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

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On 25/01/2023 12:53, 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:
...

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.

Thanks, this works for me.

Regards,
Liam


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;





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