From: Borislav Petkov <bp@alien8.de>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: x86@kernel.org, "Ingo Molnar" <mingo@kernel.org>,
linux-kernel@vger.kernel.org, "Andy Lutomirski" <luto@kernel.org>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Radim Krčmář" <rkrcmar@redhat.com>,
kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
"Rik van Riel" <riel@surriel.com>,
"Dave Hansen" <dave.hansen@linux.intel.com>
Subject: Re: [PATCH v2] x86/fpu: Disable BH while while loading FPU registers in __fpu__restore_sig()
Date: Mon, 19 Nov 2018 19:10:00 +0100 [thread overview]
Message-ID: <20181119181000.GH14688@zn.tnic> (raw)
In-Reply-To: <20181119160410.ne7oiq2gkwt6jiqg@linutronix.de>
On Mon, Nov 19, 2018 at 05:04:10PM +0100, Sebastian Andrzej Siewior wrote:
> The sequence
> fpu->initialized = 1; /* step A */
> preempt_disable(); /* step B */
> fpu__restore(fpu);
> preempt_enable();
>
> is racy in regard to a context switch.
> For 32bit frames __fpu__restore_sig() prepares the FPU state within
> fpu->state. To ensure that a context switch (switch_fpu_prepare() in
> particular) does not modify fpu->state it uses fpu__drop() which sets
> fpu->initializes to 0.
"... ->initialized to 0."
Also, a new line here pls.
> With this change the CPU's FPU state is not saved
^
comma:
,
Also, instead of "with this change" I think you mean: "After
->initialized is cleared, the CPU's FPU state..."
> to fpu->state during a context switch.
> It then loads the state to fpu->state from userland and ensures it
> sane.
"... and ensures it is sane."
> The new state is loaded via fpu__restore(). The code sets then
> fpu->initializes to 1 in order to avoid fpu__initialize() doing
fpu->initialized
> anything (overwrite the new state) which is part of fpu__restore().
<---- newline here.
> A context switch between step A and B would save CPU's current FPU
> registers to fpu->state and overwrite the newly prepared state. This
> looks like tiny race window but the Kernel Test Robot reported this back
> in 2016 while we had lazy FPU support. Borislav Petkov made the link
> between that report and another patch that has been posted.
> Since the removal of the lazy FPU support, this race goes unnoticed
> because the warning has been removed.
>
> Use local_bh_disable() around the restore sequence to avoid the race. BH
Let's write it out once: "Bottom halves need to be... "
> needs to be disabled because BH is allowed to run (even with preemption
> disabled) and might invoke kernel_fpu_begin().
... and let's put the potential example here with IPsec and softirq.
> Link: https://lkml.kernel.org/r/20160226074940.GA28911@pd.tnic
> Cc: stable@vger.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> v1…v2: A more verbose commit as message.
Very much needed, thanks!
> arch/x86/kernel/fpu/signal.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 61a949d84dfa5..d99a8ee9e185e 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -344,10 +344,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> sanitize_restored_xstate(tsk, &env, xfeatures, fx_only);
> }
>
> + local_bh_disable();
> fpu->initialized = 1;
> - preempt_disable();
> fpu__restore(fpu);
> - preempt_enable();
> + local_bh_enable();
>
> return err;
> } else {
> --
--
Regards/Gruss,
Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
next prev parent reply other threads:[~2018-11-19 18:10 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-19 16:04 [PATCH v2] x86/fpu: Disable BH while while loading FPU registers in __fpu__restore_sig() Sebastian Andrzej Siewior
2018-11-19 17:02 ` Dave Hansen
2018-11-19 17:11 ` Sebastian Andrzej Siewior
2018-11-19 17:27 ` Borislav Petkov
2018-11-19 17:31 ` Sebastian Andrzej Siewior
2018-11-19 17:41 ` Borislav Petkov
2018-11-19 17:32 ` Dave Hansen
2018-11-19 18:10 ` Borislav Petkov [this message]
2018-11-20 10:26 ` [PATCH v3] " Sebastian Andrzej Siewior
[not found] ` <20181120132531.6E80C206BB@mail.kernel.org>
2018-11-20 18:34 ` [PATCH v2] " Borislav Petkov
[not found] ` <CAMU5Q=72mNG4ikXKpLK3=SiDK3dRXtCw7wpHZ2i==awrtvRwdg@mail.gmail.com>
2018-11-21 10:56 ` Borislav Petkov
[not found] ` <CAMU5Q=5LkypghWA=23YwscaXSx5yBv7LyW1WHbY_J_hQwryNpA@mail.gmail.com>
2018-11-21 12:54 ` Boris Petkov
[not found] <87d1rk9str.fsf@yhuang-dev.intel.com>
2016-02-26 7:49 ` [lkp] [x86/fpu] 58122bf1d8: WARNING: CPU: 0 PID: 1 at arch/x86/include/asm/fpu/internal.h:529 fpu__restore+0x28f/0x9ab() Borislav Petkov
2016-02-27 12:02 ` Ingo Molnar
2016-02-27 13:13 ` Borislav Petkov
2018-11-20 12:05 ` [tip:x86/urgent] x86/fpu: Disable bottom halves while loading FPU registers tip-bot for Sebastian Andrzej Siewior
2018-11-20 16:29 ` tip-bot for Sebastian Andrzej Siewior
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