Re: [PATCH] x86/32: Use a thread flag to track SYSENTER status

From: Brian Gerst
Date: Fri Jun 30 2023 - 10:11:52 EST

I was looking at merging and simplifying the SYSENTER/SYSCALL MSR
setup code, and the ss1 hack was making that more difficult. I
haven't finished that part yet as I'm waiting for the dust to settle
on the 32-bit syscall disabling patches.

On Thu, Jun 29, 2023 at 11:18 AM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> On June 28, 2023 4:00:15 PM PDT, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> >When a task is in VM86 mode, the SYSENTER instruction must be disabled
> >to prevent kernel stack corruption. Use a thread flag to track when a
> >task needs SYSENTER disabled instead of abusing the ss1 field of the
> >TSS.
> >
> >Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
> >---
> > arch/x86/include/asm/processor.h | 26 ++------------------------
> > arch/x86/include/asm/switch_to.h | 12 ------------
> > arch/x86/include/asm/thread_info.h | 5 ++++-
> > arch/x86/include/asm/vm86.h | 8 ++++++++
> > arch/x86/kernel/cpu/common.c | 10 +---------
> > arch/x86/kernel/process.c | 4 +++-
> > arch/x86/kernel/process_32.c | 1 -
> > arch/x86/kernel/vm86_32.c | 8 ++++----
> > 8 files changed, 22 insertions(+), 52 deletions(-)
> >
> >diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> >index b216ac80ebcc..96a5e3e5bb7b 100644
> >--- a/arch/x86/include/asm/processor.h
> >+++ b/arch/x86/include/asm/processor.h
> >@@ -226,23 +226,7 @@ struct x86_hw_tss {
> > unsigned long sp0;
> > unsigned short ss0, __ss0h;
> > unsigned long sp1;
> >-
> >- /*
> >- * We don't use ring 1, so ss1 is a convenient scratch space in
> >- * the same cacheline as sp0. We use ss1 to cache the value in
> >- * MSR_IA32_SYSENTER_CS. When we context switch
> >- * MSR_IA32_SYSENTER_CS, we first check if the new value being
> >- * written matches ss1, and, if it's not, then we wrmsr the new
> >- * value and update ss1.
> >- *
> >- * The only reason we context switch MSR_IA32_SYSENTER_CS is
> >- * that we set it to zero in vm86 tasks to avoid corrupting the
> >- * stack if we were to go through the sysenter path from vm86
> >- * mode.
> >- */
> >- unsigned short ss1; /* MSR_IA32_SYSENTER_CS */
> >-
> >- unsigned short __ss1h;
> >+ unsigned short ss1, __ss1h;
> > unsigned long sp2;
> > unsigned short ss2, __ss2h;
> > unsigned long __cr3;
> >@@ -419,16 +403,11 @@ struct thread_struct {
> > unsigned long sp0;
> > #endif
> > unsigned long sp;
> >-#ifdef CONFIG_X86_32
> >- unsigned long sysenter_cs;
> >-#else
> >+#ifdef CONFIG_X86_64
> > unsigned short es;
> > unsigned short ds;
> > unsigned short fsindex;
> > unsigned short gsindex;
> >-#endif
> >-
> >-#ifdef CONFIG_X86_64
> > unsigned long fsbase;
> > unsigned long gsbase;
> > #else
> >@@ -648,7 +627,6 @@ static inline void spin_lock_prefetch(const void *x)
> > #ifdef CONFIG_X86_32
> > #define INIT_THREAD { \
> > .sp0 = TOP_OF_INIT_STACK, \
> >- .sysenter_cs = __KERNEL_CS, \
> > }
> >
> > #define KSTK_ESP(task) (task_pt_regs(task)->sp)
> >diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> >index 5c91305d09d2..53d34c6fb581 100644
> >--- a/arch/x86/include/asm/switch_to.h
> >+++ b/arch/x86/include/asm/switch_to.h
> >@@ -49,18 +49,6 @@ do { \
> > ((last) = __switch_to_asm((prev), (next))); \
> > } while (0)
> >
> >-#ifdef CONFIG_X86_32
> >-static inline void refresh_sysenter_cs(struct thread_struct *thread)
