[PATCH] RISC-V: KVM: Fix shift-out-of-bounds in make_xfence_request()

[Jiakai Xu]

The make_xfence_request() function uses a shift operation to check if a
vCPU is in the hart mask:

  if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))

However, when the difference between vcpu_id and hbase
is >= BITS_PER_LONG, the shift operation causes undefined behavior.

This was detected by UBSAN:
  UBSAN: shift-out-of-bounds in arch/riscv/kvm/tlb.c:343:23
  shift exponent 256 is too large for 64-bit type 'long unsigned int'

Fix this by adding a bounds check before the shift operation.

This bug was found by fuzzing the KVM RISC-V interface.

Fixes: 13acfec2dbcc ("RISC-V: KVM: Add remote HFENCE functions based on VCPU requests")
Signed-off-by: Jiakai Xu <jiakaiPeanut@xxxxxxxxx>
Signed-off-by: Jiakai Xu <xujiakai2025@xxxxxxxxxxx>
---
 arch/riscv/kvm/tlb.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
index ff1aeac4eb8eb..500e001513a11 100644
--- a/arch/riscv/kvm/tlb.c
+++ b/arch/riscv/kvm/tlb.c
@@ -333,14 +333,17 @@ static void make_xfence_request(struct kvm *kvm,
 	unsigned long i;
 	struct kvm_vcpu *vcpu;
 	unsigned int actual_req = req;
+	unsigned int idx;
 	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
 
 	bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (hbase != -1UL) {
-			if (vcpu->vcpu_id < hbase)
+			idx = vcpu->vcpu_id - hbase;
+
+			if (idx < 0 || idx >= BITS_PER_LONG)
 				continue;
-			if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
+			if (!(hmask & (1UL << idx)))
 				continue;
 		}
 
-- 
2.34.1