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From: Andy Lutomirski <luto@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Fenghua Yu <fenghua.yu@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Weiny Ira <ira.weiny@intel.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	X86 ML <x86@kernel.org>, Dan Williams <dan.j.williams@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>
Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions
Date: Thu, 23 Jul 2020 14:30:06 -0700
Message-ID: <CALCETrUdxpVP3dZgsZBpCODxW8yaiHguxTu=aHg_AkRbs91dWg@mail.gmail.com> (raw)
In-Reply-To: <87imeevv6b.fsf@nanos.tec.linutronix.de>

> On Jul 23, 2020, at 1:22 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Andy Lutomirski <luto@amacapital.net> writes:
>
>> Suppose some kernel code (a syscall or kernel thread) changes PKRS
>> then takes a page fault. The page fault handler needs a fresh
>> PKRS. Then the page fault handler (say a VMA’s .fault handler) changes
>> PKRS.  The we get an interrupt. The interrupt *also* needs a fresh
>> PKRS and the page fault value needs to be saved somewhere.
>>
>> So we have more than one saved value per thread, and thread_struct
>> isn’t going to solve this problem.
>
> A stack of 7 entries and an index needs 32bytes total which is a
> reasonable amount and solves the problem including scheduling from #PF
> nicely. Make it 15 and it's still only 64 bytes.
>
>> But idtentry_state is also not great for a couple reasons.  Not all
>> entries have idtentry_state, and the unwinder can’t find it for
>> debugging. For that matter, the page fault logic probably wants to
>> know the previous PKRS, so it should either be stashed somewhere
>> findable or it should be explicitly passed around.
>>
>> My suggestion is to enlarge pt_regs.  The save and restore logic can
>> probably be in C, but pt_regs is the logical place to put a register
>> that is saved and restored across all entries.
>
> Kinda, but that still sucks because schedule from #PF will get it wrong
> unless you do extra nasties.

This seems like we’re reinventing the wheel.  PKRS is not
fundamentally different from, say, RSP.  If we want to save it across
exceptions, we save it on entry and context-switch-out and restore it
on exit and context-switch-in.


>
>> Whoever does this work will have the delightful job of figuring out
>> whether BPF thinks that the layout of pt_regs is ABI and, if so,
>> fixing the resulting mess.
>>
>> The fact the new fields will go at the beginning of pt_regs will make
>> this an entertaining prospect.
>
> Good luck with all of that.

We can always cheat like this:

struct real_pt_regs {
  unsigned long pkrs;
  struct pt_regs regs;
};

and pass a pointer to regs around.  What BPF doesn't know about can't hurt it.

  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
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 [this message]
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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