Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables

From: Andy Lutomirski <luto@amacapital.net>
To: Joerg Roedel <jroedel@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Juergen Gross <jgross@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Brian Gerst <brgerst@gmail.com>,
	David Laight <David.Laight@aculab.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Eduardo Valentin <eduval@amazon.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	Will Deacon <will.deacon@arm.com>,
	"Liguori, Anthony" <aliguori@amazon.com>,
	Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
	Hugh Dickins <hughd@google.com>, Kees Cook <keescook@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	"David H . Gutteridge" <dhgutteridge@sympatico.ca>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>
Subject: Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables
Date: Fri, 20 Jul 2018 12:20:24 -1000	[thread overview]
Message-ID: <D89602E9-E620-4AF0-822C-206D7F0BA071@amacapital.net> (raw)
In-Reply-To: <20180720213700.gh6d2qd2ck6nt4ax@suse.de>

> On Jul 20, 2018, at 11:37 AM, Joerg Roedel <jroedel@suse.de> wrote:
> 
>> On Fri, Jul 20, 2018 at 12:32:10PM -0700, Andy Lutomirski wrote:
>> I'm just reading your changelog, and you said the PMDs are no longer
>> shared between the page tables.  So this presumably means that
>> vmalloc_fault() no longer actually works correctly on PTI systems.  I
>> didn't read the code to figure out *why* it doesn't work, but throwing
>> random vmalloc_sync_all() calls around is wrong.
> 
> Hmm, so the whole point of vmalloc_fault() fault is to sync changes from
> swapper_pg_dir to process page-tables when the relevant parts of the
> kernel page-table are not shared, no?
> 
> That is also the reason we don't see this on 64 bit, because there these
> parts *are* shared.
> 
> So with that reasoning vmalloc_fault() works as designed, except that
> a warning is issued when it's happens in the NMI path. That warning comes
> from
> 
>    ebc8827f75954 x86: Barf when vmalloc and kmemcheck faults happen in NMI
> 
> which went into 2.6.37 and was added because the NMI handler were not
> nesting-safe back then. Reason probably was that the handler on 64 bit
> has to use an IST stack and a nested NMI would overwrite the stack of
> the upper handler.  We don't have this problem on 32 bit as a nested NMI
> will not do another stack-switch there.
> 

Thanks for digging!  The problem was presumably that vmalloc_fault() will IRET and re-enable NMIs on the way out.  But we’ve supported page faults on user memory in NMI handlers on 32-bit and 64-bit for quite a while, and it’s fine now.

I would remove the warning, re-test, and revert the other patch.

The one case we can’t handle in vmalloc_fault() is a fault on a stack access. I don’t expect this to be a problem for PTI. It was a problem for CONFIG_VMAP_STACK, though.

> I am not sure about 64 bit, but there is a lot of assembly magic to make
> NMIs nesting-safe, so I guess the problem should be gone there too.
> 
> 
> Regards,
> 
>    Joerg