linux-kernel - Re: [PATCH 2/2 v2] staging: android: ashmem: Fix mmap size validation

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Date:   Wed, 20 Jun 2018 14:21:57 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Alistair Strachan <astrachan@...gle.com>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>, devel@...verdev.osuosl.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 2/2 v2] staging: android: ashmem: Fix mmap size validation

On Tue, Jun 19, 2018 at 9:32 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
> On Tue, Jun 19, 2018 at 05:57:35PM -0700, Alistair Strachan wrote:
> > The ashmem driver did not check that the size/offset of the vma passed
> > to its .mmap() function was not larger than the ashmem object being
> > mapped. This could cause mmap() to succeed, even though accessing parts
> > of the mapping would later fail with a segmentation fault.
> >
> > Ensure an error is returned by the ashmem_mmap() function if the vma
> > size is larger than the ashmem object size. This enables safer handling
> > of the problem in userspace.

Are we sure that this approach is a good idea? You can over-mmap
regular files. I don't like the idea of creating special mmap
semantics for files that happen to be ashmem files. Ashmem users can
detect size-changing shenanigans with ASHMEM_GET_SIZE after mmap,
since an ashmem file's size can't change after an mmap call succeeds.