From: Andrea Righi <firstname.lastname@example.org>
To: "Eric W. Biederman" <email@example.com>
Cc: Dan Carpenter <firstname.lastname@example.org>,
Bartlomiej Zolnierkiewicz <email@example.com>,
Daniel Vetter <firstname.lastname@example.org>,
Sam Ravnborg <email@example.com>,
Maarten Lankhorst <firstname.lastname@example.org>,
Peter Rosin <email@example.com>, Gerd Hoffmann <firstname.lastname@example.org>,
email@example.com, Kees Cook <firstname.lastname@example.org>,
Julia Lawall <Julia.Lawall@lip6.fr>
Subject: Re: [PATCH] fbdev: potential information leak in do_fb_ioctl()
Date: Wed, 30 Oct 2019 21:12:01 +0100 [thread overview]
Message-ID: <20191030201201.GA3209@xps-13> (raw)
On Wed, Oct 30, 2019 at 02:26:21PM -0500, Eric W. Biederman wrote:
> Andrea Righi <email@example.com> writes:
> > On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote:
> >> Dan Carpenter <firstname.lastname@example.org> writes:
> >> > The "fix" struct has a 2 byte hole after ->ywrapstep and the
> >> > "fix = info->fix;" assignment doesn't necessarily clear it. It depends
> >> > on the compiler.
> >> >
> >> > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held")
> >> > Signed-off-by: Dan Carpenter <email@example.com>
> >> > ---
> >> > I have 13 more similar places to patch... I'm not totally sure I
> >> > understand all the issues involved.
> >> What I have done in a similar situation with struct siginfo, is that
> >> where the structure first appears I have initialized it with memset,
> >> and then field by field.
> >> Then when the structure is copied I copy the structure with memcpy.
> >> That ensures all of the bytes in the original structure are initialized
> >> and that all of the bytes are copied.
> >> The goal is to avoid memory that has values of the previous users of
> >> that memory region from leaking to userspace. Which depending on who
> >> the previous user of that memory region is could tell userspace
> >> information about what the kernel is doing that it should not be allowed
> >> to find out.
> >> I tried to trace through where "info" and thus presumably "info->fix" is
> >> coming from and only made it as far as register_framebuffer. Given
> >> that I suspect a local memset, and then a field by field copy right
> >> before copy_to_user might be a sound solution. But ick. That is a lot
> >> of fields to copy.
> > I know it might sound quite inefficient, but what about making struct
> > fb_fix_screeninfo __packed?
> > This doesn't solve other potential similar issues, but for this
> > particular case it could be a reasonable and simple fix.
> It is part of the user space ABI. As such you can't move the fields.
Oh, that's right! Then memset() + memcpy() is probably the best option,
since copying all those fields one by one looks quite ugly to me...
next prev parent reply other threads:[~2019-10-30 20:12 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-29 18:23 [PATCH] fbdev: potential information leak in do_fb_ioctl() Dan Carpenter
2019-10-29 18:35 ` Joe Perches
2019-10-29 19:02 ` Eric W. Biederman
2019-10-30 7:43 ` Andrea Righi
2019-10-30 19:26 ` Eric W. Biederman
2019-10-30 20:12 ` Andrea Righi [this message]
2019-10-31 18:16 ` Joe Perches
2019-10-31 22:12 ` Eric W. Biederman
2020-01-03 13:07 ` Bartlomiej Zolnierkiewicz
2020-01-13 11:08 ` [PATCH v2] " Dan Carpenter
2020-01-15 14:31 ` Bartlomiej Zolnierkiewicz
2020-01-13 12:49 ` [PATCH] " Arnd Bergmann
2020-01-15 13:09 ` Bartlomiej Zolnierkiewicz
2020-01-15 13:16 ` Arnd Bergmann
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