From 2995590964da93e1fd9a91550f9c9d9fab28f160 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Tue, 11 Jul 2017 10:33:39 -0500 Subject: [PATCH] x86/entry/64: Initialize the top of the IRQ stack before switching stacks The OOPS unwinder wants the word at the top of the IRQ stack to point back to the previous stack at all times when the IRQ stack is in use. There's currently a one-instruction window in ENTER_IRQ_STACK during which this isn't the case. Fix it by writing the old RSP to the top of the IRQ stack before jumping. This currently writes the pointer to the stack twice, which is a bit ugly. We could get rid of this by replacing irq_stack_ptr with irq_stack_ptr_minus_eight (better name welcome). OTOH, there may be all kinds of odd microarchitectural considerations in play that affect performance by a few cycles here. Reported-by: Mike Galbraith Reported-by: Josh Poimboeuf Signed-off-by: Andy Lutomirski Signed-off-by: Josh Poimboeuf Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Jiri Slaby Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: live-patching@vger.kernel.org Link: http://lkml.kernel.org/r/aae7e79e49914808440ad5310ace138ced2179ca.1499786555.git.jpoimboe@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/entry/entry_64.S | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 0d4483ae6360..b56f7f23f21c 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -469,6 +469,7 @@ END(irq_entries_start) DEBUG_ENTRY_ASSERT_IRQS_OFF movq %rsp, \old_rsp incl PER_CPU_VAR(irq_count) + jnz .Lirq_stack_push_old_rsp_\@ /* * Right now, if we just incremented irq_count to zero, we've @@ -478,9 +479,30 @@ END(irq_entries_start) * it must be *extremely* careful to limit its stack usage. This * could include kprobes and a hypothetical future IST-less #DB * handler. + * + * The OOPS unwinder relies on the word at the top of the IRQ + * stack linking back to the previous RSP for the entire time we're + * on the IRQ stack. For this to work reliably, we need to write + * it before we actually move ourselves to the IRQ stack. + */ + + movq \old_rsp, PER_CPU_VAR(irq_stack_union + IRQ_STACK_SIZE - 8) + movq PER_CPU_VAR(irq_stack_ptr), %rsp + +#ifdef CONFIG_DEBUG_ENTRY + /* + * If the first movq above becomes wrong due to IRQ stack layout + * changes, the only way we'll notice is if we try to unwind right + * here. Assert that we set up the stack right to catch this type + * of bug quickly. */ + cmpq -8(%rsp), \old_rsp + je .Lirq_stack_okay\@ + ud2 + .Lirq_stack_okay\@: +#endif - cmovzq PER_CPU_VAR(irq_stack_ptr), %rsp +.Lirq_stack_push_old_rsp_\@: pushq \old_rsp .endm -- 2.20.1