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

[Thomas Gleixner]

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