linux-kernel - Re: [RFC PATCH 3/3] mm: perform the mapping_map_writable() check after call

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Message-ID: <20230421090628.347b6qojxvfsgoqk@quack3>
Date:   Fri, 21 Apr 2023 11:06:28 +0200
From:   Jan Kara <jack@...e.cz>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFC PATCH 3/3] mm: perform the mapping_map_writable() check
 after call_mmap()

On Mon 03-04-23 23:28:32, Lorenzo Stoakes wrote:
> In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to
> clear VM_MAYWRITE, we must be able to invoke the appropriate vm_ops->mmap()
> handler to do so. We would otherwise fail the mapping_map_writable() check
> before we had the opportunity to avoid it.
> 
> This patch moves this check after the call_mmap() invocation. Only memfd
> actively denies write access causing a potential failure here (in
> memfd_add_seals()), so there should be no impact on non-memfd cases.
> 
> This patch makes the userland-visible change that MAP_SHARED, PROT_READ
> mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  mm/mmap.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c96dcce90772..a166e9f3c474 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2596,17 +2596,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	vma->vm_pgoff = pgoff;
>  
>  	if (file) {
> -		if (is_shared_maywrite(vm_flags)) {
> -			error = mapping_map_writable(file->f_mapping);
> -			if (error)
> -				goto free_vma;
> -		}
> -
>  		vma->vm_file = get_file(file);
>  		error = call_mmap(file, vma);
>  		if (error)
>  			goto unmap_and_free_vma;
>  
> +		if (vma_is_shared_maywrite(vma)) {
> +			error = mapping_map_writable(file->f_mapping);
> +			if (error)
> +				goto unmap_and_free_vma;

Shouldn't we rather jump to close_and_free_vma?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR