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

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Message-ID: <20170508204858.GT29622@ZenIV.linux.org.uk>
Date:   Mon, 8 May 2017 21:48:58 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jann Horn <jannh@...gle.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Kees Cook <keescook@...omium.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Howells <dhowells@...hat.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
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
> 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.