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Wed, 10 Jul 2024 14:14:06 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGIL5NZakoeWxrceiZO5dPOGtHYUbvR+7u663g+7/eUG7NXVlKeuTVAdylslhMzFr1TdO1NJw== X-Received: by 2002:a05:6a00:1d83:b0:70b:17a9:e98a with SMTP id d2e1a72fcca58-70b43626d6amr7509431b3a.33.1720646046328; Wed, 10 Jul 2024 14:14:06 -0700 (PDT) Received: from [10.35.209.243] ([208.115.86.71]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-70b4397c02fsm4390228b3a.143.2024.07.10.14.14.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Jul 2024 14:14:06 -0700 (PDT) Message-ID: Date: Wed, 10 Jul 2024 23:14:04 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 7/8] mm: secretmem: use AS_INACCESSIBLE to prohibit GUP To: Patrick Roy , Mike Rapoport Cc: seanjc@google.com, pbonzini@redhat.com, akpm@linux-foundation.org, dwmw@amazon.co.uk, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, willy@infradead.org, graf@amazon.com, derekmn@amazon.com, kalyazin@amazon.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, dmatlack@google.com, tabba@google.com, chao.p.peng@linux.intel.com, xmarcalx@amazon.co.uk, James Gowans References: <20240709132041.3625501-1-roypat@amazon.co.uk> <20240709132041.3625501-8-roypat@amazon.co.uk> <0dc45181-de7e-4d97-9178-573c6f683f55@redhat.com> <258b3b76-cf87-4dfc-bcfa-b2af94aba811@amazon.co.uk> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.07.24 11:50, Patrick Roy wrote: > > > On 7/10/24 08:32, Mike Rapoport wrote: >> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. >> >> >> >> On Tue, Jul 09, 2024 at 11:09:29PM +0200, David Hildenbrand wrote: >>> On 09.07.24 15:20, Patrick Roy wrote: >>>> Inside of vma_is_secretmem and secretmem_mapping, instead of checking >>>> whether a vm_area_struct/address_space has the secretmem ops structure >>>> attached to it, check whether the address_space has the AS_INACCESSIBLE >>>> bit set. Then set the AS_INACCESSIBLE flag for secretmem's >>>> address_space. >>>> >>>> This means that get_user_pages and friends are disables for all >>>> adress_spaces that set AS_INACCESIBLE. The AS_INACCESSIBLE flag was >>>> introduced in commit c72ceafbd12c ("mm: Introduce AS_INACCESSIBLE for >>>> encrypted/confidential memory") specifically for guest_memfd to indicate >>>> that no reads and writes should ever be done to guest_memfd >>>> address_spaces. Disallowing gup seems like a reasonable semantic >>>> extension, and means that potential future mmaps of guest_memfd cannot >>>> be GUP'd. >>>> >>>> Signed-off-by: Patrick Roy >>>> --- >>>> include/linux/secretmem.h | 13 +++++++++++-- >>>> mm/secretmem.c | 6 +----- >>>> 2 files changed, 12 insertions(+), 7 deletions(-) >>>> >>>> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h >>>> index e918f96881f5..886c8f7eb63e 100644 >>>> --- a/include/linux/secretmem.h >>>> +++ b/include/linux/secretmem.h >>>> @@ -8,10 +8,19 @@ extern const struct address_space_operations secretmem_aops; >>>> static inline bool secretmem_mapping(struct address_space *mapping) >>>> { >>>> - return mapping->a_ops == &secretmem_aops; >>>> + return mapping->flags & AS_INACCESSIBLE; >>>> +} >>>> + >>>> +static inline bool vma_is_secretmem(struct vm_area_struct *vma) >>>> +{ >>>> + struct file *file = vma->vm_file; >>>> + >>>> + if (!file) >>>> + return false; >>>> + >>>> + return secretmem_mapping(file->f_inode->i_mapping); >>>> } >>> >>> That sounds wrong. You should leave *secretmem alone and instead have >>> something like inaccessible_mapping that is used where appropriate. >>> >>> vma_is_secretmem() should not suddenly succeed on something that is not >>> mm/secretmem.c >> >> I'm with David here. >> > > Right, that makes sense. My thinking here was that if memfd_secret and > potential mappings of guest_memfd have the same behavior wrt GUP, then > it makes sense to just have them rely on the same checks. But I guess I > didn't follow that thought to its logical conclusion of renaming the > "secretmem" checks into "inaccessible" checks and moving them out of > secretmem.h. > > Or do you mean to just leave secretmem untouched and add separate > "inaccessible" checks? But then we'd have two different ways of > disabling GUP for specific VMAs that both rely on checks in exactly the > same places :/ You can just replace the vma_is_secretmem in relevant places by checks if inaccessible address spaces. No need for the additional vma_is_secretmem check then. BUT, as raised in my other reply, I wonder if adding support for secretmem in KVM (I assume) would be simpler+cleaner. > >>> -- >>> Cheers, >>> >>> David / dhildenb >>> >> >> -- >> Sincerely yours, >> Mike. > -- Cheers, David / dhildenb