From: Solar Designer <solar@openwall.com>
To: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Jann Horn <jannh@google.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
Dominik Brodowski <linux@dominikbrodowski.net>,
Kernel Hardening <kernel-hardening@lists.openwall.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] x86/asm: Pin sensitive CR0 bits
Date: Wed, 27 Feb 2019 11:44:07 +0100 [thread overview]
Message-ID: <20190227104407.GA18804@openwall.com> (raw)
In-Reply-To: <20190226233647.28547-2-keescook@chromium.org>
On Tue, Feb 26, 2019 at 03:36:45PM -0800, Kees Cook wrote:
> static inline void native_write_cr0(unsigned long val)
> {
> - asm volatile("mov %0,%%cr0": : "r" (val), "m" (__force_order));
> + bool warn = false;
> +
> +again:
> + val |= X86_CR0_WP;
> + /*
> + * In order to have the compiler not optimize away the check
> + * in the WARN_ONCE(), mark "val" as being also an output ("+r")
This comment is now slightly out of date: the check is no longer "in the
WARN_ONCE()". Ditto about the comment for CR4.
> + * by this asm() block so it will perform an explicit check, as
> + * if it were "volatile".
> + */
> + asm volatile("mov %0,%%cr0": "+r" (val) : "m" (__force_order) : );
> + /*
> + * If the MOV above was used directly as a ROP gadget we can
> + * notice the lack of pinned bits in "val" and start the function
> + * from the beginning to gain the WP bit for sure. And do it
> + * without first taking the exception for a WARN().
> + */
> + if ((val & X86_CR0_WP) != X86_CR0_WP) {
> + warn = true;
> + goto again;
> + }
> + WARN_ONCE(warn, "Attempt to unpin X86_CR0_WP, cr0 bypass attack?!\n");
> }
Alexander
next prev parent reply other threads:[~2019-02-27 10:44 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-26 23:36 [PATCH 0/3] x86/asm: More pinning Kees Cook
2019-02-26 23:36 ` [PATCH 1/3] x86/asm: Pin sensitive CR0 bits Kees Cook
2019-02-27 10:44 ` Solar Designer [this message]
2019-02-27 19:45 ` Kees Cook
2019-02-26 23:36 ` [PATCH 2/3] x86/asm: Avoid taking an exception before cr4 restore Kees Cook
2019-02-26 23:36 ` [PATCH 3/3] lkdtm: Check for SMEP clearing protections Kees Cook
2019-02-26 23:40 ` Kees Cook
2019-02-26 23:45 ` [PATCH v2] " Kees Cook
2019-02-27 8:20 ` [PATCH 0/3] x86/asm: More pinning Greg KH
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