Re: [PATCH] memfd: Check for non-NULL file_seals in memfd_create() syscall

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Wed,  7 Jun 2023 15:24:27 +0200 Roberto Sassu <roberto.sassu@xxxxxxxxxxxxxxx> wrote:

> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> 
> Ensure that file_seals is non-NULL before using it in the memfd_create()
> syscall. One situation in which memfd_file_seals_ptr() could return a NULL
> pointer is when CONFIG_SHMEM=n.

Thanks.  Has thie crash actually been demonstrated?

> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -371,12 +371,15 @@ SYSCALL_DEFINE2(memfd_create,
>  
>  		inode->i_mode &= ~0111;
>  		file_seals = memfd_file_seals_ptr(file);
> -		*file_seals &= ~F_SEAL_SEAL;
> -		*file_seals |= F_SEAL_EXEC;
> +		if (file_seals) {
> +			*file_seals &= ~F_SEAL_SEAL;
> +			*file_seals |= F_SEAL_EXEC;
> +		}
>  	} else if (flags & MFD_ALLOW_SEALING) {
>  		/* MFD_EXEC and MFD_ALLOW_SEALING are set */
>  		file_seals = memfd_file_seals_ptr(file);
> -		*file_seals &= ~F_SEAL_SEAL;
> +		if (file_seals)
> +			*file_seals &= ~F_SEAL_SEAL;
>  	}
>  
>  	fd_install(fd, file);