Re: [PATCH v2] netfs: Fix netfs_page_mkwrite() to check folio->mapping is valid

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2024-06-25 at 13:29 +0100, David Howells wrote:
>     
> Fix netfs_page_mkwrite() to check that folio->mapping is valid once
> it has
> taken the folio lock (as filemap_page_mkwrite() does).  Without this,
> generic/247 occasionally oopses with something like the following:
> 
>     BUG: kernel NULL pointer dereference, address: 0000000000000000
>     #PF: supervisor read access in kernel mode
>     #PF: error_code(0x0000) - not-present page
> 
>     RIP: 0010:trace_event_raw_event_netfs_folio+0x61/0xc0
>     ...
>     Call Trace:
>      <TASK>
>      ? __die_body+0x1a/0x60
>      ? page_fault_oops+0x6e/0xa0
>      ? exc_page_fault+0xc2/0xe0
>      ? asm_exc_page_fault+0x22/0x30
>      ? trace_event_raw_event_netfs_folio+0x61/0xc0
>      trace_netfs_folio+0x39/0x40
>      netfs_page_mkwrite+0x14c/0x1d0
>      do_page_mkwrite+0x50/0x90
>      do_pte_missing+0x184/0x200
>      __handle_mm_fault+0x42d/0x500
>      handle_mm_fault+0x121/0x1f0
>      do_user_addr_fault+0x23e/0x3c0
>      exc_page_fault+0xc2/0xe0
>      asm_exc_page_fault+0x22/0x30
> 
> This is due to the invalidate_inode_pages2_range() issued at the end
> of the
> DIO write interfering with the mmap'd writes.
> 
> Fixes: 102a7e2c598c ("netfs: Allow buffered shared-writeable mmap
> through netfs_page_mkwrite()")
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: Jeff Layton <jlayton@kernel.org>
> cc: netfs@lists.linux.dev
> cc: v9fs@lists.linux.dev
> cc: linux-afs@lists.infradead.org
> cc: linux-cifs@vger.kernel.org
> cc: linux-mm@kvack.org
> cc: linux-fsdevel@vger.kernel.org
> ---
> Changes
> =======
> ver #2)
>  - Actually unlock the folio rather than returning VM_FAULT_LOCKED
> with
>    VM_FAULT_NOPAGE.
> 
>  fs/netfs/buffered_write.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c
> index 07bc1fd43530..270f8ebf8328 100644
> --- a/fs/netfs/buffered_write.c
> +++ b/fs/netfs/buffered_write.c
> @@ -523,6 +523,7 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault
> *vmf, struct netfs_group *netfs_gr
>  	struct netfs_group *group;
>  	struct folio *folio = page_folio(vmf->page);
>  	struct file *file = vmf->vma->vm_file;
> +	struct address_space *mapping = file->f_mapping;
>  	struct inode *inode = file_inode(file);
>  	struct netfs_inode *ictx = netfs_inode(inode);
>  	vm_fault_t ret = VM_FAULT_RETRY;
> @@ -534,6 +535,11 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault
> *vmf, struct netfs_group *netfs_gr
>  
>  	if (folio_lock_killable(folio) < 0)
>  		goto out;
> +	if (folio->mapping != mapping) {
> +		folio_unlock(folio);
> +		ret = VM_FAULT_NOPAGE;
> +		goto out;
> +	}
>  
>  	if (folio_wait_writeback_killable(folio)) {
>  		ret = VM_FAULT_LOCKED;
> @@ -549,7 +555,7 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault
> *vmf, struct netfs_group *netfs_gr
>  	group = netfs_folio_group(folio);
>  	if (group != netfs_group && group !=
> NETFS_FOLIO_COPY_TO_CACHE) {
>  		folio_unlock(folio);
> -		err = filemap_fdatawait_range(inode->i_mapping,
> +		err = filemap_fdatawait_range(mapping,
>  					      folio_pos(folio),
>  					      folio_pos(folio) +
> folio_size(folio));
>  		switch (err) {
> 
> 

Reviewed-by: Jeff Layton <jlayton@kernel.org>