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References: <20240507-b4-sio-vfs_fallocate-v1-1-322f84b97ad5@google.com> <20240509155356.w274h4blmcykxej6@quack3>
In-Reply-To: <20240509155356.w274h4blmcykxej6@quack3>
From: Justin Stitt <justinstitt@google.com>
Date: Thu, 9 May 2024 15:10:07 -0700
Message-ID: <CAFhGd8opxHhTdZhDg_hq7XWQFxJ34nLDxTd-nBBgye9BLohnqw@mail.gmail.com>
Subject: Re: [PATCH] fs: remove accidental overflow during wraparound check
To: Jan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>, Christian Brauner <brauner@kernel.org>, 
	Nathan Chancellor <nathan@kernel.org>, Bill Wendling <morbo@google.com>, linux-fsdevel@vger.kernel.org, 
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev, 
	linux-hardening@vger.kernel.org
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On Thu, May 9, 2024 at 8:53=E2=80=AFAM Jan Kara <jack@suse.cz> wrote:
> > @@ -319,8 +320,12 @@ int vfs_fallocate(struct file *file, int mode, lof=
f_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 >=
=3D 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.

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