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Subject[PATCH 3.2 023/153] x86/entry/64: Don't use IST entry for #BP stack
3.2.102-rc1 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>
[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@decadent.org.uk>
---
--- 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();
}
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