linux-kernel - Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <Z14Ce7cziCn5Fluq@casper.infradead.org>
Date: Sat, 14 Dec 2024 22:11:07 +0000
From: Matthew Wilcox <willy@...radead.org>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Linus Walleij <linus.walleij@...aro.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Arnd Bergmann <arnd@...nel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org,
	linux-rt-devel@...ts.linux.dev, Ard Biesheuvel <ardb@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Jason Baron <jbaron@...mai.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

On Fri, Dec 13, 2024 at 09:11:57AM +0000, Russell King (Oracle) wrote:
> On Fri, Dec 13, 2024 at 01:27:00AM +0100, Linus Walleij wrote:
> > On Wed, Dec 11, 2024 at 4:22 PM Sebastian Andrzej Siewior
> > <bigeasy@...utronix.de> wrote:
> > > On 2024-12-11 14:29:29 [+0100], Linus Walleij wrote:
> > > > So fast GUP is supposed to be lockless, and should just not
> > > > have this problem. So it can't be addressing gup_fast_pgd_range()
> > > > right?
> > > …
> > > > I'm more asking if HIGHPTE even acquires a spinlock anymore
> > > > as it is supposed to be "fast"/lockless. If it does, it is clearly violating
> > > > the "fast" promise of the fast GUP API and should not exist.
> > >
> > > This is lockless on x86. The problem is ARM's
> > > arch_kmap_local_high_get(). This is where the lock is from.
> > 
> > Aha that calls down to kmap_high_get() that that issues
> > lock_kmap_any(flags).
> > 
> > But is it really sound that the "fast" API does this? It feels
> > like a violation of the whole design of the fast stuff.
> 
> If there's no way to do it lockless, then it has to take the lock.

Well, no, it's allowed to fail.  It could trylock and fail if it can't
get the lock.