From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A4891C18DF5 for ; Tue, 20 Nov 2018 12:43:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 69A9C206BB for ; Tue, 20 Nov 2018 12:43:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="r9nG6HiW" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 69A9C206BB Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727259AbeKTXMD (ORCPT ); Tue, 20 Nov 2018 18:12:03 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:59614 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725843AbeKTXMD (ORCPT ); Tue, 20 Nov 2018 18:12:03 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=bDC1jsxvFOd8I5xXFBDzemAk6occ6ogzcepkNcGkf7U=; b=r9nG6HiWuCsZQBRPL3NoAUKNO nwNXdYVo9SsmR7+Gf7fqwvCtJUA1SXaPrYbZR6irKlSxajJPwW5Rh/ODmQ1ZJ6xBn9lNAt+OzDHPE xYqf7fEzv9ssoqqx6JcHG8cAzImyekK7ubZ9bU26hukv2ZbScTCmqfqKR+R7Z84vgskp0fa/Z1Mod BRT48rGfLvQun+zMkoVLRnO48LGyfSIHCrmBA6+z4yEjOx3qcqg7m5A5TjQI7s0DWWqjKzv38kvgo sXXPZsWIPuSv+33ekrIdirTKVAxNHz+kVCrmt50nCUuvX3UaShkmKTl4i5eGVsVXHuyPlRfSS8jsV W2hwkQA2g==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gP5MP-0003Qm-V4; Tue, 20 Nov 2018 12:42:49 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 51D6F2029F87F; Tue, 20 Nov 2018 13:42:32 +0100 (CET) Date: Tue, 20 Nov 2018 13:42:32 +0100 From: Peter Zijlstra To: Nadav Amit Cc: Ingo Molnar , linux-kernel@vger.kernel.org, x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Borislav Petkov , Dave Hansen , linux_dti@icloud.com, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org Subject: Re: [PATCH v5 00/10] x86/alternative: text_poke() fixes Message-ID: <20181120124232.GK2131@hirez.programming.kicks-ass.net> References: <20181113130730.44844-1-namit@vmware.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181113130730.44844-1-namit@vmware.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 13, 2018 at 05:07:20AM -0800, Nadav Amit wrote: > v4->v5: > - Fix Xen breakage [Damian Tometzki] > - BUG_ON() when poking_mm initialization fails [PeterZ] > - Better comments on "x86/mm: temporary mm struct" > - Cleaner removal of the custom poker I'll re-iterate my position: it is impossible for the text not to match, and if it somehow does not match, something went sideways in an unrecoverably fashion. text_poke() must not fail, ever. If it does, our text is inconsistent and we must abort/panic/bug. The only way I will accept anything else is if someone can come up with a sensible scenario of text_poke() failing and recovering from it. AFAICT there is no possible way to gracefully recover. Consider a jump label with multiple patch sites; we patch the first, then fail. In order to restore to a sane state, we must undo the patching of the first, but undoing text_poke() fails again. Then what? Allowing text_poke() to fail only creates an unfixable mess. Esp. since there is no sane scenario under which is can fail. --- --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -695,7 +695,7 @@ void __init_or_module text_poke_early(vo __ro_after_init struct mm_struct *poking_mm; __ro_after_init unsigned long poking_addr; -static int __text_poke(void *addr, const void *opcode, size_t len) +static void __text_poke(void *addr, const void *opcode, size_t len) { bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE; temporary_mm_state_t prev; @@ -731,13 +731,10 @@ static int __text_poke(void *addr, const * The lock is not really needed, but this allows to avoid open-coding. */ ptep = get_locked_pte(poking_mm, poking_addr, &ptl); - /* - * If we failed to allocate a PTE, fail. This should *never* happen, - * since we preallocate the PTE. + * This must not fail; preallocated in poking_init(). */ - if (WARN_ON_ONCE(!ptep)) - goto out; + VM_BUG_ON(!ptep) pte = mk_pte(pages[0], PAGE_KERNEL); set_pte_at(poking_mm, poking_addr, ptep, pte); @@ -795,12 +792,14 @@ static int __text_poke(void *addr, const unuse_temporary_mm(prev); pte_unmap_unlock(ptep, ptl); -out: - if (memcmp(addr, opcode, len)) - r = -EFAULT; + + /* + * If the text doesn't match what we just wrote; something is + * fundamentally screwy, there's nothing we can really do about that. + */ + BUG_ON(memcmp(addr, opcode, len)); local_irq_restore(flags); - return r; } /** @@ -814,21 +813,10 @@ static int __text_poke(void *addr, const * in a way that permits an atomic write. It also makes sure we fit on a single * page. */ -int text_poke(void *addr, const void *opcode, size_t len) +void text_poke(void *addr, const void *opcode, size_t len) { - int r; - lockdep_assert_held(&text_mutex); - - r = __text_poke(addr, opcode, len); - - /* - * TODO: change the callers to consider the return value and remove this - * historical assertion. - */ - BUG_ON(r); - - return r; + __text_poke(addr, opcode, len); } /** @@ -847,7 +835,7 @@ int text_poke(void *addr, const void *op */ int text_poke_kgdb(void *addr, const void *opcode, size_t len) { - return __text_poke(addr, opcode, len); + __text_poke(addr, opcode, len); } static void do_sync_core(void *info) --- a/arch/x86/kernel/kgdb.c +++ b/arch/x86/kernel/kgdb.c @@ -767,10 +767,8 @@ int kgdb_arch_set_breakpoint(struct kgdb */ if (mutex_is_locked(&text_mutex)) return -EBUSY; - err = text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr, + text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE); - if (err) - return err; bpt->type = BP_POKE_BREAKPOINT; return err; @@ -788,11 +786,8 @@ int kgdb_arch_remove_breakpoint(struct k */ if (mutex_is_locked(&text_mutex)) goto knl_write; - err = text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr, - BREAK_INSTR_SIZE); - if (err) - goto knl_write; - return err; + text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE); + return 0; knl_write: return probe_kernel_write((char *)bpt->bpt_addr,