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Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jbKzH-0004Hc-15; Wed, 20 May 2020 09:26:07 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jbKzE-0004H8-QQ for linux-arm-kernel@lists.infradead.org; Wed, 20 May 2020 09:26:06 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 89FD030E; Wed, 20 May 2020 02:26:03 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 390563F68F; Wed, 20 May 2020 02:26:02 -0700 (PDT) Date: Wed, 20 May 2020 10:26:00 +0100 From: Dave Martin To: Will Deacon Subject: Re: [PATCH v6] arm64: Expose FAR_EL1 tag bits in sigcontext Message-ID: <20200520092558.GI5031@arm.com> References: <20200508020106.136652-1-pcc@google.com> <20200513180914.50892-1-pcc@google.com> <20200513202808.GY21779@arm.com> <20200518095313.GZ21779@arm.com> <20200520085502.GC24293@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200520085502.GC24293@willie-the-truck> User-Agent: Mutt/1.5.23 (2014-03-12) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200520_022604_941709_EFDA0EFA X-CRM114-Status: GOOD ( 30.03 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Andrey Konovalov , Kevin Brodsky , Kostya Serebryany , Linux ARM , Catalin Marinas , Vincenzo Frascino , Peter Collingbourne , Evgenii Stepanov , Richard Henderson Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, May 20, 2020 at 09:55:03AM +0100, Will Deacon wrote: > On Tue, May 19, 2020 at 03:00:12PM -0700, Peter Collingbourne wrote: > > On Mon, May 18, 2020 at 2:53 AM Dave Martin wrote: > > > On Thu, May 14, 2020 at 05:58:21PM -0700, Peter Collingbourne wrote: > > > > diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c > > > > index baa88dc02e5c..5867f2fdbe64 100644 > > > > --- a/arch/arm64/kernel/signal.c > > > > +++ b/arch/arm64/kernel/signal.c > > > > @@ -648,6 +648,7 @@ static int setup_sigframe(struct > > > > rt_sigframe_user_layout *user, > > > > __put_user_error(ESR_MAGIC, &esr_ctx->head.magic, err); > > > > __put_user_error(sizeof(*esr_ctx), &esr_ctx->head.size, err); > > > > __put_user_error(current->thread.fault_code, > > > > &esr_ctx->esr, err); > > > > + current->thread.fault_code = 0; > > > > > > Perhaps, but we'd need to be careful. For example, can we run out of > > > user stack before this and deliver a SIGSEGV, but with the old > > > fault_code still set? Then we'd emit the old fault code with the > > > new "can't deliver signal" signal, which doesn't make sense. > > > > > > Stuff may also go wrong with signal prioritisation. > > > > > > If a higher-priority signal (say SIGINT) comes in after a data abort > > > enters the kernel but before the resulting SIGSEGV is dequeued for > > > delivery, wouldn't we deliver SIGINT first, with the bogus fault code? > > > With your change we'd then have cleared the fault code by the time we > > > deliver the SIGSEGV it actually relates to, if I've understood right. > > > > > > Today, I think we just attach that fault code to every signal that's > > > delivered until something overwrites or resets it, which means that > > > a signal that needs fault_code gets it, at the expense of attaching > > > it to a bunch of other random signals too. > > > > > > > > > Checking the signal number and si_code might help us to know what we > > > should be doing with fault_code. We need to have sure userspace can't > > > trick us with a non kernel generated signal here. It would also be > > > necessary to check how PTRACE_SETSIGINFO interacts with this. > > > > With these possible interactions in mind I think we should store the > > fault code and fault address in kernel_siginfo instead of > > thread_struct (and clear these fields when we receive a siginfo from > > userspace, i.e. in copy_siginfo_from_user which is used by > > ptrace(PTRACE_SETSIGINFO) among other places). That way, the > > information is clearly associated with the signal itself and not the > > thread, so we don't need to worry about our signal being delivered out > > of order. > > Hmm, I can't see a way to do that that isn't horribly invasive in the core > signal code. Can you? > > But generally, I agree: the per-thread handling of fault_address and > fault_code appears to be quite broken in the face of signal prioritisation > and signals that don't correspond directly to hardware trap. It would be > nice to have some tests for this... > > If we want to pile on more bodges, perhaps we could stash the signal number > to which the fault_{address,code} relate, and then check that at delivery > and clear on a match. I hate it. I agree with Daniel's suggestion in principle, but I was also concerned about whether it would be too invasive elsewhere. Question though: does the core code take special care to make sure that a force_sig cannot be outprioritised by a regular signal? If so, perhaps we get away with it. I ask this, because the same same issue may be hitting other arches otherwise. Cheers ---Dave _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel