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Date:   Mon, 30 Oct 2023 15:12:10 -0700
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Subject: Re: [PATCH v13 08/35] KVM: Introduce KVM_SET_USER_MEMORY_REGION2
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Xiaoyao Li <xiaoyao.li@intel.com>,
        Xu Yilun <yilun.xu@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Anish Moorthy <amoorthy@google.com>,
        David Matlack <dmatlack@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
        "=?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?=" <mic@digikod.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Ackerley Tng <ackerleytng@google.com>,
        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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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