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From: Ira Weiny <ira.weiny@intel.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, Dan Williams <dan.j.williams@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Fenghua Yu <fenghua.yu@intel.com>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions
Date: Fri, 24 Jul 2020 12:43:56 -0700
Message-ID: <20200724194355.GA844234@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <D866BD75-42A2-43B2-B07A-55BCC3781FEC@amacapital.net>

On Fri, Jul 24, 2020 at 10:29:23AM -0700, Andy Lutomirski wrote:
> 
> > On Jul 24, 2020, at 10:23 AM, Ira Weiny <ira.weiny@intel.com> wrote:
> > 
> > On Thu, Jul 23, 2020 at 10:15:17PM +0200, Thomas Gleixner wrote:
> >> Thomas Gleixner <tglx@linutronix.de> writes:
> >> 
> >>> Ira Weiny <ira.weiny@intel.com> writes:
> >>>> On Fri, Jul 17, 2020 at 12:06:10PM +0200, Peter Zijlstra wrote:
> >>>>>> On Fri, Jul 17, 2020 at 12:20:56AM -0700, ira.weiny@intel.com wrote:
> >>>>> I've been really digging into this today and I'm very concerned that I'm
> >>>>> completely missing something WRT idtentry_enter() and idtentry_exit().
> >>>>> 
> >>>>> I've instrumented idt_{save,restore}_pkrs(), and __dev_access_{en,dis}able()
> >>>>> with trace_printk()'s.
> >>>>> 
> >>>>> With this debug code, I have found an instance where it seems like
> >>>>> idtentry_enter() is called without a corresponding idtentry_exit().  This has
> >>>>> left the thread ref counter at 0 which results in very bad things happening
> >>>>> when __dev_access_disable() is called and the ref count goes negative.
> >>>>> 
> >>>>> Effectively this seems to be happening:
> >>>>> 
> >>>>> ...
> >>>>>    // ref == 0
> >>>>>    dev_access_enable()  // ref += 1 ==> disable protection
> >>>>>        // exception  (which one I don't know)
> >>>>>            idtentry_enter()
> >>>>>                // ref = 0
> >>>>>                _handler() // or whatever code...
> >>>>>            // *_exit() not called [at least there is no trace_printk() output]...
> >>>>>            // Regardless of trace output, the ref is left at 0
> >>>>>    dev_access_disable() // ref -= 1 ==> -1 ==> does not enable protection
> >>>>>    (Bad stuff is bound to happen now...)
> >>> 
> >>> Well, if any exception which calls idtentry_enter() would return without
> >>> going through idtentry_exit() then lots of bad stuff would happen even
> >>> without your patches.
> >>> 
> >>>> Also is there any chance that the process could be getting scheduled and that
> >>>> is causing an issue?
> >>> 
> >>> Only from #PF, but after the fault has been resolved and the tasks is
> >>> scheduled in again then the task returns through idtentry_exit() to the
> >>> place where it took the fault. That's not guaranteed to be on the same
> >>> CPU. If schedule is not aware of the fact that the exception turned off
> >>> stuff then you surely get into trouble. So you really want to store it
> >>> in the task itself then the context switch code can actually see the
> >>> state and act accordingly.
> >> 
> >> Actually thats nasty as well as you need a stack of PKRS values to
> >> handle nested exceptions. But it might be still the most reasonable
> >> thing to do. 7 PKRS values plus an index should be really sufficient,
> >> that's 32bytes total, not that bad.
> > 
> > I've thought about this a bit more and unless I'm wrong I think the
> > idtentry_state provides for that because each nested exception has it's own
> > idtentry_state doesn't it?
> 
> Only the ones that use idtentry_enter() instead of, say, nmi_enter().

Oh agreed...

But with this patch we are still better off than just preserving during context
switch.

I need to update the commit message here to make this clear though.