> >-{
> >- /* Only happens when SEP is enabled, no need to test "SEP"arately: */
> >- if (unlikely(this_cpu_read(cpu_tss_rw.x86_tss.ss1) == thread->sysenter_cs))
> >- return;
> >-
> >- this_cpu_write(cpu_tss_rw.x86_tss.ss1, thread->sysenter_cs);
> >- wrmsr(MSR_IA32_SYSENTER_CS, thread->sysenter_cs, 0);
> >-}
> >-#endif
> >-
> > /* This is used when switching tasks or entering/exiting vm86 mode. */
> > static inline void update_task_stack(struct task_struct *task)
> > {
> >diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> >index f1cccba52eb9..7f7332290ee1 100644
> >--- a/arch/x86/include/asm/thread_info.h
> >+++ b/arch/x86/include/asm/thread_info.h
> >@@ -78,6 +78,7 @@ struct thread_info {
> > * - these are process state flags that various assembly files
> > * may need to access
> > */
> >+#define TIF_DISABLE_SYSENTER 0 /* disable SYSENTER for VM86 mode */
> > #define TIF_NOTIFY_RESUME 1 /* callback before returning to user */
> > #define TIF_SIGPENDING 2 /* signal pending */
> > #define TIF_NEED_RESCHED 3 /* rescheduling necessary */
> >@@ -101,6 +102,7 @@ struct thread_info {
> > #define TIF_LAZY_MMU_UPDATES 27 /* task is updating the mmu lazily */
> > #define TIF_ADDR32 29 /* 32-bit address space on 64 bits */
> >
> >+#define _TIF_DISABLE_SYSENTER (1 << TIF_DISABLE_SYSENTER)
> > #define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME)
> > #define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
> > #define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
> >@@ -126,7 +128,8 @@ struct thread_info {
> > /* flags to check in __switch_to() */
> > #define _TIF_WORK_CTXSW_BASE \
> > (_TIF_NOCPUID | _TIF_NOTSC | _TIF_BLOCKSTEP | \
> >- _TIF_SSBD | _TIF_SPEC_FORCE_UPDATE)
> >+ _TIF_SSBD | _TIF_SPEC_FORCE_UPDATE | \
> >+ _TIF_DISABLE_SYSENTER)
> >
> > /*
> > * Avoid calls to __switch_to_xtra() on UP as STIBP is not evaluated.
> >diff --git a/arch/x86/include/asm/vm86.h b/arch/x86/include/asm/vm86.h
> >index 9e8ac5073ecb..9e3cdbfe6e45 100644
> >--- a/arch/x86/include/asm/vm86.h
> >+++ b/arch/x86/include/asm/vm86.h
> >@@ -72,6 +72,12 @@ static inline int invalid_vm86_irq(int irq)
> >
> > void release_vm86_irqs(struct task_struct *);
> >
> >+static inline void refresh_sysenter_cs(bool disable_sysenter)
> >+{
> >+ /* Only happens when SEP is enabled, no need to test "SEP"arately: */
> >+ wrmsr(MSR_IA32_SYSENTER_CS, disable_sysenter ? 0 : __KERNEL_CS , 0);
> >+}
> >+
> > #else
> >
> > #define handle_vm86_fault(a, b)
> >@@ -86,6 +92,8 @@ static inline void save_v86_state(struct kernel_vm86_regs *a, int b) { }
> >
> > #define free_vm86(t) do { } while(0)
> >
> >+static inline void refresh_sysenter_cs(bool disable_sysenter) { }
> >+
> > #endif /* CONFIG_VM86 */
> >
> > #endif /* _ASM_X86_VM86_H */
> >diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> >index cf5275a67f39..aaf636e4d1de 100644
> >--- a/arch/x86/kernel/cpu/common.c
> >+++ b/arch/x86/kernel/cpu/common.c
> >@@ -1933,22 +1933,14 @@ static void identify_cpu(struct cpuinfo_x86 *c)
> > #ifdef CONFIG_X86_32
> > void enable_sep_cpu(void)
> > {
> >- struct tss_struct *tss;
> > int cpu;
> >
> > if (!boot_cpu_has(X86_FEATURE_SEP))
> > return;
> >
> > cpu = get_cpu();
> >- tss = &per_cpu(cpu_tss_rw, cpu);
> >
> >- /*
> >- * We cache MSR_IA32_SYSENTER_CS's value in the TSS's ss1 field --
> >- * see the big comment in struct x86_hw_tss's definition.
