[v2,0/2] Relax canonical checks on some arch msrs

Message

Maxim Levitsky Aug. 2, 2024, 3:16 p.m. UTC

Recently we came up upon a failure where likely the guest writes
0xff4547ceb1600000 to MSR_KERNEL_GS_BASE and later on, qemu
sets this value via KVM_PUT_MSRS, and is rejected by the
kernel, likely due to not being canonical in 4 level paging.

I did some reverse engineering and to my surprise I found out
that both Intel and AMD have very loose checks in regard to
non canonical addresses written to this and several other msrs,
when the CPU supports 5 level paging.

Patch #1 addresses this, making KVM tolerate this.

Patch #2 is just a fix for a semi theoretical bug, found
while trying to debug the issue.

V2: addressed a very good feedback from Chao Gao. Thanks!

Best regards,
	Maxim Levitsky

Maxim Levitsky (2):
  KVM: x86: relax canonical check for some x86 architectural msrs
  KVM: SVM: fix emulation of msr reads/writes of MSR_FS_BASE and
    MSR_GS_BASE

 arch/x86/kvm/svm/svm.c | 12 ++++++++++++
 arch/x86/kvm/x86.c     | 11 ++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)