Feedback-ID: ie3994620:Fastmail
Date:   Wed, 25 Jan 2023 15:53:21 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Sean Christopherson <seanjc@google.com>
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
Subject: Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM
Message-ID: <20230125125321.yvsivupbbaqkb7a5@box.shutemov.name>
References: <20221202061347.1070246-1-chao.p.peng@linux.intel.com>
 <Y8H5Z3e4hZkFxAVS@google.com>
 <48953bf2-cee9-f818-dc50-5fb5b9b410bf@oracle.com>
 <Y9B1yiRR8DpANAEo@google.com>
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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))
 			continue;
 
+		spin_lock(&inode->i_lock);
+		if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
+			spin_unlock(&inode->i_lock);
+			continue;
+		}
+
+		rm = inode->i_mapping->private_data;
+		memfd = rm->memfd;
+
+		if (memfd->f_mapping != mapping) {
+			spin_unlock(&inode->i_lock);
+			continue;
+		}
+		spin_unlock(&inode->i_lock);
+
 		xa_for_each_range(&rm->bindings, index, notifier, start, end)
 			notifier->ops->error(notifier, start, end);
 		break;
-- 
  Kiryl Shutsemau / Kirill A. Shutemov