[RFC PATCH 73/73] x86/pvm: Disable some unsupported syscalls and features

Lai Jiangshan
2024-02-26T14:40:05Z

From: Hou Wenlong <houwenlong.hwl@antgroup.com>

n the PVM guest, the LDT won't be loaded into hardware, rendering it
ineffective. Consequently, the modify_ldt() syscall should be disabled.
Additionally, the VSYSCALL address is not within the allowed address
range, making full emulation of the vsyscall page unsupported in the PVM
guest. It is recommended to use XONLY mode instead. Furthermore,
SYSENTER (Intel) and SYSCALL32 (AMD) are not supported by the
hypervisor, so they should not be used in VDSO.

Suggested-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
---
arch/x86/entry/vsyscall/vsyscall_64.c | 4 ++++
arch/x86/kernel/ldt.c | 3 +++
arch/x86/kernel/pvm.c | 4 ++++
3 files changed, 11 insertions(+)

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index f469f8dc36d4..dc6bc7fb490e 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -378,6 +378,10 @@ void __init map_vsyscall(void)
extern char __vsyscall_page;
unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);

+ /* Full emulation is not supported in PVM guest, use XONLY instead. */
+ if (vsyscall_mode == EMULATE && boot_cpu_has(X86_FEATURE_KVM_PVM_GUEST))
+ vsyscall_mode = XONLY;
+
/*
* For full emulation, the page needs to exist for real. In
* execute-only mode, there is no PTE at all backing the vsyscall
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index adc67f98819a..d75815491d7e 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -669,6 +669,9 @@ SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
{
int ret = -ENOSYS;

+ if (cpu_feature_enabled(X86_FEATURE_KVM_PVM_GUEST))
+ return (unsigned int)ret;
+
switch (func) {
case 0:
ret = read_ldt(ptr, bytecount);
diff --git a/arch/x86/kernel/pvm.c b/arch/x86/kernel/pvm.c
index 567ea19d569c..b172bd026594 100644
--- a/arch/x86/kernel/pvm.c
+++ b/arch/x86/kernel/pvm.c
@@ -457,6 +457,10 @@ void __init pvm_early_setup(void)
setup_force_cpu_cap(X86_FEATURE_KVM_PVM_GUEST);
setup_force_cpu_cap(X86_FEATURE_PV_GUEST);

+ /* Don't use SYSENTER (Intel) and SYSCALL32 (AMD) in vdso. */
+ setup_clear_cpu_cap(X86_FEATURE_SYSENTER32);
+ setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
+
/* PVM takes care of %gs when switching to usermode for us */
pv_ops.cpu.load_gs_index = pvm_load_gs_index;
pv_ops.cpu.cpuid = pvm_cpuid;
--
2.19.1.6.gb485710b