linux-kernel - RE: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get

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Message-ID: <76bb85ceeb854e3ab68d87f846515306@AcuMS.aculab.com>
Date: Fri, 8 Nov 2024 17:12:53 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Josh Poimboeuf' <jpoimboe@...nel.org>, "x86@...nel.org" <x86@...nel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Thomas
 Gleixner" <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Peter
 Zijlstra <peterz@...radead.org>, Pawan Gupta
	<pawan.kumar.gupta@...ux.intel.com>, Waiman Long <longman@...hat.com>, Dave
 Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, Linus
 Torvalds <torvalds@...ux-foundation.org>, Michael Ellerman
	<mpe@...erman.id.au>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>, "Andrew Cooper" <andrew.cooper3@...rix.com>,
	Mark Rutland <mark.rutland@....com>, "Kirill A . Shutemov"
	<kirill@...temov.name>
Subject: RE: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit
 __get_user()

From: Josh Poimboeuf
> Sent: 29 October 2024 03:28
> 
> On Mon, Oct 28, 2024 at 06:56:15PM -0700, Josh Poimboeuf wrote:
> > The barrier_nospec() in 64-bit __get_user() is slow.  Instead use
> > pointer masking to force the user pointer to all 1's if a previous
> > access_ok() mispredicted true for an invalid address.
> 
> Linus pointed out that __get_user() may be used by some code to access
> both kernel and user space and in fact I found one such usage in
> vc_read_mem()....
> 
> So I self-NAK this patch for now.
> 
> Still, it would be great if patch 1 could get merged as that gives a
> significant performance boost.

I'm a bit late to the party and still a week behind :-(

But I've wondered if access_ok() ought to be implemented using an
'asm goto with output' - much like get_user().

Then the use would be:
	masked_address = access_ok(maybe_bad_address, size, jump_label);
with later user accesses using the masked_address.

Once you've done that __get_user() doesn't need to contain address masking.

Given that clac/stac iare so slow should there are be something that
combines stac with access_ok() bracketed with a 'user_access_end'
or an actual fault.

I've sure there is code (maybe reading iovec[] or in sys_poll())
that wants to do multiple get/put_user in a short loop rather that
calling copy_to/from_user().

	David

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