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

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Date: Thu, 4 Apr 2024 11:25:22 -0400
From: Chuck Lever <chuck.lever@...cle.com>
To: Jan Kara <jack@...e.cz>
Cc: Kees Cook <keescook@...omium.org>, Christian Brauner <brauner@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        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 Thu, Apr 04, 2024 at 11:19:00AM +0200, Jan Kara wrote:
> 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

I agree, an in-code comment is needed.

Acked-by: Chuck Lever <chuck.lever@...cle.com>


> > ---
> > 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

-- 
Chuck Lever