linux-kernel - Re: question about save_xstate_sig()

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 23 Jan 2015 13:07:23 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?

On 01/23/2015 11:34 AM, Rik van Riel wrote:
> While working on a patch series to defer FPU state loading until
> kernel -> user space transition, and be more lazy with FPU state
> while in the kernel, I came across this code in save_xstate_sig().
> 
> Not only is this broken with my new code, but it looks like it may
> be broken with the current code, too...
> 
> Specifically, save_user_xstate() may page fault and sleep. After
> returning from the page fault, there is no guarantee that the
> FPU state will be restored into the CPU, when the system is not
> running with eager fpu mode.
> 
> In that case, what prevents us from saving random FPU register state
> to the user's stack frame?  Potentially state containing data from
> other programs...
> 

If the FPU state is not current, we'll have CR0.TS = 1 and the XSAVE
will cause an #NM exception, which will cause the FPU state to be
swapped in.

	-hpa


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/