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11 Jan 2019 12:42:15 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.56,466,1539673200"; d="scan'208";a="266466292" Received: from ray.jf.intel.com (HELO [10.7.201.19]) ([10.7.201.19]) by orsmga004.jf.intel.com with ESMTP; 11 Jan 2019 12:42:14 -0800 Subject: Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame Ownership To: Khalid Aziz , juergh@gmail.com, tycho@tycho.ws, jsteckli@amazon.de, ak@linux.intel.com, torvalds@linux-foundation.org, liran.alon@oracle.com, keescook@google.com, konrad.wilk@oracle.com Cc: deepa.srinivasan@oracle.com, chris.hyser@oracle.com, tyhicks@canonical.com, dwmw@amazon.co.uk, andrew.cooper3@citrix.com, jcm@redhat.com, boris.ostrovsky@oracle.com, kanth.ghatraju@oracle.com, joao.m.martins@oracle.com, jmattson@google.com, pradeep.vincent@oracle.com, john.haxby@oracle.com, tglx@linutronix.de, kirill.shutemov@linux.intel.com, hch@lst.de, steven.sistare@oracle.com, kernel-hardening@lists.openwall.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andy Lutomirski , Peter Zijlstra References: <31fe7522-0a59-94c8-663e-049e9ad2bff6@intel.com> <7e3b2c4b-51ff-2027-3a53-8c798c2ca588@oracle.com> From: Dave Hansen Openpgp: preference=signencrypt Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <7e3b2c4b-51ff-2027-3a53-8c798c2ca588@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >> The second process could easily have the page's old TLB entry. It could >> abuse that entry as long as that CPU doesn't context switch >> (switch_mm_irqs_off()) or otherwise flush the TLB entry. > > That is an interesting scenario. Working through this scenario, physmap > TLB entry for a page is flushed on the local processor when the page is > allocated to userspace, in xpfo_alloc_pages(). When the userspace passes > page back into kernel, that page is mapped into kernel space using a va > from kmap pool in xpfo_kmap() which can be different for each new > mapping of the same page. The physical page is unmapped from kernel on > the way back from kernel to userspace by xpfo_kunmap(). So two processes > on different CPUs sharing same physical page might not be seeing the > same virtual address for that page while they are in the kernel, as long > as it is an address from kmap pool. ret2dir attack relies upon being > able to craft a predictable virtual address in the kernel physmap for a > physical page and redirect execution to that address. Does that sound right? All processes share one set of kernel page tables. Or, did your patches change that somehow that I missed? Since they share the page tables, they implicitly share kmap*() mappings. kmap_atomic() is not *used* by more than one CPU, but the mapping is accessible and at least exists for all processors. I'm basically assuming that any entry mapped in a shared page table is exploitable on any CPU regardless of where we logically *want* it to be used.