linux-kernel - Re: [PATCH 1/5] KVM: rseq: Update rseq when processing NOTIFY

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Message-ID: <425456d3-4772-2a1b-9cf3-a5b750b95c2e@redhat.com>
Date:   Mon, 6 Sep 2021 12:28:12 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <hca@...ux.ibm.com>, gor <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Oleg Nesterov <oleg@...hat.com>, rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        shuah <shuah@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-csky <linux-csky@...r.kernel.org>,
        linux-mips <linux-mips@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        Peter Foley <pefoley@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 1/5] KVM: rseq: Update rseq when processing NOTIFY_RESUME
 on xfer to KVM guest

On 20/08/21 20:51, Mathieu Desnoyers wrote:
>> Ah, or is it the case that rseq_cs is non-NULL if and only if userspace is in an
>> rseq critical section, and because syscalls in critical sections are illegal, by
>> definition clearing rseq_cs is a nop unless userspace is misbehaving.
> Not quite, as I described above. But we want it to stay set so the CONFIG_DEBUG_RSEQ
> code executed when returning from ioctl to userspace will be able to validate that
> it is not nested within a rseq critical section.
> 
>> If that's true, what about explicitly checking that at NOTIFY_RESUME?  Or is it
>> not worth the extra code to detect an error that will likely be caught anyways?
> The error will indeed already be caught on return from ioctl to userspace, so I
> don't see any added value in duplicating this check.

Sean, can you send a v2 (even for this patch only would be okay)?

Thanks,

Paolo