linux-hardening - Re: [PATCH] fs: Set file_handle::handle_bytes before referencing file_handle::f

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Date: Thu, 4 Apr 2024 11:19:00 +0200
From: Jan Kara <jack@...e.cz>
To: Kees Cook <keescook@...omium.org>
Cc: Christian Brauner <brauner@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Chuck Lever <chuck.lever@...cle.com>,
	Jeff Layton <jlayton@...nel.org>,
	Amir Goldstein <amir73il@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-hardening@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: Set file_handle::handle_bytes before referencing
 file_handle::f_handle

On Wed 03-04-24 14:54:03, Kees Cook wrote:
> With adding __counted_by(handle_bytes) to struct file_handle, we need
> to explicitly set it in the one place it wasn't yet happening prior to
> accessing the flex array "f_handle".
> 
> Fixes: 1b43c4629756 ("fs: Annotate struct file_handle with __counted_by() and use struct_size()")
> Signed-off-by: Kees Cook <keescook@...omium.org>

OK, so this isn't really a functional bug AFAIU but the compiler will
wrongly complain we are accessing handle->f_handle beyond claimed array
size (because handle->handle_bytes == 0 at that point). Am I right? If
that's the case, please add a short comment explaining this (because it
looks odd we set handle->handle_bytes and then reset it a few lines later).
With the comment feel free to add:

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

								Honza

> ---
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Chuck Lever <chuck.lever@...cle.com>
> Cc: Jeff Layton <jlayton@...nel.org>
> Cc: Amir Goldstein <amir73il@...il.com>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-nfs@...r.kernel.org
> Cc: linux-hardening@...r.kernel.org
> ---
>  fs/fhandle.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index 53ed54711cd2..08ec2340dd22 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -40,6 +40,7 @@ static long do_sys_name_to_handle(const struct path *path,
>  			 GFP_KERNEL);
>  	if (!handle)
>  		return -ENOMEM;
> +	handle->handle_bytes = f_handle.handle_bytes;
>  
>  	/* convert handle size to multiple of sizeof(u32) */
>  	handle_dwords = f_handle.handle_bytes >> 2;
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR