parent reply other threads:[~2018-02-07 20:24 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
2018-02-07 20:24 ` Borislav Petkov [this message]
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
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