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

[Liam Merwick]

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;