[PATCH v13 20/35] x86/fred: Allow single-step trap and NMI when starting a new task

Xin Li
2023-12-05T11:21:29Z

From: "H. Peter Anvin (Intel)" <hpa@zytor.com>

Entering a new task is logically speaking a return from a system call
(exec, fork, clone, etc.). As such, if ptrace enables single stepping
a single step exception should be allowed to trigger immediately upon
entering user space. This is not optional.

NMI should *never* be disabled in user space. As such, this is an
optional, opportunistic way to catch errors.

Allow single-step trap and NMI when starting a new task, thus once
the new task enters user space, single-step trap and NMI are both
enabled immediately.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
---

Changes since v8:
* Use high-order 48 bits above the lowest 16 bit SS only when FRED
is enabled (Thomas Gleixner).
---
arch/x86/kernel/process_64.c | 38 ++++++++++++++++++++++++++++++------
1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 7f66c0b14de6..7062b84dd467 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -56,6 +56,7 @@
#include <asm/resctrl.h>
#include <asm/unistd.h>
#include <asm/fsgsbase.h>
+#include <asm/fred.h>
#ifdef CONFIG_IA32_EMULATION
/* Not included via unistd.h */
#include <asm/unistd_32_ia32.h>
@@ -528,7 +529,7 @@ void x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase)
static void
start_thread_common(struct pt_regs *regs, unsigned long new_ip,
unsigned long new_sp,
- unsigned int _cs, unsigned int _ss, unsigned int _ds)
+ u16 _cs, u16 _ss, u16 _ds)
{
WARN_ON_ONCE(regs != current_pt_regs());

@@ -545,11 +546,36 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
loadsegment(ds, _ds);
load_gs_index(0);

- regs->ip = new_ip;
- regs->sp = new_sp;
- regs->cs = _cs;
- regs->ss = _ss;
- regs->flags = X86_EFLAGS_IF;
+ regs->ip = new_ip;
+ regs->sp = new_sp;
+ regs->csx = _cs;
+ regs->ssx = _ss;
+ /*
+ * Allow single-step trap and NMI when starting a new task, thus
+ * once the new task enters user space, single-step trap and NMI
+ * are both enabled immediately.
+ *
+ * Entering a new task is logically speaking a return from a
+ * system call (exec, fork, clone, etc.). As such, if ptrace
+ * enables single stepping a single step exception should be
+ * allowed to trigger immediately upon entering user space.
+ * This is not optional.
+ *
+ * NMI should *never* be disabled in user space. As such, this
+ * is an optional, opportunistic way to catch errors.
+ *
+ * Paranoia: High-order 48 bits above the lowest 16 bit SS are
+ * discarded by the legacy IRET instruction on all Intel, AMD,
+ * and Cyrix/Centaur/VIA CPUs, thus can be set unconditionally,
+ * even when FRED is not enabled. But we choose the safer side
+ * to use these bits only when FRED is enabled.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_FRED)) {
+ regs->fred_ss.swevent = true;
+ regs->fred_ss.nmi = true;
+ }
+
+ regs->flags = X86_EFLAGS_IF | X86_EFLAGS_FIXED;
}

void
--
2.43.0