Received-SPF: pass (google.com: domain of linux-kernel+bounces-176005-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Precedence: bulk
MIME-Version: 1.0
References: <20240509-b4-sio-read_write-v1-1-06bec2022697@google.com> <20240510151508.hajqjxsn7rghk3dj@quack3>
In-Reply-To: <20240510151508.hajqjxsn7rghk3dj@quack3>
From: Justin Stitt <justinstitt@google.com>
Date: Fri, 10 May 2024 09:21:06 -0700
Message-ID: <CAFhGd8qbUYXmgiFuLGQ7dWXFUtZacvT82wD4jSS-xNTvtzXKGQ@mail.gmail.com>
Subject: Re: [PATCH] fs: fix unintentional arithmetic wraparound in offset calculation
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
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, May 10, 2024 at 8:15=E2=80=AFAM Jan Kara <jack@suse.cz> wrote:
>
> On Thu 09-05-24 21:34:58, Justin Stitt wrote:
> > ---
> >  fs/read_write.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/read_write.c b/fs/read_write.c
> > index d4c036e82b6c..10c3eaa5ef55 100644
> > --- a/fs/read_write.c
> > +++ b/fs/read_write.c
> > @@ -88,7 +88,7 @@ generic_file_llseek_size(struct file *file, loff_t of=
fset, int whence,
> >  {
> >       switch (whence) {
> >       case SEEK_END:
> > -             offset +=3D eof;
> > +             offset =3D min_t(loff_t, offset, maxsize - eof) + eof;
>
> Well, but by this you change the behavior of seek(2) for huge offsets.
> Previously we'd return -EINVAL (from following vfs_setpos()), now we set
> position to maxsize. I don't think that is desirable?

RIght, we shouldn't change the current behavior. This patch needs rethinkin=
g.

>
> Also the addition in SEEK_CUR could overflow in the same way AFAICT so we
> could treat that in one patch so that the whole function is fixed at once=
?

Yep let's include that one as well. However, I'm going to hold off on
sending a new version until the discussion about how to handle
overflow comes to a conclusion; as suggested by Greg [1]. I made too
many assumptions about how folks want overflow to be handled. In the
case of this patch, a simple check_add_overflow() should be okay and
match the behavior, but let's wait and see.

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

[1]: https://lore.kernel.org/all/2024051039-bankable-liking-e836@gregkh/

Thanks
Justin