ping Re: [PATCH v4 0/2] x86: Fix ARCH_REQ_XCOMP_PERM and update the test

[Paolo Bonzini]

Thomas, Dave,

can this series be included in 5.18 and CCed to stable?

The bug makes the __state_perm field completely wrong.  As a result, 
arch_prctl(ARCH_GET_XCOMP_PERM) only returns the features that were 
requested last, forgetting what was already in __state_perm (the 
"permitted" argument to __xstate_request_perm).

In KVM, it is a bit worse.  It affects 
arch_prctl(ARCH_GET_XCOMP_GUEST_PERM) in the same way and also 
ioctl(KVM_GET_SUPPORTED_CPUID), but the bug can also make KVM return the 
wrong xsave state size to userspace.  It's likely to go unnoticed by 
userspace until Intel adds non-dynamic states above a dynamic one, but 
potentially userspace could allocate a buffer that is too small.

Paolo

On 1/29/22 18:36, Chang S. Bae wrote:
> Changes from V3:
> * Rebased onto 5.17-rc1.
>
> V3: https://lore.kernel.org/lkml/20211110003209.21666-1-chang.seok.bae@xxxxxxxxx/
>
> ---
>
> The recent x86 dynamic state support incorporates the arch_prctl option to
> request permission before using a dynamic state.
>
> It was designed to add the requested feature in the group leader's
> permission bitmask so that every thread can reference this master bitmask.
> The group leader is assumed to be unchanged here. The mainline is the case
> as a group leader is identified at fork() or exec() time only.
>
> This master bitmask should include non-dynamic features always, as they
> are permitted by default. Users may check them via ARCH_GET_XCOMP_PERM.
>
> But, in hindsight, the implementation does overwrite the bitmask with the
> requested bit only, instead of adding the bit to the existing one. This
> overwrite effectively revokes the permission that is granted already.
>
> Fix the code and also update the selftest to disclose the issue if there
> is.
>
> Chang S. Bae (1):
>    selftests/x86/amx: Update the ARCH_REQ_XCOMP_PERM test
>
> Yang Zhong (1):
>    x86/arch_prctl: Fix the ARCH_REQ_XCOMP_PERM implementation
>
>   arch/x86/kernel/fpu/xstate.c      |  2 +-
>   tools/testing/selftests/x86/amx.c | 16 ++++++++++++++--
>   2 files changed, 15 insertions(+), 3 deletions(-)
>
>
> base-commit: e783362eb54cd99b2cac8b3a9aeac942e6f6ac07