From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f200.google.com (mail-pg1-f200.google.com [209.85.215.200]) by kanga.kvack.org (Postfix) with ESMTP id 7BB966B3346 for ; Sat, 25 Aug 2018 00:23:51 -0400 (EDT) Received: by mail-pg1-f200.google.com with SMTP id o16-v6so7094457pgv.21 for ; Fri, 24 Aug 2018 21:23:51 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id t2-v6si9000522pgm.626.2018.08.24.21.23.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 24 Aug 2018 21:23:49 -0700 (PDT) Received: from mail-wm0-f52.google.com (mail-wm0-f52.google.com [74.125.82.52]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id CC7AC2174D for ; Sat, 25 Aug 2018 04:23:48 +0000 (UTC) Received: by mail-wm0-f52.google.com with SMTP id n11-v6so3362550wmc.2 for ; Fri, 24 Aug 2018 21:23:48 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <8E0D8C66-6F21-4890-8984-B6B3082D4CC5@gmail.com> References: <20180822153012.173508681@infradead.org> <20180822154046.823850812@infradead.org> <20180822155527.GF24124@hirez.programming.kicks-ass.net> <20180823134525.5f12b0d3@roar.ozlabs.ibm.com> <776104d4c8e4fc680004d69e3a4c2594b638b6d1.camel@au1.ibm.com> <20180823133958.GA1496@brain-police> <20180824084717.GK24124@hirez.programming.kicks-ass.net> <20180824180438.GS24124@hirez.programming.kicks-ass.net> <56A9902F-44BE-4520-A17C-26650FCC3A11@gmail.com> <9A38D3F4-2F75-401D-8B4D-83A844C9061B@gmail.com> <8E0D8C66-6F21-4890-8984-B6B3082D4CC5@gmail.com> From: Andy Lutomirski Date: Fri, 24 Aug 2018 21:23:26 -0700 Message-ID: Subject: Re: TLB flushes on fixmap changes Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Nadav Amit Cc: Linus Torvalds , Paolo Bonzini , Jiri Kosina , Masami Hiramatsu , Peter Zijlstra , Will Deacon , Benjamin Herrenschmidt , Nick Piggin , Andrew Lutomirski , the arch/x86 maintainers , Borislav Petkov , Rik van Riel , Jann Horn , Adin Scannell , Dave Hansen , Linux Kernel Mailing List , linux-mm , David Miller , Martin Schwidefsky , Michael Ellerman On Fri, Aug 24, 2018 at 7:29 PM, wrote: > > > On August 24, 2018 5:58:43 PM PDT, Linus Torvalds wrote: >>Adding a few people to the cc. >> >>On Fri, Aug 24, 2018 at 1:24 PM Nadav Amit >>wrote: >>> > >>> > Can you actually find something that changes the fixmaps after boot >>> > (again, ignoring kmap)? >>> >>> At least the alternatives mechanism appears to do so. >>> >>> IIUC the following path is possible when adding a module: >>> >>> jump_label_add_module() >>> ->__jump_label_update() >>> ->arch_jump_label_transform() >>> ->__jump_label_transform() >>> ->text_poke_bp() >>> ->text_poke() >>> ->set_fixmap() >> >>Yeah, that looks a bit iffy. >> >>But making the tlb flush global wouldn't help. This is running on a >>local core, and if there are other CPU's that can do this at the same >>time, then they'd just fight about the same mapping. >> >>Honestly, I think it's ok just because I *hope* this is all serialized >>anyway (jump_label_lock? But what about other users of text_poke?). > > The users should hold text_mutex. > >> >>But I'd be a lot happier about it if it either used an explicit lock >>to make sure, or used per-cpu fixmap entries. > > My concern is that despite the lock, one core would do a speculative page walk and cache a translation that soon after would become stale. > >> >>And the tlb flush is done *after* the address is used, which is bogus >>anyway. > > It seems to me that it is intended to remove the mapping that might be a security issue. > > But anyhow, set_fixmap and clear_fixmap perform a local TLB flush, (in __set_pte_vaddr()) so locally things should be fine. > >> >>> And a similar path can happen when static_key_enable/disable() is >>called. >> >>Same comments. >> >>How about replacing that >> >> local_irq_save(flags); >> ... do critical things here ... >> local_irq_restore(flags); >> >>in text_poke() with >> >> static DEFINE_SPINLOCK(poke_lock); >> >> spin_lock_irqsave(&poke_lock, flags); >> ... do critical things here ... >> spin_unlock_irqrestore(&poke_lock, flags); >> >>and moving the local_flush_tlb() to after the set_fixmaps, but before >>the access through the virtual address. >> >>But changing things to do a global tlb flush would just be wrong. > > As I noted, I think that locking and local flushes as they are right now are fine (besides the redundant flush). > > My concern is merely that speculative page walks on other cores would cache stale entries. > > This is almost certainly a bug, or even two bugs. Bug 1: why on Earth do we flush in __set_pte_vaddr()? We should flush when *clearing* or when modifying an existing fixmap entry. Right now, if we do text_poke() after boot, then the TLB entry will stick around and will be a nice exploit target. Bug 2: what you're describing. It's racy. Couldn't text_poke() use kmap_atomic()? Or, even better, just change CR3?