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Date: Fri, 12 May 2017 16:15:56 -0700
From: Andy Lutomirski <>
To: Al Viro <>
Cc: Jann Horn <>, Ingo Molnar <>, 
	Kees Cook <>, Thomas Garnier <>, 
	Martin Schwidefsky <>, Heiko Carstens <>, 
	Dave Hansen <>, Arnd Bergmann <>, 
	Thomas Gleixner <>, David Howells <>, 
	René Nyffenegger <>, 
	Andrew Morton <>, 
	"Paul E . McKenney" <>, "Eric W . Biederman" <>, 
	Oleg Nesterov <>, Pavel Tikhomirov <>, 
	Ingo Molnar <>, "H . Peter Anvin" <>, Andy Lutomirski <>, 
	Paolo Bonzini <>, Rik van Riel <>, 
	Josh Poimboeuf <>, Borislav Petkov <>, Brian Gerst <>, 
	"Kirill A . Shutemov" <>, 
	Christian Borntraeger <>, Russell King <>, 
	Will Deacon <>, Catalin Marinas <>, 
	Mark Rutland <>, James Morse <>, 
	linux-s390 <>, LKML <>, 
	Linux API <>, "the arch/x86 maintainers" <>, 
	"" <>, 
	Kernel Hardening <>, 
	Linus Torvalds <>, Peter Zijlstra <>
Subject: Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

On Mon, May 8, 2017 at 1:48 PM, Al Viro <> wrote:
> On Mon, May 08, 2017 at 04:06:35PM +0200, Jann Horn wrote:
>> I think Kees might be talking about
>>, fixed in
>> commit e6978e4bf181fb3b5f8cb6f71b4fe30fbf1b655c. The issue was that
>> perf code that can run in pretty much any context called access_ok().
> And that commit has *NOT* solved the problem.  perf_callchain_user()
> can be called synchronously, without passing through that code.
> Tracepoint shite...
> That set_fs() should be done in get_perf_callchain(), just around the call of
> perf_callchain_user().  Along with pagefault_disable(), actually.

Even that's not quite enough because of a different issue: perf nmis
can hit during scheduling or when we're in lazy mm, leading to the
entirely wrong set of page tables being used.  We need
nmi_uaccess_begin() and nmi_uaccess_end(), and the former needs to be
allowed to fail.

AFAIK this isn't presently a security problem because it mainly
affects kernel threads, and you need to be root to profile them, but
maybe there's some race where it does matter.

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