linux-kernel - Re: [PATCH] amd64-agp: fix arbitrary kernel memory writes

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Message-ID: <CAKgHYH2qhVMFdnxnjuO6TjGHrzsP4oCNPgEBww6KPPLOhfU9mA@mail.gmail.com>
Date:   Wed, 29 May 2019 23:14:29 +0800
From:   Yang Xiao <92siuyang@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     airlied@...ux.ie, arnd@...db.de,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] amd64-agp: fix arbitrary kernel memory writes

I am not so sure about taking off the cast, just to be in line with
patch in 194b3da873fd.
The comment can be deleted.

On Wed, May 29, 2019 at 4:35 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Wed, May 29, 2019 at 12:52:01PM +0800, Young Xiao wrote:
> > pg_start is copied from userspace on AGPIOC_BIND and AGPIOC_UNBIND ioctl
> > cmds of agp_ioctl() and passed to agpioc_bind_wrap().  As said in the
> > comment, (pg_start + mem->page_count) may wrap in case of AGPIOC_BIND,
> > and it is not checked at all in case of AGPIOC_UNBIND.  As a result, user
> > with sufficient privileges (usually "video" group) may generate either
> > local DoS or privilege escalation.
> >
> > See commit 194b3da873fd ("agp: fix arbitrary kernel memory writes")
> > for details.
> >
> > Signed-off-by: Young Xiao <92siuyang@...il.com>
> > ---
> >  drivers/char/agp/amd64-agp.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/char/agp/amd64-agp.c b/drivers/char/agp/amd64-agp.c
> > index c69e39f..5daa0e3 100644
> > --- a/drivers/char/agp/amd64-agp.c
> > +++ b/drivers/char/agp/amd64-agp.c
> > @@ -60,7 +60,8 @@ static int amd64_insert_memory(struct agp_memory *mem, off_t pg_start, int type)
> >
> >       /* Make sure we can fit the range in the gatt table. */
> >       /* FIXME: could wrap */
> > -     if (((unsigned long)pg_start + mem->page_count) > num_entries)
> > +     if (((pg_start + mem->page_count) > num_entries) ||
> > +         ((pg_start + mem->page_count) < pg_start))
>
> Why did you take off the cast for the first test?
>
> And if this really does fix this issue, should you remove the FIXME
> line?
>
> thanks,
>
> greg k-h



-- 
Best regards!

Young
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