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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_MUTT 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 35134C43381 for ; Fri, 1 Mar 2019 15:33:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0126F2084D for ; Fri, 1 Mar 2019 15:33:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388938AbfCAPc7 (ORCPT ); Fri, 1 Mar 2019 10:32:59 -0500 Received: from mga01.intel.com ([192.55.52.88]:64184 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388109AbfCAPc7 (ORCPT ); Fri, 1 Mar 2019 10:32:59 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 01 Mar 2019 07:32:58 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.58,428,1544515200"; d="scan'208";a="278951009" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.181]) by orsmga004.jf.intel.com with ESMTP; 01 Mar 2019 07:32:58 -0800 Date: Fri, 1 Mar 2019 07:32:58 -0800 From: Sean Christopherson To: Jann Horn Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andrew Morton , Josh Poimboeuf , syzbot , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Masahiro Yamada , Michal Marek , linux-kbuild@vger.kernel.org Subject: Re: [PATCH 1/2] x86/unwind: handle NULL pointer calls better in frame unwinder Message-ID: <20190301153258.GD22584@linux.intel.com> References: <20190301031201.7416-1-jannh@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190301031201.7416-1-jannh@google.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 01, 2019 at 04:12:00AM +0100, Jann Horn wrote: > When the frame unwinder is invoked for an oops caused by a call to NULL, > it currently skips the parent function because BP still points to the > parent's stack frame; the (nonexistent) current function only has the first > half of a stack frame, and BP doesn't point to it yet. > > Add a special case for IP==0 that calculates a fake BP from SP, then uses > the real BP for the next frame. > > Note that this handles first_frame specially: We return information about > the parent function as long as the saved IP is >=first_frame, even if the > fake BP points below it. > > With an artificially-added NULL call in prctl_set_seccomp(), before this > patch, the trace is: > > Call Trace: > ? prctl_set_seccomp+0x3a/0x50 > __x64_sys_prctl+0x457/0x6f0 > ? __ia32_sys_prctl+0x750/0x750 > do_syscall_64+0x72/0x160 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > After this patch, the trace is: > > Call Trace: > prctl_set_seccomp+0x3a/0x50 > __x64_sys_prctl+0x457/0x6f0 > ? __ia32_sys_prctl+0x750/0x750 > do_syscall_64+0x72/0x160 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > Signed-off-by: Jann Horn > --- > arch/x86/include/asm/unwind.h | 6 ++++++ > arch/x86/kernel/unwind_frame.c | 25 ++++++++++++++++++++++--- > 2 files changed, 28 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h > index 1f86e1b0a5cd..499578f7e6d7 100644 > --- a/arch/x86/include/asm/unwind.h > +++ b/arch/x86/include/asm/unwind.h > @@ -23,6 +23,12 @@ struct unwind_state { > #elif defined(CONFIG_UNWINDER_FRAME_POINTER) > bool got_irq; > unsigned long *bp, *orig_sp, ip; > + /* > + * If non-NULL: The current frame is incomplete and doesn't contain a > + * valid BP. When looking for the next frame, use this instead of the > + * non-existent saved BP. > + */ > + unsigned long *next_bp; > struct pt_regs *regs; > #else > unsigned long *sp; > diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c > index 3dc26f95d46e..9b9fd4826e7a 100644 > --- a/arch/x86/kernel/unwind_frame.c > +++ b/arch/x86/kernel/unwind_frame.c > @@ -320,10 +320,14 @@ bool unwind_next_frame(struct unwind_state *state) > } > > /* Get the next frame pointer: */ > - if (state->regs) > + if (state->next_bp) { > + next_bp = state->next_bp; > + state->next_bp = NULL; > + } else if (state->regs) { > next_bp = (unsigned long *)state->regs->bp; > - else > + } else { > next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task, *state->bp); > + } > > /* Move to the next frame if it's safe: */ > if (!update_stack_state(state, next_bp)) > @@ -398,6 +402,21 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task, > > bp = get_frame_pointer(task, regs); > > + /* > + * If we crash with IP==0, the last successfully executed instruction > + * was probably an indirect function call with a NULL function pointer. > + * That means that SP points into the middle of an incomplete frame: > + * *SP is a return pointer, and *(SP-sizeof(unsigned long)) is where we > + * would have written a frame pointer if we hadn't crashed. > + * Pretend that the frame is complete and that BP points to it, but save > + * the real BP so that we can use it when looking for the next frame. > + */ > + if (regs && regs->ip == 0 && Would it make sense to do 'regs->ip < PAGE_SIZE', a la show_fault_oops()? E.g. to handle bugs where a function pointer gets loaded with NULL+offset. > + (unsigned long *)kernel_stack_pointer(regs) >= first_frame) { > + state->next_bp = bp; > + bp = ((unsigned long *)kernel_stack_pointer(regs)) - 1; > + } > + > /* Initialize stack info and make sure the frame data is accessible: */ > get_stack_info(bp, state->task, &state->stack_info, > &state->stack_mask); > @@ -410,7 +429,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task, > */ > while (!unwind_done(state) && > (!on_stack(&state->stack_info, first_frame, sizeof(long)) || > - state->bp < first_frame)) > + (state->next_bp == NULL && state->bp < first_frame))) > unwind_next_frame(state); > } > EXPORT_SYMBOL_GPL(__unwind_start); > -- > 2.21.0.352.gf09ad66450-goog >