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x86/fred: Allow single-step trap and NMI when starting a new task

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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>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20231205105030.8698-21-xin3.li@intel.com
This commit is contained in:
H. Peter Anvin (Intel) 2023-12-05 02:50:09 -08:00 committed by Borislav Petkov (AMD)
parent df8838737b
commit ad41a14cc2
1 changed files with 32 additions and 6 deletions
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38
arch/x86/kernel/process_64.c
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@ -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