From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f70.google.com (mail-oi0-f70.google.com [209.85.218.70]) by kanga.kvack.org (Postfix) with ESMTP id BB7D16B3D3F for ; Sun, 26 Aug 2018 18:48:41 -0400 (EDT) Received: by mail-oi0-f70.google.com with SMTP id p11-v6so12999311oih.17 for ; Sun, 26 Aug 2018 15:48:41 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id e22-v6sor8113027oib.39.2018.08.26.15.48.40 for (Google Transport Security); Sun, 26 Aug 2018 15:48:40 -0700 (PDT) MIME-Version: 1.0 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> <20180826112341.f77a528763e297cbc36058fa@kernel.org> In-Reply-To: From: Jann Horn Date: Mon, 27 Aug 2018 00:48:13 +0200 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: Andy Lutomirski , Kees Cook Cc: mhiramat@kernel.org, Nadav Amit , Linus Torvalds , Paolo Bonzini , jkosina@suse.cz, Peter Zijlstra , Will Deacon , benh@au1.ibm.com, npiggin@gmail.com, the arch/x86 maintainers , Borislav Petkov , Rik van Riel , Adin Scannell , Dave Hansen , kernel list , Linux-MM , "David S. Miller" , Martin Schwidefsky , Michael Ellerman On Sun, Aug 26, 2018 at 6:21 AM Andy Lutomirski wrote: > > On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu wrote: > > On Fri, 24 Aug 2018 21:23:26 -0700 > > Andy Lutomirski wrote: > >> Couldn't text_poke() use kmap_atomic()? Or, even better, just change CR3? > > > > No, since kmap_atomic() is only for x86_32 and highmem support kernel. > > In x86-64, it seems that returns just a page address. That is not > > good for text_poke, since it needs to make a writable alias for RO > > code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache? > > > > I just re-read text_poke(). It's, um, horrible. Not only is the > implementation overcomplicated and probably buggy, but it's SLOOOOOW. > It's totally the wrong API -- poking one instruction at a time > basically can't be efficient on x86. The API should either poke lots > of instructions at once or should be text_poke_begin(); ...; > text_poke_end();. > > Anyway, the attached patch seems to boot. Linus, Kees, etc: is this > too scary of an approach? With the patch applied, text_poke() is a > fantastic exploit target. On the other hand, even without the patch > applied, text_poke() is every bit as juicy. Twiddling CR0.WP is incompatible with Xen PV, right? It can't let you do it because you're not actually running in ring 0 (but in ring 1 or 3), so CR0.WP has no influence on what you can access; and it must not let you bypass write protection because you have read-only access to host page tables. I think this code has to be compatible with Xen PV, right? In theory Xen PV could support this by emulating X86 instructions, but I don't see anything related to CR0.WP in their emulation code. From xen/arch/x86/pv/emul-priv-op.c: case 0: /* Write CR0 */ if ( (val ^ read_cr0()) & ~X86_CR0_TS ) { gdprintk(XENLOG_WARNING, "Attempt to change unmodifiable CR0 flags\n"); break; } do_fpu_taskswitch(!!(val & X86_CR0_TS)); return X86EMUL_OKAY; Having a special fallback path for "patch kernel code while running under Xen PV" would be kinda ugly.