Date: Fri, 11 Jan 2019 12:42:14 -0800 From: Dave Hansen <dave.hansen@...el.com> To: Khalid Aziz <khalid.aziz@...cle.com>, juergh@...il.com, tycho@...ho.ws, jsteckli@...zon.de, ak@...ux.intel.com, torvalds@...ux-foundation.org, liran.alon@...cle.com, keescook@...gle.com, konrad.wilk@...cle.com Cc: deepa.srinivasan@...cle.com, chris.hyser@...cle.com, tyhicks@...onical.com, dwmw@...zon.co.uk, andrew.cooper3@...rix.com, jcm@...hat.com, boris.ostrovsky@...cle.com, kanth.ghatraju@...cle.com, joao.m.martins@...cle.com, jmattson@...gle.com, pradeep.vincent@...cle.com, john.haxby@...cle.com, tglx@...utronix.de, kirill.shutemov@...ux.intel.com, hch@....de, steven.sistare@...cle.com, kernel-hardening@...ts.openwall.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame Ownership >> 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.
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