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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 3D91AC63777 for ; Thu, 3 Dec 2020 11:56:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C5860208FE for ; Thu, 3 Dec 2020 11:56:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387978AbgLCL4Y (ORCPT ); Thu, 3 Dec 2020 06:56:24 -0500 Received: from ozlabs.org ([203.11.71.1]:42825 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387556AbgLCL4X (ORCPT ); Thu, 3 Dec 2020 06:56:23 -0500 Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 4CmvRd42wQz9ryj; Thu, 3 Dec 2020 22:55:41 +1100 (AEDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ellerman.id.au; s=201909; t=1606996541; bh=LVO4s9uIVveCgBubOjnui+gjJXlUQnDqrs3xghQjPUM=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=O5uBCrwFrtUjkNJP7Q8upqCfsLkZ3j11YiRaq284cVEQNcphSRlneehNq3Jlh1Cf2 6wrPIF25Fp9o2+u6PK83mudOUq5ZybC7X6j+MX60saikZFjPzSA5xeK4XtoQbBQEEt TUTgTF4u/uqu1XaEQ8PbIVnGFplvfmz6sveMdpQs1Y0yht0TBZy0ZP8V29KzxYu5hd IwkdO+U/XHs7kslQYBSqPg77l8Rn/Vly+Z8RusvNDKw4UAgUZ04JOzTRzT2SEDzZ51 pGFbmbgLXBaq5Qp4OL68NR4sP/RHaBXwIzFIjq/Awk0ASXlXbOhwioKBf6gjJRP6Wq +hQbSSxPOPk8A== From: Michael Ellerman To: Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH] powerpc/mm: Don't see NULL pointer dereference as a KUAP fault In-Reply-To: <8b865b93d25c15c8e6d41e71c368bfc28da4489d.1606816701.git.christophe.leroy@csgroup.eu> References: <8b865b93d25c15c8e6d41e71c368bfc28da4489d.1606816701.git.christophe.leroy@csgroup.eu> Date: Thu, 03 Dec 2020 22:55:40 +1100 Message-ID: <87mtyvumrn.fsf@mpe.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Christophe Leroy writes: > Sometimes, NULL pointer dereferences are expected. Even when they > are accidental they are unlikely an exploit attempt because the > first page is never mapped. The first page can be mapped if mmap_min_addr is 0. Blocking all faults to the first page would potentially break any program that does that. Also if there is something mapped at 0 it's a good chance it is an exploit attempt :) cheers > The exemple below shows what we get when invoking the "show task" > sysrq handler, by writing 't' in /proc/sysrq-trigger > > [ 1117.202054] ------------[ cut here ]------------ > [ 1117.202102] Bug: fault blocked by AP register ! > [ 1117.202261] WARNING: CPU: 0 PID: 377 at arch/powerpc/include/asm/nohash/32/kup-8xx.h:66 do_page_fault+0x4a8/0x5ec > [ 1117.202310] Modules linked in: > [ 1117.202428] CPU: 0 PID: 377 Comm: sh Tainted: G W 5.10.0-rc5-s3k-dev-01340-g83f53be2de31-dirty #4175 > [ 1117.202499] NIP: c0012048 LR: c0012048 CTR: 00000000 > [ 1117.202573] REGS: cacdbb88 TRAP: 0700 Tainted: G W (5.10.0-rc5-s3k-dev-01340-g83f53be2de31-dirty) > [ 1117.202625] MSR: 00021032 CR: 24082222 XER: 20000000 > [ 1117.202899] > [ 1117.202899] GPR00: c0012048 cacdbc40 c2929290 00000023 c092e554 00000001 c09865e8 c092e640 > [ 1117.202899] GPR08: 00001032 00000000 00000000 00014efc 28082224 100d166a 100a0920 00000000 > [ 1117.202899] GPR16: 100cac0c 100b0000 1080c3fc 1080d685 100d0000 100d0000 00000000 100a0900 > [ 1117.202899] GPR24: 100d0000 c07892ec 00000000 c0921510 c21f4440 0000005c c0000000 cacdbc80 > [ 1117.204362] NIP [c0012048] do_page_fault+0x4a8/0x5ec > [ 1117.204461] LR [c0012048] do_page_fault+0x4a8/0x5ec > [ 1117.204509] Call Trace: > [ 1117.204609] [cacdbc40] [c0012048] do_page_fault+0x4a8/0x5ec (unreliable) > [ 1117.204771] [cacdbc70] [c00112f0] handle_page_fault+0x8/0x34 > [ 1117.204911] --- interrupt: 301 at copy_from_kernel_nofault+0x70/0x1c0 > [ 1117.204979] NIP: c010dbec LR: c010dbac CTR: 00000001 > [ 1117.205053] REGS: cacdbc80 TRAP: 0301 Tainted: G W (5.10.0-rc5-s3k-dev-01340-g83f53be2de31-dirty) > [ 1117.205104] MSR: 00009032 CR: 28082224 XER: 00000000 > [ 1117.205416] DAR: 0000005c DSISR: c0000000 > [ 1117.205416] GPR00: c0045948 cacdbd38 c2929290 00000001 00000017 00000017 00000027 0000000f > [ 1117.205416] GPR08: c09926ec 00000000 00000000 3ffff000 24082224 > [ 1117.206106] NIP [c010dbec] copy_from_kernel_nofault+0x70/0x1c0 > [ 1117.206202] LR [c010dbac] copy_from_kernel_nofault+0x30/0x1c0 > [ 1117.206258] --- interrupt: 301 > [ 1117.206372] [cacdbd38] [c004bbb0] kthread_probe_data+0x44/0x70 (unreliable) > [ 1117.206561] [cacdbd58] [c0045948] print_worker_info+0xe0/0x194 > [ 1117.206717] [cacdbdb8] [c00548ac] sched_show_task+0x134/0x168 > [ 1117.206851] [cacdbdd8] [c005a268] show_state_filter+0x70/0x100 > [ 1117.206989] [cacdbe08] [c039baa0] sysrq_handle_showstate+0x14/0x24 > [ 1117.207122] [cacdbe18] [c039bf18] __handle_sysrq+0xac/0x1d0 > [ 1117.207257] [cacdbe48] [c039c0c0] write_sysrq_trigger+0x4c/0x74 > [ 1117.207407] [cacdbe68] [c01fba48] proc_reg_write+0xb4/0x114 > [ 1117.207550] [cacdbe88] [c0179968] vfs_write+0x12c/0x478 > [ 1117.207686] [cacdbf08] [c0179e60] ksys_write+0x78/0x128 > [ 1117.207826] [cacdbf38] [c00110d0] ret_from_syscall+0x0/0x34 > [ 1117.207938] --- interrupt: c01 at 0xfd4e784 > [ 1117.208008] NIP: 0fd4e784 LR: 0fe0f244 CTR: 10048d38 > [ 1117.208083] REGS: cacdbf48 TRAP: 0c01 Tainted: G W (5.10.0-rc5-s3k-dev-01340-g83f53be2de31-dirty) > [ 1117.208134] MSR: 0000d032 CR: 44002222 XER: 00000000 > [ 1117.208470] > [ 1117.208470] GPR00: 00000004 7fc34090 77bfb4e0 00000001 1080fa40 00000002 7400000f fefefeff > [ 1117.208470] GPR08: 7f7f7f7f 10048d38 1080c414 7fc343c0 00000000 > [ 1117.209104] NIP [0fd4e784] 0xfd4e784 > [ 1117.209180] LR [0fe0f244] 0xfe0f244 > [ 1117.209236] --- interrupt: c01 > [ 1117.209274] Instruction dump: > [ 1117.209353] 714a4000 418200f0 73ca0001 40820084 73ca0032 408200f8 73c90040 4082ff60 > [ 1117.209727] 0fe00000 3c60c082 386399f4 48013b65 <0fe00000> 80010034 3860000b 7c0803a6 > [ 1117.210102] ---[ end trace 1927c0323393af3e ]--- > > So, avoid the big KUAP warning by bailing out of bad_kernel_fault() > before calling bad_kuap_fault() when address references the first > page. > > Signed-off-by: Christophe Leroy > --- > arch/powerpc/mm/fault.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c > index 0add963a849b..be2b4318206f 100644 > --- a/arch/powerpc/mm/fault.c > +++ b/arch/powerpc/mm/fault.c > @@ -198,6 +198,10 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code, > { > int is_exec = TRAP(regs) == 0x400; > > + // Kernel fault on first page is likely a NULL pointer dereference > + if (address < PAGE_SIZE) > + return true; > + > /* NX faults set DSISR_PROTFAULT on the 8xx, DSISR_NOEXEC_OR_G on others */ > if (is_exec && (error_code & (DSISR_NOEXEC_OR_G | DSISR_KEYFAULT | > DSISR_PROTFAULT))) { > -- > 2.25.0