Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM

Kirill A. Shutemov
2023-01-25T13:01:27Z

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