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Date:   Wed, 12 Apr 2023 18:07:28 -0700
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Subject: Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM
From:   Sean Christopherson <seanjc@google.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Liam Merwick <liam.merwick@oracle.com>,
        Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-api@vger.kernel.org, linux-doc@vger.kernel.org,
        qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Arnd Bergmann <arnd@arndb.de>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Miaohe Lin <linmiaohe@huawei.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>, Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
        Steven Price <steven.price@arm.com>,
        "Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com,
        ak@linux.intel.com, david@redhat.com, aarcange@redhat.com,
        ddutile@redhat.com, dhildenb@redhat.com,
        Quentin Perret <qperret@google.com>, tabba@google.com,
        Michael Roth <michael.roth@amd.com>, mhocko@suse.com,
        wei.w.wang@intel.com
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On Wed, Jan 25, 2023, Kirill A. Shutemov wrote:
> On Wed, Jan 25, 2023 at 12:20:26AM +0000, Sean Christopherson wrote:
> > On Tue, Jan 24, 2023, Liam Merwick wrote:
> > > On 14/01/2023 00:37, Sean Christopherson wrote:
> > > > On Fri, Dec 02, 2022, Chao Peng wrote:
> > > > > This patch series implements KVM guest private memory for confidential
> > > > > computing scenarios like Intel TDX[1]. If a TDX host accesses
> > > > > TDX-protected guest memory, machine check can happen which can further
> > > > > crash the running host system, this is terrible for multi-tenant
> > > > > configurations. The host accesses include those from KVM userspace like
> > > > > QEMU. This series addresses KVM userspace induced crash by introducing
> > > > > new mm and KVM interfaces so KVM userspace can still manage guest memory
> > > > > via a fd-based approach, but it can never access the guest memory
> > > > > content.
> > > > > 
> > > > > The patch series touches both core mm and KVM code. I appreciate
> > > > > Andrew/Hugh and Paolo/Sean can review and pick these patches. Any other
> > > > > reviews are always welcome.
> > > > >    - 01: mm change, target for mm tree
> > > > >    - 02-09: KVM change, target for KVM tree
> > > > 
> > > > A version with all of my feedback, plus reworked versions of Vishal's selftest,
> > > > is available here:
> > > > 
> > > >    git@github.com:sean-jc/linux.git x86/upm_base_support
> > > > 
> > > > It compiles and passes the selftest, but it's otherwise barely tested.  There are
> > > > a few todos (2 I think?) and many of the commits need changelogs, i.e. it's still
> > > > a WIP.
> > > > 
> > > 
> > > When running LTP (https://github.com/linux-test-project/ltp) on the v10
> > > bits (and also with Sean's branch above) I encounter the following NULL
> > > pointer dereference with testcases/kernel/syscalls/madvise/madvise01
> > > (100% reproducible).
> > > 
> > > It appears that in restrictedmem_error_page()
> > > inode->i_mapping->private_data is NULL in the
> > > list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) but I
> > > don't know why.
> > 
> > Kirill, can you take a look?  Or pass the buck to someone who can? :-)
> 
> The patch below should help.
> 
> diff --git a/mm/restrictedmem.c b/mm/restrictedmem.c
> index 15c52301eeb9..39ada985c7c0 100644
> --- a/mm/restrictedmem.c
> +++ b/mm/restrictedmem.c
> @@ -307,14 +307,29 @@ void restrictedmem_error_page(struct page *page, struct address_space *mapping)
>  
>  	spin_lock(&sb->s_inode_list_lock);
>  	list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) {
> -		struct restrictedmem *rm = inode->i_mapping->private_data;
>  		struct restrictedmem_notifier *notifier;
> -		struct file *memfd = rm->memfd;
> +		struct restrictedmem *rm;
>  		unsigned long index;
> +		struct file *memfd;
>  
> -		if (memfd->f_mapping != mapping)
> +		if (atomic_read(&inode->i_count))

Kirill, should this be

		if (!atomic_read(&inode->i_count))
			continue;

i.e. skip unreferenced inodes, not skip referenced inodes?