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Subject[PATCH 4.9 53/67] x86/entry/64: Dont use IST entry for #BP stack
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@kernel.org>

commit d8ba61ba58c88d5207c1ba2f7d9a2280e7d03be9 upstream.

There's nothing IST-worthy about #BP/int3. We don't allow kprobes
in the small handful of places in the kernel that run at CPL0 with
an invalid stack, and 32-bit kernels have used normal interrupt
gates for #BP forever.

Furthermore, we don't allow kprobes in places that have usergs while
in kernel mode, so "paranoid" is also unnecessary.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/entry/entry_64.S | 2 +-
arch/x86/kernel/traps.c | 24 +++++++++++-------------
2 files changed, 12 insertions(+), 14 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -943,7 +943,7 @@ apicinterrupt3 HYPERVISOR_CALLBACK_VECTO
#endif /* CONFIG_HYPERV */

idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
-idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
+idtentry int3 do_int3 has_error_code=0
idtentry stack_segment do_stack_segment has_error_code=1

#ifdef CONFIG_XEN
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -526,7 +526,6 @@ do_general_protection(struct pt_regs *re
}
NOKPROBE_SYMBOL(do_general_protection);

-/* May run on IST stack. */
dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
{
#ifdef CONFIG_DYNAMIC_FTRACE
@@ -541,7 +540,15 @@ dotraplinkage void notrace do_int3(struc
if (poke_int3_handler(regs))
return;

+ /*
+ * Use ist_enter despite the fact that we don't use an IST stack.
+ * We can be called from a kprobe in non-CONTEXT_KERNEL kernel
+ * mode or even during context tracking state changes.
+ *
+ * This means that we can't schedule. That's okay.
+ */
ist_enter(regs);
+
RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
@@ -558,17 +565,11 @@ dotraplinkage void notrace do_int3(struc
SIGTRAP) == NOTIFY_STOP)
goto exit;

- /*
- * Let others (NMI) know that the debug stack is in use
- * as we may switch to the interrupt stack.
- */
- debug_stack_usage_inc();
preempt_disable();
cond_local_irq_enable(regs);
do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, error_code, NULL);
cond_local_irq_disable(regs);
preempt_enable_no_resched();
- debug_stack_usage_dec();
exit:
ist_exit(regs);
}
@@ -989,19 +990,16 @@ void __init trap_init(void)
cpu_init();

/*
- * X86_TRAP_DB and X86_TRAP_BP have been set
- * in early_trap_init(). However, ITS works only after
- * cpu_init() loads TSS. See comments in early_trap_init().
+ * X86_TRAP_DB was installed in early_trap_init(). However,
+ * IST works only after cpu_init() loads TSS. See comments
+ * in early_trap_init().
*/
set_intr_gate_ist(X86_TRAP_DB, &debug, DEBUG_STACK);
- /* int3 can be called from all */
- set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);

x86_init.irqs.trap_init();

#ifdef CONFIG_X86_64
memcpy(&debug_idt_table, &idt_table, IDT_ENTRIES * 16);
set_nmi_gate(X86_TRAP_DB, &debug);
- set_nmi_gate(X86_TRAP_BP, &int3);
#endif
}

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