linux-kernel - Re: [PATCH 00/11 v3] x86: load FPU registers on return to userland

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Message-ID: <20181005115514.acxsyghmzlhhqjx7@linutronix.de>
Date:   Fri, 5 Oct 2018 13:55:14 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 00/11 v3] x86: load FPU registers on return to userland

On 2018-10-04 12:45:08 [-0400], Rik van Riel wrote:
> Wait, so any thread can bypass its memory protection
> keys, even if there is a seccomp filter preventing
> it from calling the PKRU syscalls?

We have SYS_pkey_alloc +free and SYS_pkey_mprotect. For read/ write of
the register value, libc is using and opcodes.

> Is that intended?

Either that or it ended like that because someone failed to attend a
meeting where this was discussed. Here is something from pkeys(7):

| Protection  keys  have  the  potential  to  add  a  layer  of security and
| reliability to applications.  But they have not been primarily designed as a
| security feature.  For instance, WRPKRU is a completely unprivileged
| instruction, so pkeys are useless in any case that an attacker controls the
| PKRU register or can execute arbitrary instructions.

Sebastian