From: Josh Poimboeuf <jpoimboe@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
tim.c.chen@linux.intel.com, pjt@google.com, jikos@kernel.org,
gregkh@linux-foundation.org, dave.hansen@intel.com,
riel@redhat.com, luto@amacapital.net,
torvalds@linux-foundation.org, ak@linux.intel.com,
keescook@google.com, peterz@infradead.org
Subject: Re: [PATCH 2/2] x86/speculation: Simplify indirect_branch_prediction_barrier()
Date: Tue, 6 Feb 2018 17:25:14 -0600 [thread overview]
Message-ID: <20180206232514.qcy4y3dzfkjo3xdg@treble> (raw)
In-Reply-To: <1517946292.3677.22.camel@infradead.org>
On Tue, Feb 06, 2018 at 07:44:52PM +0000, David Woodhouse wrote:
> On Fri, 2018-01-26 at 21:08 +0100, Borislav Petkov wrote:
> > Make it all a function which does the WRMSR instead of having a hairy
> > inline asm.
>
> ...
>
> > + alternative_input("",
> > + "call __ibp_barrier",
> > + X86_FEATURE_IBPB,
> > + ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
> > }
>
> Dammit. I know the best time to comment is *before* I add my own sign-
> off to it and before Linus has merged it but... I think this is broken.
>
> If you're calling a C function then you have to mark *all* the call-
> clobbered registers as, well, clobbered.
>
> If you really really really want to *call* something out of line, then
> it would need to be implemented in asm.
Hm. In theory I agree this seems like a bug. On x86_64 I believe we
would need to mark the following registers as clobbered: r8-r11, ax, cx,
dx, si, di, plus "memory" and "cc".
But I'm scratching my head a bit, because we seem to have this bug all
over the kernel. (Grep for ASM_CALL_CONSTRAINT to see them.)
Many of those inline asm calls have been around a long time. So why
hasn't it ever bitten us?
--
Josh
next prev parent reply other threads:[~2018-02-06 23:25 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-26 12:11 [PATCH 0/4] x86: Some cleanups Borislav Petkov
2018-01-26 12:11 ` [PATCH 1/4] x86/alternative: Print unadorned pointers Borislav Petkov
2018-01-26 15:02 ` [tip:x86/pti] " tip-bot for Borislav Petkov
2018-01-26 12:11 ` [PATCH 2/4] x86/nospec: Fix header guards names Borislav Petkov
2018-01-26 15:03 ` [tip:x86/pti] " tip-bot for Borislav Petkov
2018-01-26 12:11 ` [PATCH 3/4] x86/retpoline: Simplify vmexit_fill_RSB() Borislav Petkov
2018-01-26 12:33 ` David Woodhouse
2018-01-26 13:24 ` Borislav Petkov
2018-01-26 16:24 ` David Woodhouse
2018-01-26 16:47 ` Borislav Petkov
2018-01-26 20:06 ` Borislav Petkov
2018-01-26 20:07 ` [PATCH v2 1/2] " Borislav Petkov
2018-01-27 4:20 ` Konrad Rzeszutek Wilk
2018-01-27 9:01 ` Borislav Petkov
2018-01-27 14:04 ` Konrad Rzeszutek Wilk
2018-01-29 17:13 ` Peter Zijlstra
2018-01-26 20:08 ` [PATCH 2/2] x86/speculation: Simplify indirect_branch_prediction_barrier() Borislav Petkov
2018-01-27 12:32 ` David Woodhouse
2018-01-27 13:21 ` Borislav Petkov
2018-02-06 19:44 ` David Woodhouse
2018-02-06 23:25 ` Josh Poimboeuf [this message]
2018-02-06 23:31 ` David Woodhouse
2018-02-06 23:49 ` Josh Poimboeuf
2018-01-26 12:11 ` [PATCH 4/4] x86/bugs: Drop one "mitigation" from dmesg Borislav Petkov
2018-01-26 13:35 ` Greg KH
2018-01-26 15:03 ` [tip:x86/pti] " tip-bot for Borislav Petkov
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