linux-kernel - Re: [PATCH v3 04/10] KVM: Implement kvm_put

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <e2abc69b-74c2-64ef-e270-43d93513eaae@arm.com>
Date:   Thu, 22 Aug 2019 16:46:10 +0100
From:   Steven Price <steven.price@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        linux-doc@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 04/10] KVM: Implement kvm_put_guest()

On 22/08/2019 16:28, Sean Christopherson wrote:
> On Wed, Aug 21, 2019 at 04:36:50PM +0100, Steven Price wrote:
>> kvm_put_guest() is analogous to put_user() - it writes a single value to
>> the guest physical address. The implementation is built upon put_user()
>> and so it has the same single copy atomic properties.
> 
> What you mean by "single copy atomic"?  I.e. what guarantees does
> put_user() provide that __copy_to_user() does not?

Single-copy atomicity is defined by the Arm architecture[1] and I'm not
going to try to go into the full details here, so this is a summary.

For the sake of this feature what we care about is that the value
written/read cannot be "torn". In other words if there is a read (in
this case from another VCPU) that is racing with the write then the read
will either get the old value or the new value. It cannot return a
mixture. (This is of course assuming that the read is using a
single-copy atomic safe method).

__copy_to_user() is implemented as a memcpy() and as such cannot provide
single-copy atomicity in the general case (the buffer could easily be
bigger than the architecture can guarantee).

put_user() on the other hand is implemented (on arm64) as an explicit
store instruction and therefore is guaranteed by the architecture to be
single-copy atomic (i.e. another CPU cannot see a half-written value).

Steve

[1] https://static.docs.arm.com/ddi0487/ea/DDI0487E_a_armv8_arm.pdf#page=110