[PATCH 3.2 023/153] x86/entry/64: Don't use IST entry for #BP stack

[Ben Hutchings]

3.2.102-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Andy Lutomirski <luto@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
[carnil: Backport to 3.16:
 - Adjust finename change: arch/x86/kernel/entry_64.S
 - Context changes
]
[bwh: Rebase on top of "x86/traps: Enable DEBUG_STACK after cpu_init() for
 TRAP_DB/BP", and restore change in trap_init() instead of early_trap_init().
 Backport to 3.2:
 - Use zeroentry macro in entry_64.S
 - Drop changes related to breakpoint-in-NMI support
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1401,7 +1401,7 @@ apicinterrupt XEN_HVM_EVTCHN_CALLBACK \
 	.pushsection .kprobes.text, "ax"
 
 paranoidzeroentry_ist debug do_debug DEBUG_STACK
-paranoidzeroentry_ist int3 do_int3 DEBUG_STACK
+zeroentry int3 do_int3
 errorentry stack_segment do_stack_segment
 #ifdef CONFIG_XEN
 zeroentry xen_debug do_debug
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -314,7 +314,6 @@ gp_in_kernel:
 	die("general protection fault", regs, error_code);
 }
 
-/* May run on IST stack. */
 dotraplinkage void __kprobes do_int3(struct pt_regs *regs, long error_code)
 {
 #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
@@ -790,13 +789,11 @@ void __init trap_init(void)
 	cpu_init();
 
 	/*
-	 * X86_TRAP_DB and X86_TRAP_BP have been set
-	 * in early_trap_init(). However, DEBUG_STACK works only after
-	 * cpu_init() loads TSS. See comments in early_trap_init().
+	 * X86_TRAP_DB was installed in early_trap_init(). However,
+	 * DEBUG_STACK 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();
 }