Re: [PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[Pasha Tatashin]

On 05-05 15:39, Pratyush Yadav wrote:
> From: "Pratyush Yadav (Google)" <pratyush@xxxxxxxxxx>
> 
> When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the
> implied seal is set after the check that makes sure the memfd can not
> have any writable mappings. This means one can use SEAL_EXEC to apply
> SEAL_WRITE while having writeable mappings.
> 
> This breaks the contract that SEAL_WRITE provides and can be used by an
> attacker to pass a memfd that appears to be write sealed but can still
> be modified arbitrarily.
> 
> Fix this by adding the implied seals before the call for
> mapping_deny_writable() is done.
> 
> Fixes: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Pratyush Yadav (Google) <pratyush@xxxxxxxxxx>

Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> 

> ---
>  mm/memfd.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memfd.c b/mm/memfd.c
> index fb425f4e315f..abe13b291ddc 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -283,6 +283,12 @@ int memfd_add_seals(struct file *file, unsigned int seals)
>  		goto unlock;
>  	}
>  
> +	/*
> +	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
> +	 */
> +	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
> +		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
> +
>  	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
>  		error = mapping_deny_writable(file->f_mapping);
>  		if (error)
> @@ -295,12 +301,6 @@ int memfd_add_seals(struct file *file, unsigned int seals)
>  		}
>  	}
>  
> -	/*
> -	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
> -	 */
> -	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
> -		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
> -
>  	*file_seals |= seals;
>  	error = 0;
>  
> -- 
> 2.54.0.545.g6539524ca2-goog
>