linux-kernel - Re: [PATCH v13 08/35] KVM: Introduce KVM_SET_USER_MEMORY

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Date:   Mon, 30 Oct 2023 15:12:10 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, 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-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        "Mickaël Salaün" <mic@...ikod.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.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: [PATCH v13 08/35] KVM: Introduce KVM_SET_USER_MEMORY_REGION2

On Mon, Oct 30, 2023, Sean Christopherson wrote:
> On Mon, Oct 30, 2023, Paolo Bonzini wrote:
> > On 10/27/23 20:21, Sean Christopherson wrote:
> > > 
> > > +		if (ioctl == KVM_SET_USER_MEMORY_REGION)
> > > +			size = sizeof(struct kvm_userspace_memory_region);
> > 
> > This also needs a memset(&mem, 0, sizeof(mem)), otherwise the out-of-bounds
> > access of the commit message becomes a kernel stack read.
> 
> Ouch.  There's some irony.  Might be worth doing memset(&mem, -1, sizeof(mem))
> though as '0' is a valid file descriptor and a valid file offset.
> 
> > Probably worth adding a check on valid flags here.
> 
> Definitely needed.  There's a very real bug here.  But rather than duplicate flags
> checking or plumb @ioctl all the way to __kvm_set_memory_region(), now that we
> have the fancy guard(mutex) and there are no internal calls to kvm_set_memory_region(),
> what if we:
> 
>   1. Acquire/release slots_lock in __kvm_set_memory_region()

Gah, this won't work with x86's usage, which acquire slots_lock quite far away
from __kvm_set_memory_region() in several places.  The rest of the idea still
works, kvm_vm_ioctl_set_memory_region() will just need to take slots_lock manually.

>   2. Call kvm_set_memory_region() from x86 code for the internal memslots
>   3. Disallow *any* flags for internal memslots
>   4. Open code check_memory_region_flags in kvm_vm_ioctl_set_memory_region()
>   5. Pass @ioctl to kvm_vm_ioctl_set_memory_region() and allow KVM_MEM_PRIVATE
>      only for KVM_SET_USER_MEMORY_REGION2