Thanks,
Ira

  reply index

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-17  7:20 [PATCH RFC V2 00/17] PKS: Add Protection Keys Supervisor (PKS) support ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 01/17] x86/pkeys: Create pkeys_internal.h ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 02/17] x86/fpu: Refactor arch_set_user_pkey_access() for PKS support ira.weiny
2020-07-17  8:54   ` Peter Zijlstra
2020-07-17 20:52     ` Ira Weiny
2020-07-20  9:14       ` Peter Zijlstra
2020-07-17 22:36     ` Dave Hansen
2020-07-20  9:13       ` Peter Zijlstra
2020-07-17  7:20 ` [PATCH RFC V2 03/17] x86/pks: Enable Protection Keys Supervisor (PKS) ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 04/17] x86/pks: Preserve the PKRS MSR on context switch ira.weiny
2020-07-17  8:31   ` Peter Zijlstra
2020-07-17 21:39     ` Ira Weiny
2020-07-17  8:59   ` Peter Zijlstra
2020-07-17 22:34     ` Ira Weiny
2020-07-20  9:15       ` Peter Zijlstra
2020-07-20 18:35         ` Ira Weiny
2020-07-17  7:20 ` [PATCH RFC V2 05/17] x86/pks: Add PKS kernel API ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 06/17] x86/pks: Add a debugfs file for allocated PKS keys ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 07/17] Documentation/pkeys: Update documentation for kernel pkeys ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 08/17] x86/pks: Add PKS Test code ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 09/17] memremap: Convert devmap static branch to {inc,dec} ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 10/17] fs/dax: Remove unused size parameter ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 11/17] drivers/dax: Expand lock scope to cover the use of addresses ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 12/17] memremap: Add zone device access protection ira.weiny
2020-07-17  9:10   ` Peter Zijlstra
2020-07-18  5:06     ` Ira Weiny
2020-07-20  9:16       ` Peter Zijlstra
2020-07-17  9:17   ` Peter Zijlstra
2020-07-18  5:51     ` Ira Weiny
2020-07-17  9:20   ` Peter Zijlstra
2020-07-17  7:20 ` [PATCH RFC V2 13/17] kmap: Add stray write protection for device pages ira.weiny
2020-07-17  9:21   ` Peter Zijlstra
2020-07-19  4:13     ` Ira Weiny
2020-07-20  9:17       ` Peter Zijlstra
2020-07-21 16:31         ` Ira Weiny
2020-07-17  7:20 ` [PATCH RFC V2 14/17] dax: Stray write protection for dax_direct_access() ira.weiny
2020-07-17  9:22   ` Peter Zijlstra
2020-07-19  4:41     ` Ira Weiny
2020-07-17  7:20 ` [PATCH RFC V2 15/17] nvdimm/pmem: Stray write protection for pmem->virt_addr ira.weiny
2020-07-17  7:20 ` [PATCH RFC V2 16/17] [dax|pmem]: Enable stray write protection ira.weiny
2020-07-17  9:25   ` Peter Zijlstra
2020-07-17  7:20 ` [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions ira.weiny
2020-07-17  9:30   ` Peter Zijlstra
2020-07-21 18:01     ` Ira Weiny
2020-07-21 19:11       ` Peter Zijlstra
2020-07-17  9:34   ` Peter Zijlstra
2020-07-17 10:06   ` Peter Zijlstra
2020-07-22  5:27     ` Ira Weiny
2020-07-22  9:48       ` Peter Zijlstra
2020-07-22 21:24         ` Ira Weiny
2020-07-23 20:08       ` Thomas Gleixner
2020-07-23 20:15         ` Thomas Gleixner
2020-07-24 17:23           ` Ira Weiny
2020-07-24 17:29             ` Andy Lutomirski
2020-07-24 19:43               ` Ira Weiny [this message]
2020-07-22 16:21   ` Andy Lutomirski
2020-07-23 16:18     ` Fenghua Yu
2020-07-23 16:23       ` Dave Hansen
2020-07-23 16:52         ` Fenghua Yu
2020-07-23 17:08           ` Andy Lutomirski
2020-07-23 17:30             ` Dave Hansen
2020-07-23 20:23               ` Thomas Gleixner
2020-07-23 20:22             ` Thomas Gleixner
2020-07-23 21:30               ` Andy Lutomirski
2020-07-23 22:14                 ` Thomas Gleixner
2020-07-23 19:53   ` Thomas Gleixner
2020-07-23 22:04     ` Ira Weiny
2020-07-23 23:41       ` Thomas Gleixner
2020-07-24 21:24         ` Thomas Gleixner
2020-07-24 21:31           ` Thomas Gleixner
2020-07-25  0:09           ` Andy Lutomirski
2020-07-27 20:59           ` Ira Weiny
2020-07-24 22:19 ` [PATCH RFC V2 00/17] PKS: Add Protection Keys Supervisor (PKS) support Kees Cook

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