From: "H. Peter Anvin" <hpa@zytor.com>
To: Rik van Riel <riel@redhat.com>,
Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Fenghua Yu <fenghua.yu@intel.com>,
the arch/x86 maintainers <x86@kernel.org>,
Oleg Nesterov <oleg@redhat.com>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?
Date: Fri, 23 Jan 2015 13:07:23 -0800 [thread overview]
Message-ID: <54C2B80B.8050501@zytor.com> (raw)
In-Reply-To: <54C2A245.4010307@redhat.com>
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
next prev parent reply other threads:[~2015-01-23 21:07 UTC|newest]
Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-23 19:34 Rik van Riel
2015-01-23 20:51 ` [PATCH, RFC] x86,fpu: make signal handling xstate save & restore preemption safe Rik van Riel
2015-01-23 21:07 ` H. Peter Anvin [this message]
2015-01-24 13:39 ` question about save_xstate_sig() - WHY DOES THIS WORK? Rik van Riel
2015-01-24 20:20 ` Oleg Nesterov
2015-01-26 23:27 ` Rik van Riel
2015-01-27 19:40 ` Oleg Nesterov
2015-01-27 20:27 ` Rik van Riel
2015-01-27 20:50 ` Rik van Riel
2015-01-29 21:01 ` Oleg Nesterov
2015-01-29 20:45 ` Oleg Nesterov
2015-01-29 20:52 ` Rik van Riel
2015-01-29 21:00 ` [PATCH RFC] x86,fpu: merge save_init_fpu & unlazy_fpu Rik van Riel
2015-01-29 21:21 ` Oleg Nesterov
2015-01-29 21:07 ` [PATCH 0/3]: x86, fpu: unlazy_fpu fixes/cleanups Oleg Nesterov
2015-01-29 21:07 ` [PATCH 1/3] x86, fpu: unlazy_fpu: don't reset thread.fpu_counter Oleg Nesterov
2015-01-29 21:26 ` Rik van Riel
2015-01-29 21:08 ` [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager_fpu() Oleg Nesterov
2015-01-29 21:36 ` Rik van Riel
2015-01-29 21:49 ` Oleg Nesterov
2015-01-29 21:53 ` Rik van Riel
2015-01-29 21:54 ` Rik van Riel
2015-01-29 21:08 ` [PATCH 3/3] x86, fpu: kill save_init_fpu(), change math_error() to use unlazy_fpu() Oleg Nesterov
2015-01-29 21:54 ` Rik van Riel
2015-01-29 21:17 ` [PATCH 0/3]: x86, fpu: unlazy_fpu fixes/cleanups Dave Hansen
2015-01-29 21:33 ` Oleg Nesterov
2015-01-29 21:43 ` Dave Hansen
2015-01-29 21:56 ` Oleg Nesterov
2015-01-29 21:58 ` Rik van Riel
2015-01-29 23:26 ` Dave Hansen
2015-01-30 1:33 ` Rik van Riel
2015-02-02 18:11 ` Dave Hansen
2015-01-30 12:45 ` Oleg Nesterov
2015-01-30 13:30 ` Oleg Nesterov
2015-01-30 13:43 ` Oleg Nesterov
2015-01-30 17:49 ` [PATCH 0/3] cleanups to the disable lazy fpu restore code riel
2015-01-30 17:49 ` [PATCH 1/3] x86,fpu: move lazy restore functions up a few lines riel
2015-01-30 17:49 ` [PATCH 2/3] x86,fpu: introduce task_disable_lazy_fpu_restore helper riel
2015-01-30 17:49 ` [PATCH 3/3] x86,fpu: use disable_task_lazy_fpu_restore helper riel
2015-01-30 21:46 ` Dave Hansen
2015-01-30 21:48 ` Rik van Riel
2015-02-02 17:56 ` Rik van Riel
2015-02-02 18:00 ` [PATCH 0/6] cleanups to lazy FPU restore code riel
2015-02-02 18:00 ` [PATCH 1/6] x86,fpu: move lazy restore functions up a few lines riel
2015-02-02 18:00 ` [PATCH 2/6] x86,fpu: introduce task_disable_lazy_fpu_restore helper riel
2015-02-02 18:00 ` [PATCH 3/6] x86,fpu: use an explicit if/else in switch_fpu_prepare riel
2015-02-02 18:00 ` [PATCH 4/6] x86,fpu: use disable_task_lazy_fpu_restore helper riel
2015-02-02 19:21 ` Oleg Nesterov
2015-02-02 19:43 ` Rik van Riel
2015-02-03 19:08 ` Oleg Nesterov
2015-02-03 22:01 ` Rik van Riel
2015-02-06 16:42 ` Rik van Riel
2015-02-02 18:00 ` [PATCH 5/6] x86,fpu: also check fpu_lazy_restore when use_eager_fpu riel
2015-02-02 18:55 ` Oleg Nesterov
2015-02-02 19:19 ` Rik van Riel
2015-02-02 18:00 ` [PATCH 6/6] x86,fpu: remove redundant increments of fpu_counter riel
2015-02-02 18:34 ` Oleg Nesterov
2015-02-02 18:40 ` Rik van Riel
2015-02-18 23:40 ` Ingo Molnar
2015-02-18 23:54 ` Borislav Petkov
2015-02-19 20:09 ` Oleg Nesterov
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