linux-kernel - Re: [PATCH] ALSA: Adjust structure(snd_timer_tread) members to avoid 8 padding bytes

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Date:   Tue, 30 May 2017 08:26:01 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Manjeet Pawar <manjeet.p@...sung.com>
cc:     perex@...ex.cz, tiwai@...e.de, alsa-devel@...a-project.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        a.sahrawat@...sung.com, lalit.mohan@...sung.com,
        pankaj.m@...sung.com, Vaneet Narang <v.narang@...sung.com>
Subject: Re: [PATCH] ALSA: Adjust structure(snd_timer_tread) members to avoid
 8 padding bytes

On Tue, 30 May 2017, Manjeet Pawar wrote:
> This patch is made to fix below CVE issue where 8 padding bytes are sent to
> user without being initialized. Structure members are adjusted to avoid 8 
> padding bytes.
> 
> CVE-2016-4569/CVE-2016-4578:
> The stack object tread has a total size of 32 bytes. Its field
> event and val both contain 4 bytes padding. These 8 bytes
> padding bytes are sent to user without being initialized.
> 
> Fix patch:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cec8f96e49d9be372fdb0c3836dcf31ec71e457e
> https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/commit/?h=for-next&id=9a47e9cff994f37f7f0dbd9ae23740d0f64f9fe6
> 
> These patches does memset on structure object to initialize 8 padding bytes, which can be skipped by 
> adjusting structure members.

And by rearranging the struct members you break existing user space, which
is a NONO. The struct has to stay as is.

Thanks,

	tglx