ACK/Cmnt: [SRU][J][PATCH 0/2] A general-proteciton exception during guest migration to unsupported PKRU machine

[Stefan Bader]

On 16.03.24 04:43, Chengen Du wrote:
> BugLink: https://bugs.launchpad.net/bugs/2032164
> 
> SRU Justification:
> 
> [Impact]
> When a host that supports PKRU initiates a guest that lacks PKRU support, the flag is enabled on the guest's fpstate.
> This information is then passed to userspace through the vcpu ioctl KVM_GET_XSAVE.
> However, a problem arises when the user opts to migrate the mentioned guest to another machine that does not support PKRU.
> In this scenario, the new host attempts to restore the guest's fpu registers.
> Nevertheless, due to the absence of PKRU support on the new host, a general-protection exception takes place, leading to a guest crash.
> 
> [Fix]
> The problem is resolved by the following upstream commit:
> 18164f66e6c5 x86/fpu: Allow caller to constrain xfeatures when copying to uabi buffer
> 8647c52e9504 KVM: x86: Constrain guest-supported xfeatures only at KVM_GET_XSAVE{2}
> 
> [Test Plan]
> Several scenarios need to be conducted to confirm the migration outcome.
> 	Patched kernel with PKRU -> kernel with PKRU
> 	Patched kernel with PKRU -> kernel without PKRU
> 	Patched kernel without PKRU -> kernel with PKRU
> 	Patched kernel without PKRU -> kernel without PKRU
> 	Kernel with PKRU -> patched kernel with PKRU
> 	Kernel with PKRU -> patched kernel without PKRU
> 	Kernel without PKRU -> patched kernel with PKRU
> 	Kernel without PKRU -> patched kernel without PKRU
> 	Patched kernel with PKRU -> patched kernel without PKRU
> 
> Each scenarios shall succeed except "Kernel with PKRU -> patched kernel without PKRU" one.
> Addressing this case poses challenges because the most plausible solution is to clamp the FPU features at the destination during migration.
> However, upstream does not support this approach due to concerns about silently dropping features requested by userspace.
> This could potentially lead to other issues and violate KVM's ABI.
> 
> [Where problems could occur]
> The introduced commits will impact the guest migration process,
> potentially leading to failures and preventing the guest from operating successfully on the migration destination.
> 
> Sean Christopherson (2):
>    x86/fpu: Allow caller to constrain xfeatures when copying to uabi
>      buffer
>    KVM: x86: Constrain guest-supported xfeatures only at KVM_GET_XSAVE{2}
> 
>   arch/x86/include/asm/fpu/api.h |  3 ++-
>   arch/x86/kernel/fpu/core.c     |  5 +++--
>   arch/x86/kernel/fpu/xstate.c   |  7 +++++--
>   arch/x86/kernel/fpu/xstate.h   |  3 ++-
>   arch/x86/kvm/x86.c             | 33 +++++++++++++++++++++++++++------
>   5 files changed, 39 insertions(+), 12 deletions(-)
> 

As this was tested beforehand with good results.

Acked-by: Stefan Bader <stefan.bader at canonical.com>
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