linux-kernel - Re: [RFC PATCH v11 06/29] KVM: Introduce KVM_SET_USER_MEMORY

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Message-ID: <eb356cf1-c661-930b-2175-427a59267d1f@redhat.com>
Date:   Mon, 31 Jul 2023 17:58:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Quentin Perret <qperret@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v11 06/29] KVM: Introduce KVM_SET_USER_MEMORY_REGION2

On 7/29/23 02:03, Sean Christopherson wrote:
> KVM would need to do multiple uaccess reads, but that's not a big
> deal.  Am I missing something, or did past us just get too clever and
> miss the obvious solution?

You would have to introduce struct kvm_userspace_memory_region2 anyway, 
though not a new ioctl, for two reasons:

1) the current size of the struct is part of the userspace API via the 
KVM_SET_USER_MEMORY_REGION #define, so introducing a new struct is the 
easiest way to preserve this

2) the struct can (at least theoretically) enter the ABI of a shared 
library, and such mismatches are really hard to detect and resolve.  So 
it's better to add the padding to a new struct, and keep struct 
kvm_userspace_memory_region backwards-compatible.


As to whether we should introduce a new ioctl: doing so makes 
KVM_SET_USER_MEMORY_REGION's detection of bad flags a bit more robust; 
it's not like we cannot introduce new flags at all, of course, but 
having out-of-bounds reads as a side effect of new flags is a bit nasty. 
  Protecting programs from their own bugs gets into diminishing returns 
very quickly, but introducing a new ioctl can make exploits a bit harder 
when struct kvm_userspace_memory_region is on the stack and adjacent to 
an attacker-controlled location.

Paolo