linux-kernel - Re: [PATCH] jfs: Fix fortify moan in symlink

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Date:   Mon, 24 Oct 2022 10:28:27 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linux@...blig.org
Cc:     shaggy@...nel.org, jfs-discussion@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org,
        syzbot+5fc38b2ddbbca7f5c680@...kaller.appspotmail.com
Subject: Re: [PATCH] jfs: Fix fortify moan in symlink

On Sat, Oct 22, 2022 at 09:39:14PM +0100, linux@...blig.org wrote:
> From: "Dr. David Alan Gilbert" <linux@...blig.org>
> 
> JFS has in jfs_incore.h:
> 
>       /* _inline may overflow into _inline_ea when needed */
>       /* _inline_ea may overlay the last part of
>        * file._xtroot if maxentry = XTROOTINITSLOT
>        */
>       union {
>         struct {
>           /* 128: inline symlink */
>           unchar _inline[128];
>           /* 128: inline extended attr */
>           unchar _inline_ea[128];
>         };
>         unchar _inline_all[256];
> 
> and currently the symlink code copies into _inline;
> if this is larger than 128 bytes it triggers a fortify warning of the
> form:
> 
>   memcpy: detected field-spanning write (size 132) of single field
>      "ip->i_link" at fs/jfs/namei.c:950 (size 18446744073709551615)

Which compiler are you using for this build? This size report (SIZE_MAX)
should be impossible to reach. But also, the size is just wrong --
i_inline is 128 bytes, not SIZE_MAX. So, the detection is working
(132 > 128), but the report is broken, and I can't see how...

> 
> when it's actually OK.
> 
> Copy it into _inline_all instead.
> 
> Reported-by: syzbot+5fc38b2ddbbca7f5c680@...kaller.appspotmail.com
> Signed-off-by: Dr. David Alan Gilbert <linux@...blig.org>
> ---
>  fs/jfs/namei.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/jfs/namei.c b/fs/jfs/namei.c
> index 9db4f5789c0ec..4fbbf88435e69 100644
> --- a/fs/jfs/namei.c
> +++ b/fs/jfs/namei.c
> @@ -946,7 +946,7 @@ static int jfs_symlink(struct user_namespace *mnt_userns, struct inode *dip,
>  	if (ssize <= IDATASIZE) {
>  		ip->i_op = &jfs_fast_symlink_inode_operations;
>  
> -		ip->i_link = JFS_IP(ip)->i_inline;
> +		ip->i_link = JFS_IP(ip)->i_inline_all;
>  		memcpy(ip->i_link, name, ssize);
>  		ip->i_size = ssize - 1;
>  

Regardless, the fix looks correct to me!

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook