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Thu, 23 Jul 2020 20:15:20 +0000 (UTC) X-HE-Tag: ray83_290c21826f41 X-Filterd-Recvd-Size: 4838 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by imf22.hostedemail.com (Postfix) with ESMTP for ; Thu, 23 Jul 2020 20:15:19 +0000 (UTC) From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1595535317; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=wLeYvK4WU299PQEtv/NKty1ri+ij6okAVbv/cvUrEeY=; b=oyPtyDLEyrQxVGt8iJo2xBVK2GuF7oNCZSR0MZXlO55sJxMcbRwOTcx+pZMm8iupNKXkY6 cmNtzBQ14NEZrpefB9QYqESp/CsQv15uuPoDty1IWmk7EkFqw0BvF+SMFXwhgJXdEOq5rA 0oXjb1YnIKnXU+YnlCK6ARyPtSRZaG5FTFcxDobkytwRsenOVpi32CpfiCGflKlCMd97Xv K66n+f+ml/uyyisyllGyCA0lEfZgGSX8JWy+xVHuPNfppCO6WqD6LYPsdk6weGw638QfaJ npBu2XCpFllhkCWgpy53paqFUouUnngUtCxG+BVCjd1gsWXBw1/morHz26HXVw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1595535317; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=wLeYvK4WU299PQEtv/NKty1ri+ij6okAVbv/cvUrEeY=; b=xMq58S6bjjF6OmxCQMrCWKgry7jvqsd5b97YDXj6lRB8oTRACn+ydkU/iCO6YRSjFiVyVw OKqEl3CJJ8XV30CQ== To: Ira Weiny , Peter Zijlstra Cc: 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 In-Reply-To: <87o8o6vvt0.fsf@nanos.tec.linutronix.de> 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> <87o8o6vvt0.fsf@nanos.tec.linutronix.de> Date: Thu, 23 Jul 2020 22:15:17 +0200 Message-ID: <87lfjavvhm.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: 3D5D7180BE972 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 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: Thomas Gleixner writes: > Ira Weiny writes: >> On Fri, Jul 17, 2020 at 12:06:10PM +0200, Peter Zijlstra wrote: >>> On Fri, Jul 17, 2020 at 12:20:56AM -0700, ira.weiny@intel.com 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...) > > Well, if any exception which calls idtentry_enter() would return without > going through idtentry_exit() then lots of bad stuff would happen even > without your patches. > >> Also is there any chance that the process could be getting scheduled and that >> is causing an issue? > > Only from #PF, but after the fault has been resolved and the tasks is > scheduled in again then the task returns through idtentry_exit() to the > place where it took the fault. That's not guaranteed to be on the same > CPU. If schedule is not aware of the fact that the exception turned off > stuff then you surely get into trouble. So you really want to store it > in the task itself then the context switch code can actually see the > state and act accordingly. Actually thats nasty as well as you need a stack of PKRS values to handle nested exceptions. But it might be still the most reasonable thing to do. 7 PKRS values plus an index should be really sufficient, that's 32bytes total, not that bad. Thanks, tglx