linux-kernel - Re: [RFC v2 3/3] atomic: Add alignment check to instrumented atomic operations

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

Message-ID: <20250915100604.GZ3245006@noisy.programming.kicks-ass.net>
Date: Mon, 15 Sep 2025 12:06:04 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Finn Thain <fthain@...ux-m68k.org>
Cc: Will Deacon <will@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Boqun Feng <boqun.feng@...il.com>, Jonathan Corbet <corbet@....net>,
	Mark Rutland <mark.rutland@....com>, Arnd Bergmann <arnd@...db.de>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	linux-m68k@...r.kernel.org
Subject: Re: [RFC v2 3/3] atomic: Add alignment check to instrumented atomic
 operations

On Mon, Sep 15, 2025 at 07:38:52PM +1000, Finn Thain wrote:
> 
> On Mon, 15 Sep 2025, Peter Zijlstra wrote:
> 
> > On Sun, Sep 14, 2025 at 10:45:29AM +1000, Finn Thain wrote:
> > > From: Peter Zijlstra <peterz@...radead.org>
> > > 
> > > Add a Kconfig option for debug builds which logs a warning when an
> > > instrumented atomic operation takes place at some location that isn't
> > > a long word boundary. Some platforms don't trap for this.
> > > 
> > > Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
> > > ---
> > > This patch differs slightly from Peter's code which checked for natural
> > > alignment.
> > > ---
> > >  include/linux/instrumented.h |  4 ++++
> > >  lib/Kconfig.debug            | 10 ++++++++++
> > >  2 files changed, 14 insertions(+)
> > > 
> > > diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
> > > index 711a1f0d1a73..55f5685971a1 100644
> > > --- a/include/linux/instrumented.h
> > > +++ b/include/linux/instrumented.h
> > > @@ -7,6 +7,7 @@
> > >  #ifndef _LINUX_INSTRUMENTED_H
> > >  #define _LINUX_INSTRUMENTED_H
> > >  
> > > +#include <linux/bug.h>
> > >  #include <linux/compiler.h>
> > >  #include <linux/kasan-checks.h>
> > >  #include <linux/kcsan-checks.h>
> > > @@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
> > >  {
> > >  	kasan_check_read(v, size);
> > >  	kcsan_check_atomic_read(v, size);
> > > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > >  }
> > >  
> > >  /**
> > > @@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
> > >  {
> > >  	kasan_check_write(v, size);
> > >  	kcsan_check_atomic_write(v, size);
> > > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > >  }
> > >  
> > >  /**
> > > @@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
> > >  {
> > >  	kasan_check_write(v, size);
> > >  	kcsan_check_atomic_read_write(v, size);
> > > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > >  }
> > 
> > Right, so why aren't we trusting the size argument? And instead
> > mandating a possibly larger alignment?
> > 
> 
> It wasn't supposed to mandate a larger alignment in practice. I considered 
> doing something like (unsigned long)v & (size - 1) & (sizeof(long) - 1) 
> but decided that the extra overhead probably wouldn't be worthwhile, if in 
> practice, no-one is doing atomic ops on shorts or chars. I will revisit 
> this.

atomic_t is aligned at 4 bytes, you're now mandating it is aligned at 8
bytes (on LP64), this cannot be right.

kernel/locking/qspinlock.c:xchg_tail() does xchg_relaxed(&lock->tail,
...) which is u16. Again, you cannot mandate 8 bytes here.

> When you do atomic operations on atomic_t or atomic64_t, (sizeof(long)
> - 1) probably doesn't make much sense. But atomic operations get used on 
> scalar types (aside from atomic_t and atomic64_t) that don't have natural 
> alignment. Please refer to the other thread about this: 
> https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/

Perhaps set ARCH_SLAB_MINALIGN ?