> >- */
> >-
> >- tss->x86_tss.ss1 = __KERNEL_CS;
> >- wrmsr(MSR_IA32_SYSENTER_CS, tss->x86_tss.ss1, 0);
> >+ wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
> > wrmsr(MSR_IA32_SYSENTER_ESP, (unsigned long)(cpu_entry_stack(cpu) + 1), 0);
> > wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long)entry_SYSENTER_32, 0);
> >
> >diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> >index cc7a642f8c9d..cd2c483b91c1 100644
> >--- a/arch/x86/kernel/process.c
> >+++ b/arch/x86/kernel/process.c
> >@@ -75,7 +75,6 @@ __visible DEFINE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw) = {
> > .sp1 = TOP_OF_INIT_STACK,
> >
> > .ss0 = __KERNEL_DS,
> >- .ss1 = __KERNEL_CS,
> > #endif
> > .io_bitmap_base = IO_BITMAP_OFFSET_INVALID,
> > },
> >@@ -713,6 +712,9 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
> > /* Enforce MSR update to ensure consistent state */
> > __speculation_ctrl_update(~tifn, tifn);
> > }
> >+
> >+ if (unlikely((tifp ^ tifn) & _TIF_DISABLE_SYSENTER))
> >+ refresh_sysenter_cs((tifn & _TIF_DISABLE_SYSENTER) != 0);
> > }
> >
> > /*
> >diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> >index 708c87b88cc1..c609faa3e50e 100644
> >--- a/arch/x86/kernel/process_32.c
> >+++ b/arch/x86/kernel/process_32.c
> >@@ -196,7 +196,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
> > * case prev or next is vm86.
> > */
> > update_task_stack(next_p);
> >- refresh_sysenter_cs(next);
> > this_cpu_write(pcpu_hot.top_of_stack,
> > (unsigned long)task_stack_page(next_p) +
> > THREAD_SIZE);
> >diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
> >index e9e803a4d44c..9d25793ba999 100644
> >--- a/arch/x86/kernel/vm86_32.c
> >+++ b/arch/x86/kernel/vm86_32.c
> >@@ -143,10 +143,10 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
> > exit_vm86:
> > preempt_disable();
> > tsk->thread.sp0 = vm86->saved_sp0;
> >- tsk->thread.sysenter_cs = __KERNEL_CS;
> > update_task_stack(tsk);
> >- refresh_sysenter_cs(&tsk->thread);
> > vm86->saved_sp0 = 0;
> >+ if (test_and_clear_thread_flag(TIF_DISABLE_SYSENTER))
> >+ refresh_sysenter_cs(false);
> > preempt_enable();
> >
> > memcpy(&regs->pt, &vm86->regs32, sizeof(struct pt_regs));
> >@@ -332,8 +332,8 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
> > tsk->thread.sp0 += 16;
> >
> > if (boot_cpu_has(X86_FEATURE_SEP)) {
> >- tsk->thread.sysenter_cs = 0;
> >- refresh_sysenter_cs(&tsk->thread);
> >+ set_thread_flag(TIF_DISABLE_SYSENTER);
> >+ refresh_sysenter_cs(true);
> > }
> >
> > update_task_stack(tsk);
>
> As much as I find the ss1 hack putrid, is this purely a cleanup or is there a stronger motivation for it? Could you clarify in the patch comment?