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

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Date:   Mon, 24 Oct 2022 19:49:17 +0100
From:   "Dr. David Alan Gilbert" <linux@...blig.org>
To:     Kees Cook <keescook@...omium.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

* Kees Cook (keescook@...omium.org) wrote:
> 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?

I think that report was the same on gcc on Fedora 37 and whatever
syzkaller was running.

> 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...

Yeh, and led me down a blind alley for a while thinking something had
really managed to screwup the strlen somehow.

> 
> > 
> > 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>

Thanks!

Dave

> -- 
> Kees Cook
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
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