[PATCH v3 1/4] KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT

[Fuad Tabba <fuad.tabba@linux.dev>]

triage_sysreg_trap() applies the FGU bits through the encoding's FGT
entry in the trap xarray, and populate_nv_trap_config() only adds those
entries on a CPU with FEAT_FGT. Without it, a hidden feature that still
traps reaches its handler instead: a TLBI OS trapped through
HCR_EL2.TTLBOS lands in handle_tlbi_el1(), which expects an EL1 TLBI
only from vEL2 and WARNs before the guest gets its UNDEF. A VMM can
trigger the WARN by hiding TLBI OS and having the guest execute one.

kvm_calculate_traps() computes the FGU bits with or without FEAT_FGT,
and the entries map an encoding to its FGT bit whatever the CPU
implements. Add them unconditionally, so that a hidden feature that
traps is UNDEFINED on any CPU.

Fixes: f5a5a406b4b8b ("KVM: arm64: Propagate and handle Fine-Grained UNDEF bits")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---

Notes:
    For a nested guest on a CPU without FGT, check_fgt_bit() now runs too,
    but it forwards nothing: a guest without FEAT_FGT has its FGT registers
    RES0.
    
    f5a5a406b4b8b added the FGU check to triage_sysreg_trap() behind an
    ARM64_HAS_FGT gate that populate_nv_trap_config() already had, so the
    check never fired without FGT.

 arch/arm64/kvm/emulate-nested.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index 625604019fb32..b8529532b6e78 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -2386,9 +2386,6 @@ int __init populate_nv_trap_config(void)
 				print_nv_trap_error(fgt, "FGT bit is reserved", ret);
 			}
 
-			if (!cpus_have_final_cap(ARM64_HAS_FGT))
-				continue;
-
 			prev = xa_store(&sr_forward_xa, enc,
 					xa_mk_value(tc.val), GFP_KERNEL);
 
-- 
2.39.5