linux-kernel - Re: [PATCH v3 0/5] kvm: fix latent guest entry/exit bugs

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Message-ID: <980279cc-7836-e330-e435-4fbe5c361e0d@linux.ibm.com>
Date:   Tue, 1 Feb 2022 15:02:33 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Cc:     aleksandar.qemu.devel@...il.com, alexandru.elisei@....com,
        anup@...infault.org, aou@...s.berkeley.edu, atishp@...shpatra.org,
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        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
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Subject: Re: [PATCH v3 0/5] kvm: fix latent guest entry/exit bugs



Am 01.02.22 um 14:29 schrieb Mark Rutland:
> Several architectures have latent bugs around guest entry/exit,
> including:

Thanks for looking into this.

> 
> 1) Enabling interrupts during an RCU EQS, allowing interrupt handlers to
>     run without RCU watching.
> 
> 2) Using (potentially) instrumented code between guest_enter() and
>     guest_exit(), allowing instrumentation handlers to run without RCU
>     watching.
> 
> 3) Not informing lockdep and tracing about interrupt masking, or
>     informing in an incorrect order (e.g. relative to entering/exiting an
>     RCU EQS).
> 
> 4) Unbalanced entry/exit accounting in some cases (which may or may not
>     result in functional problems).
> 
> Overall, the architectures affected are:
> 
>    arm64, mips, powerpc, riscv, s390, x86
> 
> This series reworks the common code to make handling these issues
> earier, and for the following architectures fixes those issues by
> conversion to new helper functions:
> 
>    arm64, mips, riscv, x86
> 
> The core, arm64, and x86 patches have reviews from the relevant
> maintainers, and I think those are good-to-go. I have not yet had
> acks/reviews for the mips and riscv patches. I'm fairly certain the
> riscv patch is correct by virtue of it being so simple, and I'm
> relatively confident that the mips patch is correct (though I may have
> missed additional issues), but I have no way of testing either so I've
> placed them at the end of the series where they can easily be dropped if
> necessary.
> 
> This series does NOT fix the following architectures, which will need
> more substantial changes to architecture-specific entry logic and/or
> sequencing:
> 
>    powerpc, s390

Right, s390 is more complicated as we need to modify the page fault handling.
For the time being we should be as bad/good as before with the deprecated old
guest_enter/exit_irqoff. I will test this to be sure.


> 
> ... and I assume it would be preferable to fix the common code and
> simple cases now, such that those can be addressed in subsequent
> follow-ups.
> 
> Since v1 [1]:
> * Add arch_in_rcu_eqs()
> * Convert s390
> * Rename exit_to_guest_mode() -> guest_state_enter_irqoff()
> * Rename enter_from_guest_mode() -> guest_state_exit_irqoff()
> * Various commit message cleanups
> 
> Since v2 [2]:
> * Rebase to v5.17-rc2
> * Fixup mips exit handling
> * Drop arch_in_rcu_eqs() & s390 patches
> 
> I've pushed the series (based on v5.17-rc2) to my kvm/entry-rework branch:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=kvm/entry-rework
>    git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git kvm/entry-rework
> 
> This version of the series is tagged as kvm-entry-rework-20220201.
> 
> [1] https://lore.kernel.org/r/20220111153539.2532246-1-mark.rutland@arm.com/
> [2] https://lore.kernel.org/r/20220119105854.3160683-1-mark.rutland@arm.com/
> 
> Thanks,
> 
> 
> Mark Rutland (5):
>    kvm: add guest_state_{enter,exit}_irqoff()
>    kvm/arm64: rework guest entry logic
>    kvm/x86: rework guest entry logic
>    kvm/riscv: rework guest entry logic
>    kvm/mips: rework guest entry logic
> 
>   arch/arm64/kvm/arm.c     |  51 +++++++++++-------
>   arch/mips/kvm/mips.c     |  50 +++++++++++++++--
>   arch/riscv/kvm/vcpu.c    |  44 +++++++++------
>   arch/x86/kvm/svm/svm.c   |   4 +-
>   arch/x86/kvm/vmx/vmx.c   |   4 +-
>   arch/x86/kvm/x86.c       |   4 +-
>   arch/x86/kvm/x86.h       |  45 ----------------
>   include/linux/kvm_host.h | 112 +++++++++++++++++++++++++++++++++++++--
>   8 files changed, 222 insertions(+), 92 deletions(-)
>