[PATCH v9 16/36] x86/fred: Allow single-step trap and NMI when starting a new task

[Xin Li <xin3.li@xxxxxxxxx>]

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

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@xxxxxxxxx>
Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---

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 | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 6d5fed29f552..0b47871a6141 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>
@@ -507,8 +508,18 @@ 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)
 {
+       /*
+        * 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.
+        */
+       const unsigned long ssx_flags = cpu_feature_enabled(X86_FEATURE_FRED) ?
+               (FRED_SSX_SOFTWARE_INITIATED | FRED_SSX_NMI) : 0;
+
        WARN_ON_ONCE(regs != current_pt_regs());
 
        if (static_cpu_has(X86_BUG_NULL_SEG)) {
@@ -522,11 +533,11 @@ 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 | ssx_flags;
+       regs->flags     = X86_EFLAGS_IF | X86_EFLAGS_FIXED;
 }
 
 void
-- 
2.34.1