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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 43D43C433ED for ; Tue, 13 Apr 2021 18:00:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 151756121E for ; Tue, 13 Apr 2021 18:00:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347417AbhDMSA6 (ORCPT ); Tue, 13 Apr 2021 14:00:58 -0400 Received: from mail.kernel.org ([198.145.29.99]:47172 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237095AbhDMSA5 (ORCPT ); Tue, 13 Apr 2021 14:00:57 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id AAB07613B6; Tue, 13 Apr 2021 18:00:35 +0000 (UTC) Date: Tue, 13 Apr 2021 19:00:33 +0100 From: Catalin Marinas To: Liam Howlett Cc: Andre Przywara , Will Deacon , Peter Collingbourne , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "netdev@vger.kernel.org" , "bpf@vger.kernel.org" , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Amit Daniel Kachhap Subject: Re: [PATCH] arch/arm64/kernel/traps: Use find_vma_intersection() in traps for setting si_code Message-ID: <20210413180030.GA31164@arm.com> References: <20210407150940.542103-1-Liam.Howlett@Oracle.com> <20210412174343.GG2060@arm.com> <20210413143035.7zrct6a3up42uaoo@revolver> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210413143035.7zrct6a3up42uaoo@revolver> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, Apr 13, 2021 at 04:52:34PM +0000, Liam Howlett wrote: > * Catalin Marinas [210412 13:44]: > > On Wed, Apr 07, 2021 at 03:11:06PM +0000, Liam Howlett wrote: > > > find_vma() will continue to search upwards until the end of the virtual > > > memory space. This means the si_code would almost never be set to > > > SEGV_MAPERR even when the address falls outside of any VMA. The result > > > is that the si_code is not reliable as it may or may not be set to the > > > correct result, depending on where the address falls in the address > > > space. > > > > > > Using find_vma_intersection() allows for what is intended by only > > > returning a VMA if it falls within the range provided, in this case a > > > window of 1. > > > > > > Signed-off-by: Liam R. Howlett > > > --- > > > arch/arm64/kernel/traps.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c > > > index a05d34f0e82a..a44007904a64 100644 > > > --- a/arch/arm64/kernel/traps.c > > > +++ b/arch/arm64/kernel/traps.c > > > @@ -383,9 +383,10 @@ void force_signal_inject(int signal, int code, unsigned long address, unsigned i > > > void arm64_notify_segfault(unsigned long addr) > > > { > > > int code; > > > + unsigned long ut_addr = untagged_addr(addr); > > > > > > mmap_read_lock(current->mm); > > > - if (find_vma(current->mm, untagged_addr(addr)) == NULL) > > > + if (find_vma_intersection(current->mm, ut_addr, ut_addr + 1) == NULL) > > > code = SEGV_MAPERR; > > > else > > > code = SEGV_ACCERR; [...] > > I don't think your change is entirely correct either. We can have a > > fault below the vma of a stack (with VM_GROWSDOWN) and > > find_vma_intersection() would return NULL but it should be a SEGV_ACCERR > > instead. > > I'm pretty sure I am missing something. From what you said above, I > think this means that there can be a user cache fault below the stack > which should notify the user application that they are not allowed to > expand the stack by sending a SIGV_ACCERR in the si_code? Is this > expected behaviour or am I missing a code path to this function? My point was that find_vma() may return a valid vma where addr < vm_end but also addr < vm_addr. It's the responsibility of the caller to check that that vma can be expanded (VM_GROWSDOWN) and we do something like this in __do_page_fault(). find_vma_intersection(), OTOH, requires addr >= vm_start. If we hit this case (addr < vm_start), normally we'd first need to check whether it's expandable and, if not, return MAPERR. If it's expandable, it should be ACCERR since something else caused the fault. Now, I think at least for user_cache_maint_handler(), we can assume that __do_page_fault() handled any expansion already, so we don't need to check it here. In this case, your find_vma_intersection() check should work. Are there other cases where we invoke arm64_notify_segfault() without a prior fault? I think in swp_handler() we can bail out early before we even attempted the access so we may report MAPERR but ACCERR is a better indication. Also in sys_rt_sigreturn() we always call it as arm64_notify_segfault(regs->sp). I'm not sure that's correct in all cases, see restore_altstack(). I guess this code needs some tidying up. > > Maybe this should employ similar checks as __do_page_fault() (with > > expand_stack() and VM_GROWSDOWN). > > You mean the code needs to detect endianness and to check if this is an > attempt to expand the stack for both cases? Nothing to do with endianness, just the relation between the address and the vma->vm_start and whether the vma can be expanded down. -- Catalin