linux-kernel - Re: [PATCH v10 1/3] x86/syscalls: Check address limit on user-mode return

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Date:   Tue, 20 Jun 2017 13:24:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Garnier <thgarnie@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Oleg Nesterov <oleg@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Petr Mladek <pmladek@...e.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Dave Hansen <dave.hansen@...el.com>,
        David Howells <dhowells@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Pratyush Anand <panand@...hat.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v10 1/3] x86/syscalls: Check address limit on user-mode return

On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> Ensure the address limit is a user-mode segment before returning to
> user-mode. Otherwise a process can corrupt kernel-mode memory and elevate
> privileges [1].
>
> The set_fs function sets the TIF_SETFS flag to force a slow path on
> return. In the slow path, the address limit is checked to be USER_DS if
> needed.
>
> The addr_limit_user_check function is added as a cross-architecture
> function to check the address limit.
>
> [1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>
> Signed-off-by: Thomas Garnier <thgarnie@...gle.com>

Thanks for reworking this series!

The bad state correctly BUGs under the LKDTM test:

[   21.171586] lkdtm: Performing direct entry CORRUPT_USER_DS
[   21.172791] lkdtm: setting bad task size limit
[   21.173742] ------------[ cut here ]------------
[   21.174641] kernel BUG at ./include/linux/syscalls.h:220!
...
[   21.193166] Call Trace:
[   21.193617]  ? trace_hardirqs_on_thunk+0x1a/0x1c
[   21.194443]  entry_SYSCALL64_slow_path+0x25/0x25


Tested-by: Kees Cook <keescook@...omium.org>

-Kees

-- 
Kees Cook
Pixel Security