linux-kernel - Re: [PATCH v4 2/3] mm: update memfd seal write check to include F_SEAL

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Message-ID: <20231013103208.kdffpyerufr4ygnw@quack3>
Date:   Fri, 13 Oct 2023 12:32:08 +0200
From:   Jan Kara <jack@...e.cz>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.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>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andy Lutomirski <luto@...nel.org>, Jan Kara <jack@...e.cz>,
        linux-fsdevel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v4 2/3] mm: update memfd seal write check to include
 F_SEAL_WRITE

On Thu 12-10-23 18:04:29, Lorenzo Stoakes wrote:
> The seal_check_future_write() function is called by shmem_mmap() or
> hugetlbfs_file_mmap() to disallow any future writable mappings of an memfd
> sealed this way.
> 
> The F_SEAL_WRITE flag is not checked here, as that is handled via the
> mapping->i_mmap_writable mechanism and so any attempt at a mapping would
> fail before this could be run.
> 
> However we intend to change this, meaning this check can be performed for
> F_SEAL_WRITE mappings also.
> 
> The logic here is equally applicable to both flags, so update this function
> to accommodate both and rename it accordingly.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>

For some reason only this one patch landed in my inbox but I've checked all
three on lore and they look good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

to all of them. Thanks!

								Honza

> ---
>  fs/hugetlbfs/inode.c |  2 +-
>  include/linux/mm.h   | 15 ++++++++-------
>  mm/shmem.c           |  2 +-
>  3 files changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 06693bb1153d..5c333373dcc9 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -112,7 +112,7 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
>  	vm_flags_set(vma, VM_HUGETLB | VM_DONTEXPAND);
>  	vma->vm_ops = &hugetlb_vm_ops;
>  
> -	ret = seal_check_future_write(info->seals, vma);
> +	ret = seal_check_write(info->seals, vma);
>  	if (ret)
>  		return ret;
>  
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bae234d18d81..26d7dc3b342b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4078,25 +4078,26 @@ static inline void mem_dump_obj(void *object) {}
>  #endif
>  
>  /**
> - * seal_check_future_write - Check for F_SEAL_FUTURE_WRITE flag and handle it
> + * seal_check_write - Check for F_SEAL_WRITE or F_SEAL_FUTURE_WRITE flags and
> + *                    handle them.
>   * @seals: the seals to check
>   * @vma: the vma to operate on
>   *
> - * Check whether F_SEAL_FUTURE_WRITE is set; if so, do proper check/handling on
> - * the vma flags.  Return 0 if check pass, or <0 for errors.
> + * Check whether F_SEAL_WRITE or F_SEAL_FUTURE_WRITE are set; if so, do proper
> + * check/handling on the vma flags.  Return 0 if check pass, or <0 for errors.
>   */
> -static inline int seal_check_future_write(int seals, struct vm_area_struct *vma)
> +static inline int seal_check_write(int seals, struct vm_area_struct *vma)
>  {
> -	if (seals & F_SEAL_FUTURE_WRITE) {
> +	if (seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
>  		/*
>  		 * New PROT_WRITE and MAP_SHARED mmaps are not allowed when
> -		 * "future write" seal active.
> +		 * write seals are active.
>  		 */
>  		if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE))
>  			return -EPERM;
>  
>  		/*
> -		 * Since an F_SEAL_FUTURE_WRITE sealed memfd can be mapped as
> +		 * Since an F_SEAL_[FUTURE_]WRITE sealed memfd can be mapped as
>  		 * MAP_SHARED and read-only, take care to not allow mprotect to
>  		 * revert protections on such mappings. Do this only for shared
>  		 * mappings. For private mappings, don't need to mask
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 6503910b0f54..cab053831fea 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2405,7 +2405,7 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
>  	struct shmem_inode_info *info = SHMEM_I(inode);
>  	int ret;
>  
> -	ret = seal_check_future_write(info->seals, vma);
> +	ret = seal_check_write(info->seals, vma);
>  	if (ret)
>  		return ret;
>  
> -- 
> 2.42.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR