linux-kernel - Re: [PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct map

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: <DDZV32U60137.1HE9JGMU6P1KD@google.com>
Date: Tue, 04 Nov 2025 11:08:23 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Mike Rapoport <rppt@...nel.org>, Brendan Jackman <jackmanb@...gle.com>
Cc: "Roy, Patrick" <roypat@...zon.co.uk>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"corbet@....net" <corbet@....net>, "maz@...nel.org" <maz@...nel.org>, 
	"oliver.upton@...ux.dev" <oliver.upton@...ux.dev>, "joey.gouly@....com" <joey.gouly@....com>, 
	"suzuki.poulose@....com" <suzuki.poulose@....com>, "yuzenghui@...wei.com" <yuzenghui@...wei.com>, 
	"catalin.marinas@....com" <catalin.marinas@....com>, "will@...nel.org" <will@...nel.org>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, 
	"hpa@...or.com" <hpa@...or.com>, "luto@...nel.org" <luto@...nel.org>, 
	"peterz@...radead.org" <peterz@...radead.org>, "willy@...radead.org" <willy@...radead.org>, 
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "david@...hat.com" <david@...hat.com>, 
	"lorenzo.stoakes@...cle.com" <lorenzo.stoakes@...cle.com>, 
	"Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>, "vbabka@...e.cz" <vbabka@...e.cz>, 
	"surenb@...gle.com" <surenb@...gle.com>, "mhocko@...e.com" <mhocko@...e.com>, "song@...nel.org" <song@...nel.org>, 
	"jolsa@...nel.org" <jolsa@...nel.org>, "ast@...nel.org" <ast@...nel.org>, 
	"daniel@...earbox.net" <daniel@...earbox.net>, "andrii@...nel.org" <andrii@...nel.org>, 
	"martin.lau@...ux.dev" <martin.lau@...ux.dev>, "eddyz87@...il.com" <eddyz87@...il.com>, 
	"yonghong.song@...ux.dev" <yonghong.song@...ux.dev>, 
	"john.fastabend@...il.com" <john.fastabend@...il.com>, "kpsingh@...nel.org" <kpsingh@...nel.org>, 
	"sdf@...ichev.me" <sdf@...ichev.me>, "haoluo@...gle.com" <haoluo@...gle.com>, "jgg@...pe.ca" <jgg@...pe.ca>, 
	"jhubbard@...dia.com" <jhubbard@...dia.com>, "peterx@...hat.com" <peterx@...hat.com>, 
	"jannh@...gle.com" <jannh@...gle.com>, "pfalcato@...e.de" <pfalcato@...e.de>, 
	"shuah@...nel.org" <shuah@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	"kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>, 
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>, 
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>, "Cali, Marco" <xmarcalx@...zon.co.uk>, 
	"Kalyazin, Nikita" <kalyazin@...zon.co.uk>, "Thomson, Jack" <jackabt@...zon.co.uk>, 
	"derekmn@...zon.co.uk" <derekmn@...zon.co.uk>, "tabba@...gle.com" <tabba@...gle.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>
Subject: Re: [PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct map

On Mon Nov 3, 2025 at 10:50 AM UTC, Mike Rapoport wrote:
> On Mon, Nov 03, 2025 at 10:35:38AM +0000, Brendan Jackman wrote:
>> On Sat Nov 1, 2025 at 9:39 AM UTC, Mike Rapoport wrote:
>> > On Fri, Oct 31, 2025 at 05:30:12PM +0000, Brendan Jackman wrote:
>> >> On Wed Sep 24, 2025 at 3:22 PM UTC, Patrick Roy wrote:
>> >> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> >> > index 1d0585616aa3..73a15cade54a 100644
>> >> > --- a/include/linux/kvm_host.h
>> >> > +++ b/include/linux/kvm_host.h
>> >> > @@ -731,6 +731,12 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
>> >> >  bool kvm_arch_supports_gmem_mmap(struct kvm *kvm);
>> >> >  #endif
>> >> >  
>> >> > +#ifdef CONFIG_KVM_GUEST_MEMFD
>> >> > +#ifndef kvm_arch_gmem_supports_no_direct_map
>> >> > +#define kvm_arch_gmem_supports_no_direct_map can_set_direct_map
>> >> > +#endif
>> >> > +#endif /* CONFIG_KVM_GUEST_MEMFD */
>> >> 
>> >> The test robot seems happy so I think I'm probably mistaken here, but
>> >> AFAICS can_set_direct_map only exists when ARCH_HAS_SET_DIRECT_MAP,
>> >> which powerpc doesn't set.
>> >
>> > We have stubs returning 0 for architectures that don't have
>> > ARCH_HAS_SET_DIRECT_MAP.
>> 
>> I can't see any such stub for can_set_direct_map() specifically?
>
> include/linux/set_memory.h:
>
> #ifndef CONFIG_ARCH_HAS_SET_DIRECT_MAP
> static inline int set_direct_map_invalid_noflush(struct page *page)
> {
> 	return 0;
> }
> static inline int set_direct_map_default_noflush(struct page *page)
> {
> 	return 0;
> }
>
> static inline int set_direct_map_valid_noflush(struct page *page,
> 					       unsigned nr, bool valid)
> {
> 	return 0;
> }
>
> static inline bool kernel_page_present(struct page *page)
> {
> 	return true;
> }
> #else /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
> /*
>  * Some architectures, e.g. ARM64 can disable direct map modifications at
>  * boot time. Let them overrive this query.
>  */
> #ifndef can_set_direct_map
> static inline bool can_set_direct_map(void)
> {
> 	return true;
> }
> #define can_set_direct_map can_set_direct_map

But this is for CONFIG_ARCH_HAS_DIRECT_MAP? I am reading this as a stub
to fill in for archs that have set_direct_map_*, but don't have runtime
disablement like arm64.

Whereas my concern is archs that don't have set_direct_map_* at all,
i.e. where we need to unconditionally fail
GUEST_MEMFG_FLAG_NO_DIRECT_MAP.

(Or would we prefer to just not define it at all on those archs? Not
sure what the norms are there, I guess that's a question for KVM/arch
maintainers).