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Subject[PATCH v2 2/3] x86/entry/64: Teach idtentry to use the IRQ stack
Date
We don't specifically need IST for things like kprobes, but we do
want to avoid rare, surprising extra stack usage if a kprobe hits
with a deep stack.

Teach idtentry to use the IRQ stack for selected entries.

This implementation uses the IRQ stack even if we entered from user
mode. This disallows tricks like ist_begin_non_atomic. If we ever
need such a trick in one of these entries, we can rework this. For
now, let's keep it simple.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/entry/entry_64.S | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 3227fb6a5b94..af3573e75ed4 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -756,13 +756,17 @@ apicinterrupt IRQ_WORK_VECTOR irq_work_interrupt smp_irq_work_interrupt
*/
#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss) + (TSS_ist + ((x) - 1) * 8)

-.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
+.macro idtentry sym do_sym has_error_code:req irqstack=0 paranoid=0 shift_ist=-1
ENTRY(\sym)
/* Sanity check */
.if \shift_ist != -1 && \paranoid == 0
.error "using shift_ist requires paranoid=1"
.endif

+ .if \irqstack && \paranoid
+ .error "using irqstack requires !paranoid"
+ .endif
+
ASM_CLAC
PARAVIRT_ADJUST_EXCEPTION_FRAME

@@ -804,8 +808,16 @@ ENTRY(\sym)
subq $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
.endif

+ .if \irqstack
+ ENTER_IRQ_STACK old_rsp=%rcx
+ .endif
+
call \do_sym

+ .if \irqstack
+ LEAVE_IRQ_STACK
+ .endif
+
.if \shift_ist != -1
addq $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
.endif
--
2.4.3


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