This is the backport of recently upstreamed series that moves VERW execution to a later point in exit-to-user path. This is needed because in some cases it may be possible for data accessed after VERW executions may end into MDS affected CPU buffers. Moving VERW closer to ring transition reduces the attack surface.
Patch 1/6 includes a minor fix that is queued for upstream: https://lore.kernel.org/lkml/170899674562.398.6398007479766564897.tip-bot2@t...
Patch 1,2,5 and 6 needed conflict resolution.
I saw a few new warnings:
arch/x86/entry/entry.o: warning: objtool: mds_verw_sel+0x0: unreachable instruction
I tried using REACHABLE, but that did not fix the warning.
For the below warning:
vmlinux.o: warning: objtool: .altinstr_replacement+0x17: unsupported relocation in alternatives section
not sure if this is related to this series or a pre-existing warning, I will check later without this series.
I am not too concerned because the alternative did substitute verw correctly:
entry_SYSCALL_64: ... 0xffffffff8200013d <+253>: swapgs 0xffffffff82000140 <+256>: verw 0xffffffff82000000 0xffffffff82000148 <+264>: sysretq 0xffffffff8200014b <+267>: int3
--- Pawan Gupta (5): x86/bugs: Add asm helpers for executing VERW x86/entry_64: Add VERW just before userspace transition x86/entry_32: Add VERW just before userspace transition x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key KVM/VMX: Move VERW closer to VMentry for MDS mitigation
Sean Christopherson (1): KVM/VMX: Use BT+JNC, i.e. EFLAGS.CF to select VMRESUME vs. VMLAUNCH
Documentation/x86/mds.rst | 38 +++++++++++++++++++++++++----------- arch/x86/entry/entry.S | 22 +++++++++++++++++++++ arch/x86/entry/entry_32.S | 3 +++ arch/x86/entry/entry_64.S | 11 +++++++++++ arch/x86/entry/entry_64_compat.S | 1 + arch/x86/include/asm/cpufeatures.h | 2 +- arch/x86/include/asm/entry-common.h | 1 - arch/x86/include/asm/nospec-branch.h | 25 ++++++++++++------------ arch/x86/kernel/cpu/bugs.c | 15 ++++++-------- arch/x86/kernel/nmi.c | 3 --- arch/x86/kvm/vmx/run_flags.h | 7 +++++-- arch/x86/kvm/vmx/vmenter.S | 9 ++++++--- arch/x86/kvm/vmx/vmx.c | 12 ++++++++---- 13 files changed, 103 insertions(+), 46 deletions(-) --- base-commit: 81e1dc2f70014b9523dd02ca763788e4f81e5bac change-id: 20240226-delay-verw-backport-6-1-y-4b0cec84087c