linux-kernel - Re: [PATCH v4 12/12] mm: bail out of lazy_mmu_mode

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: <f7ed51e6-c3f6-402c-b328-8af5f970006d@arm.com>
Date: Mon, 10 Nov 2025 11:48:24 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Ryan Roberts <ryan.roberts@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Alexander Gordeev <agordeev@...ux.ibm.com>,
 Andreas Larsson <andreas@...sler.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>, Borislav Petkov
 <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 David Hildenbrand <david@...hat.com>, "David S. Miller"
 <davem@...emloft.net>, David Woodhouse <dwmw2@...radead.org>,
 "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
 Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
 Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Suren Baghdasaryan
 <surenb@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>,
 Yeoreum Yun <yeoreum.yun@....com>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
 xen-devel@...ts.xenproject.org, x86@...nel.org
Subject: Re: [PATCH v4 12/12] mm: bail out of lazy_mmu_mode_* in interrupt
 context

On 07/11/2025 15:42, Ryan Roberts wrote:
> On 29/10/2025 10:09, Kevin Brodsky wrote:
>> The lazy MMU mode cannot be used in interrupt context. This is
>> documented in <linux/pgtable.h>, but isn't consistently handled
>> across architectures.
>>
>> arm64 ensures that calls to lazy_mmu_mode_* have no effect in
>> interrupt context, because such calls do occur in certain
>> configurations - see commit b81c688426a9 ("arm64/mm: Disable barrier
>> batching in interrupt contexts"). Other architectures do not check
>> this situation, most likely because it hasn't occurred so far.
>>
>> Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
>> disabled while in interrupt mode (see queue_pte_barriers() and
>> xen_get_lazy_mode() respectively).
>>
>> Let's handle this in the new generic lazy_mmu layer, in the same
>> fashion as arm64: bail out of lazy_mmu_mode_* if in_interrupt(), and
>> have in_lazy_mmu_mode() return false to disable any optimisation.
>> Also remove the arm64 handling that is now redundant; x86/Xen has
>> its own internal tracking so it is left unchanged.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>> ---
>>  arch/arm64/include/asm/pgtable.h | 17 +----------------
>>  include/linux/pgtable.h          | 16 ++++++++++++++--
>>  include/linux/sched.h            |  3 +++
>>  3 files changed, 18 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 61ca88f94551..96987a49e83b 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -62,37 +62,22 @@ static inline void emit_pte_barriers(void)
>>  
>>  static inline void queue_pte_barriers(void)
>>  {
>> -	if (in_interrupt()) {
>> -		emit_pte_barriers();
>> -		return;
>> -	}
>> -
>>  	if (in_lazy_mmu_mode())
>>  		test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);
>>  	else
>>  		emit_pte_barriers();
>>  }
>>  
>> -static inline void arch_enter_lazy_mmu_mode(void)
>> -{
>> -	if (in_interrupt())
>> -		return;
>> -}
>> +static inline void arch_enter_lazy_mmu_mode(void) {}
>>  
>>  static inline void arch_flush_lazy_mmu_mode(void)
>>  {
>> -	if (in_interrupt())
>> -		return;
>> -
>>  	if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
>>  		emit_pte_barriers();
>>  }
>>  
>>  static inline void arch_leave_lazy_mmu_mode(void)
>>  {
>> -	if (in_interrupt())
>> -		return;
>> -
>>  	arch_flush_lazy_mmu_mode();
>>  }
> Ahh ok, by the time you get to the final state, I think a most of my
> comments/concerns are solved. Certainly this now looks safe for the interrupt
> case, whereas I think the intermediate state when you initially introduce
> nesting is broken. So perhaps you want to look at how to rework it to prevent that.

Agreed, as discussed on patch 7. I might split this patch - first add
the in_interrupt() checks before patch 7, and then remove the
now-redundant checks on arm64.

- Kevin