linux-hardening - Re: [PATCH] fs: fix unintentional arithmetic wraparound in offset calculation

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Date: Fri, 10 May 2024 09:21:06 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Jan Kara <jack@...e.cz>
Cc: 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: fix unintentional arithmetic wraparound in offset calculation

On Fri, May 10, 2024 at 8:15 AM Jan Kara <jack@...e.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 offset, int whence,
> >  {
> >       switch (whence) {
> >       case SEEK_END:
> > -             offset += eof;
> > +             offset = 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 rethinking.

>
> 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@...e.com>
> SUSE Labs, CR

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

Thanks
Justin