From b301f8dd05f04b4501b44b4b698f9e65fab533a1 Mon Sep 17 00:00:00 2001
From: Alexander Potapenko <glider@google.com>
Date: Fri, 21 Nov 2025 15:34:10 +0100
Subject: executor: sys/linux: implement SYZOS_API_NESTED_AMD_VMCB_WRITE_MASK

The new command allows mutation of AMD VMCB block with plain 64-bit writes.
In addition to VM ID and VMCB offset, @nested_amd_vmcb_write_mask takes
three 64-bit numbers: the set mask, the unset mask, and the flip mask.
This allows to make bitwise modifications to VMCB without disturbing
the execution too much.

Also add sys/linux/test/amd64-syz_kvm_nested_amd_vmcb_write_mask to test the
new command behavior.
---
 .../test/amd64-syz_kvm_nested_amd_vmcb_write_mask  | 43 ++++++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 sys/linux/test/amd64-syz_kvm_nested_amd_vmcb_write_mask

(limited to 'sys/linux/test')

diff --git a/sys/linux/test/amd64-syz_kvm_nested_amd_vmcb_write_mask b/sys/linux/test/amd64-syz_kvm_nested_amd_vmcb_write_mask
new file mode 100644
index 000000000..4ccf23953
--- /dev/null
+++ b/sys/linux/test/amd64-syz_kvm_nested_amd_vmcb_write_mask
@@ -0,0 +1,43 @@
+#
+# 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$x86(r1, &(0x7f0000c00000/0x400000)=nil)
+
+# Create a nested AMD VM that performs HLT and INVD to test vmcb_write_mask.
+# This is the AMD equivalent of the amd64-syz_kvm_nested_vmwrite_mask test.
+# 1. L2 executes HLT -> exit to L1 (HLT is intercepted by default).
+# 2. L1 disables HLT intercept via vmcb_write_mask.
+# 3. L1 resumes L2.
+# 4. L2 executes HLT (no L2->L1 exit), causing an L1->L0 KVM_EXIT_HLT.
+# 5. L0 resumes L1, which resumes L2.
+# 6. L2 executes INVD -> exit to L1.
+#
+# The HLT intercept bit is bit 24 in the VMCB's intercept vector 3 (offset 0xC).
+# We unset this bit (0x1000000).
+#
+r3 = syz_kvm_add_vcpu$x86(r2, &AUTO={0x0, &AUTO=[@enable_nested={AUTO, AUTO, 0x0}, @nested_create_vm={AUTO, AUTO, 0x0}, @nested_load_code={AUTO, AUTO, {0x0, "f40f08"}}, @nested_vmlaunch={AUTO, AUTO, 0x0}, @nested_amd_vmcb_write_mask={AUTO, AUTO, {0x0, @control_area=0xc, 0x0, 0x1000000, 0x0}}, @nested_vmresume={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)
+
+# L2 VM executes HLT. Exit reason is mapped to 0xe2e20001.
+#
+ioctl$KVM_RUN(r3, AUTO, 0x0)
+syz_kvm_assert_syzos_uexit$x86(r5, 0xe2e20001)
+
+# L1 disables HLT intercept and resumes L2. L2 executes HLT, causing KVM_EXIT_HLT (0x5) from L1 to L0.
+# This confirms that the vmcb_write_mask command was successful.
+#
+ioctl$KVM_RUN(r3, AUTO, 0x0)
+syz_kvm_assert_syzos_kvm_exit$x86(r5, 0x5)
+
+# After resuming, L2 executes INVD. Exit reason is mapped to 0xe2e20002.
+#
+ioctl$KVM_RUN(r3, AUTO, 0x0)
+syz_kvm_assert_syzos_uexit$x86(r5, 0xe2e20002)
+
+# guest_main should finish with guest_uexit(-1).
+#
+ioctl$KVM_RUN(r3, AUTO, 0x0)
+syz_kvm_assert_syzos_uexit$x86(r5, 0xffffffff)
-- 
cgit mrf-deployment