Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

[Andy Lutomirski]

On Mon, May 8, 2017 at 1:48 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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.
>

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.