From: Ira Weiny <ira.weiny@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: 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 10:23:44 -0700 [thread overview]
Message-ID: <20200724172344.GO844235@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <87lfjavvhm.fsf@nanos.tec.linutronix.de>
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?
Ira
next prev parent reply other threads:[~2020-07-24 17:23 UTC|newest]
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 [this message]
2020-07-24 17:29 ` Andy Lutomirski
2020-07-24 19:43 ` Ira Weiny
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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