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Wed, 22 Jul 2020 09:48:59 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 93C513011C6; Wed, 22 Jul 2020 11:48:53 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D378821A7D50E; Wed, 22 Jul 2020 11:48:53 +0200 (CEST) Date: Wed, 22 Jul 2020 11:48:53 +0200 From: Peter Zijlstra To: Ira Weiny Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andy Lutomirski , Dave Hansen , x86@kernel.org, Dan Williams , Vishal Verma , Andrew Morton , Fenghua Yu , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions Message-ID: <20200722094853.GM10769@hirez.programming.kicks-ass.net> References: <20200717072056.73134-1-ira.weiny@intel.com> <20200717072056.73134-18-ira.weiny@intel.com> <20200717100610.GH10769@hirez.programming.kicks-ass.net> <20200722052709.GB478587@iweiny-DESK2.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200722052709.GB478587@iweiny-DESK2.sc.intel.com> X-Rspamd-Queue-Id: 545A318FEA0A5 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jul 21, 2020 at 10:27:09PM -0700, Ira Weiny wrote: > I've been really digging into this today and I'm very concerned that I'm > completely missing something WRT idtentry_enter() and idtentry_exit(). > > I've instrumented idt_{save,restore}_pkrs(), and __dev_access_{en,dis}able() > with trace_printk()'s. > > With this debug code, I have found an instance where it seems like > idtentry_enter() is called without a corresponding idtentry_exit(). This has > left the thread ref counter at 0 which results in very bad things happening > when __dev_access_disable() is called and the ref count goes negative. > > Effectively this seems to be happening: > > ... > // ref == 0 > dev_access_enable() // ref += 1 ==> disable protection > // exception (which one I don't know) > idtentry_enter() > // ref = 0 > _handler() // or whatever code... > // *_exit() not called [at least there is no trace_printk() output]... > // Regardless of trace output, the ref is left at 0 > dev_access_disable() // ref -= 1 ==> -1 ==> does not enable protection > (Bad stuff is bound to happen now...) > ... > > The ref count ends up completely messed up after this sequence... and the PKRS > register gets out of sync as well. This is starting to make some sense of how > I was getting what seemed like random crashes before. > > Unfortunately I don't understand the idt entry/exit code well enough to see > clearly what is going on. Is there any reason idtentry_exit() is not called > after idtentry_enter()? Perhaps some NMI/MCE or 'not normal' exception code > path I'm not seeing? In my searches I see a corresponding exit for every > enter. But MCE is pretty hard to follow. > > Also is there any chance that the process could be getting scheduled and that > is causing an issue? Ooh, I think I see the problem. We also use idtentry_enter() for #PF, and #PF can schedule, exactly as you observed. Argh! This then means you need to go frob something in arch/x86/include/asm/idtentry.h, which is somewhat unfortunate. Thomas, would it make sense to add a type parameter to idtentry_{enter,exit}() ?