From e2d17597c5c0f0be4b4b6e992d9434edef02c2f0 Mon Sep 17 00:00:00 2001
From: Alexander Potapenko <glider@google.com>
Date: Mon, 19 Jan 2026 09:38:53 +0100
Subject: executor: sys/linux: SYZOS: add AMD SET_INTERCEPT primitive

This patch introduces SYZOS_API_NESTED_AMD_SET_INTERCEPT to SYZOS.

This primitive enables the fuzzer to surgically modify intercept vectors
in the AMD VMCB (Virtual Machine Control Block) Control Area.
It implements a read-modify-write operation on 32-bit VMCB offsets,
allowing the L1 hypervisor (SYZOS) to deterministically set or clear
specific intercept bits (e.g., for RDTSC, HLT, or exceptions) for the L2
guest.

This capability allows syzkaller to systematically explore KVM's nested
SVM emulation logic by toggling intercepts on and off, rather than
relying on static defaults or random memory corruption.
---
 sys/linux/dev_kvm_amd64.txt                        |  8 +++++
 .../test/amd64-syz_kvm_nested_amd_set_intercept    | 39 ++++++++++++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 sys/linux/test/amd64-syz_kvm_nested_amd_set_intercept

(limited to 'sys')

diff --git a/sys/linux/dev_kvm_amd64.txt b/sys/linux/dev_kvm_amd64.txt
index acd80540c..0d90ceeab 100644
--- a/sys/linux/dev_kvm_amd64.txt
+++ b/sys/linux/dev_kvm_amd64.txt
@@ -160,6 +160,13 @@ syzos_api_nested_amd_inject_event {
 	flags		int64[0:3]
 }
 
+syzos_api_nested_amd_set_intercept {
+	vm_id		syzos_api_vm_id
+	offset		int64
+	bit_mask	int64
+	action		int64[0:1]
+}
+
 # IDs here must match those in executor/common_kvm_amd64_syzos.h.
 syzos_api_call$x86 [
 	uexit				syzos_api$x86[0, intptr]
@@ -183,6 +190,7 @@ syzos_api_call$x86 [
 	nested_amd_stgi			syzos_api$x86[382, void]
 	nested_amd_clgi			syzos_api$x86[383, void]
 	nested_amd_inject_event		syzos_api$x86[384, syzos_api_nested_amd_inject_event]
+	nested_amd_set_intercept	syzos_api$x86[385, syzos_api_nested_amd_set_intercept]
 ] [varlen]
 
 kvm_text_x86 [
diff --git a/sys/linux/test/amd64-syz_kvm_nested_amd_set_intercept b/sys/linux/test/amd64-syz_kvm_nested_amd_set_intercept
new file mode 100644
index 000000000..91293ba0d
--- /dev/null
+++ b/sys/linux/test/amd64-syz_kvm_nested_amd_set_intercept
@@ -0,0 +1,39 @@
+#
+# requires: arch=amd64 -threaded
+#
+r0 = openat$kvm(0, &AUTO='/dev/kvm\x00', 0x0, 0x0)
+r1 = ioctl$KVM_CREATE_VM(r0, AUTO, 0x0)
+r2 = syz_kvm_setup_syzos_vm$x86(r1, &(0x7f0000c00000/0x400000)=nil)
+
+# Test AMD Nested Intercepts: The RDTSC Liberation
+#
+# 1. L1 Setup:
+#    - SYZOS initializes VMCB with ALL intercepts enabled.
+#    - We call SET_INTERCEPT to *CLEAR* the RDTSC intercept.
+#      - Offset: 0xC (Control Vector 1)
+#      - Bit: 14 (0x4000) = RDTSC
+#      - Action: 0 (Clear/Disable)
+#
+# 2. L2 Payload:
+#    - "rdtsc" (0x0f 0x31)
+#    - "hlt"   (0xf4)
+#
+# 3. Execution Flow:
+#    - If RDTSC is intercepted: Exit Reason = RDTSC (0x4).
+#    - If RDTSC is native: executes RDTSC -> executes HLT -> Exit Reason = HLT (0x1).
+#
+r3 = syz_kvm_add_vcpu$x86(r2, &AUTO={0x0, &AUTO=[@enable_nested={AUTO, AUTO, 0x0}, @nested_create_vm={AUTO, AUTO, 0x0}, @nested_amd_set_intercept={AUTO, AUTO, {0x0, 0xC, 0x4000, 0x0}}, @nested_load_code={AUTO, AUTO, {0x0, "0f31f4"}}, @nested_vmlaunch={AUTO, AUTO, 0x0}], AUTO})
+r4 = ioctl$KVM_GET_VCPU_MMAP_SIZE(r0, AUTO)
+r5 = mmap$KVM_VCPU(&(0x7f0000009000/0x1000)=nil, r4, 0x3, 0x1, r3, 0x0)
+
+# Run the VCPU.
+# Expectation: Success (Native Execution) -> HLT Exit.
+# Code: 0xe2e20001 (SYZOS_NESTED_EXIT_REASON_HLT)
+#
+ioctl$KVM_RUN(r3, AUTO, 0x0)
+syz_kvm_assert_syzos_uexit$x86(r3, r5, 0xe2e20001)
+
+# Cleanup
+#
+ioctl$KVM_RUN(r3, AUTO, 0x0)
+syz_kvm_assert_syzos_uexit$x86(r3, r5, 0xffffffffffffffff)
-- 
cgit mrf-deployment