linux-hardening - Re: [PATCH] fs: remove accidental overflow during wraparound check

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Date: Sun, 12 May 2024 10:05:11 +0200
From: Jan Kara <jack@...e.cz>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] fs: remove accidental overflow during wraparound check

On Thu 09-05-24 15:10:07, Justin Stitt wrote:
> On Thu, May 9, 2024 at 8:53 AM Jan Kara <jack@...e.cz> wrote:
> > > @@ -319,8 +320,12 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> > >       if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
> > >               return -ENODEV;
> > >
> > > -     /* Check for wrap through zero too */
> > > -     if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
> > > +     /* Check for wraparound */
> > > +     if (check_add_overflow(offset, len, &sum))
> > > +             return -EFBIG;
> > > +
> > > +     /* Now, check bounds */
> > > +     if (sum > inode->i_sb->s_maxbytes || sum < 0)
> > >               return -EFBIG;
> >
> > But why do you check for sum < 0? We know from previous checks offset >= 0
> > && len > 0 so unless we overflow, sum is guaranteed to be > 0.
> 
> Fair enough. I suppose with the overflow check in place we can no
> longer have a sum less than zero there. If nothing else, it tells
> readers of this code what the domain of (offset+len) is. I don't mind
> sending a new version, though.

Well, for normal readers offset+len is always a positive number. That's
what you expect. If you see a check for offset+len < 0, you start wondering
what are you missing... only to find you miss nothing and the check is
pointless. So yes, please send a version without the pointless check.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR