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Date: Mon, 8 May 2017 21:48:58 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Jann Horn <jannh@google.com>
Cc: Ingo Molnar <mingo@kernel.org>, Kees Cook <keescook@chromium.org>,
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Subject: Re: [PATCH v9 1/4] syscalls: Verify address limit before returning
 to user-mode
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On Mon, May 08, 2017 at 04:06:35PM +0200, Jann Horn wrote:

> I think Kees might be talking about
> https://bugs.chromium.org/p/project-zero/issues/detail?id=822, 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.