linux-hardening - Re: [PATCH 1/6] netfs: Eliminate Clang randstruct warning

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Date:   Tue, 03 May 2022 17:31:57 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Kees Cook <keescook@...omium.org>, Bill Wendling <morbo@...gle.com>
Cc:     David Howells <dhowells@...hat.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-hardening@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 1/6] netfs: Eliminate Clang randstruct warning

On Tue, 2022-05-03 at 13:54 -0700, Kees Cook wrote:
> Clang's structure layout randomization feature gets upset when it sees
> struct inode (which is randomized) cast to struct netfs_i_context. This
> is due to seeing the inode pointer as being treated as an array of inodes,
> rather than "something else, following struct inode".
> 
> Since netfs can't use container_of() (since it doesn't know what the
> true containing struct is), it uses this direct offset instead. Adjust
> the code to better reflect what is happening: an arbitrary pointer is
> being adjusted and cast to something else: use a "void *" for the math.
> The resulting binary output is the same, but Clang no longer sees an
> unexpected cross-structure cast:
> 
> In file included from ../fs/nfs/inode.c:50:
> In file included from ../fs/nfs/fscache.h:15:
> In file included from ../include/linux/fscache.h:18:
> ../include/linux/netfs.h:298:9: error: casting from randomized structure pointer type 'struct inode *' to 'struct netfs_i_context *'
>         return (struct netfs_i_context *)(inode + 1);
>                ^
> 1 error generated.
> 
> Cc: David Howells <dhowells@...hat.com>
> Cc: Jeff Layton <jlayton@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  include/linux/netfs.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index c7bf1eaf51d5..0c33b715cbfd 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -295,7 +295,7 @@ extern void netfs_stats_show(struct seq_file *);
>   */
>  static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
>  {
> -	return (struct netfs_i_context *)(inode + 1);
> +	return (void *)inode + sizeof(*inode);
>  }
>  
>  /**
> @@ -307,7 +307,7 @@ static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
>   */
>  static inline struct inode *netfs_inode(struct netfs_i_context *ctx)
>  {
> -	return ((struct inode *)ctx) - 1;
> +	return (void *)ctx - sizeof(struct inode);
>  }
>  
>  /**

Looks reasonable.

Reviewed-by: Jeff Layton <jlayton@...nel.org>