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

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Message-ID: <CAHk-=wh=_RbcUfnjxL-X82wiCphU3_=d1qQ15JXy+49jmB6BVg@mail.gmail.com>
Date: Sat, 16 Nov 2024 15:08:58 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <David.Laight@...lab.com>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, "x86@...nel.org" <x86@...nel.org>, 
	"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>, 
	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()

On Sat, 16 Nov 2024 at 13:38, David Laight <David.Laight@...lab.com> wrote:
>
> If __get_user() is the same as get_user() [..]

No, the problem is that it's the same from a performance angle (and
now it's actually slower), but some hacky code paths depend on
__get_user() not checking the address.

They then use that to read from either user space _or_ kernel space.

Wrong? Yes. Architecture-specific? Yes. But it sadly happens.

             Linus