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Date: Mon, 8 May 2017 21:48:58 +0100
From: Al Viro <>
To: Jann Horn <>
Cc: 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 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.

BTW, that's a nice example demonstrating why doing that on the kernel
boundary is wrong.  Wider (in theory) area being "protected" => easier
to miss the ways not crossing its border.

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