linux-kernel - Re: [PATCH v2 0/6] x86/alternatives: text

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Date:   Thu, 6 Sep 2018 19:17:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jiri Kosina <jkosina@...e.cz>,
        Andy Lutomirski <luto@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2 0/6] x86/alternatives: text_poke() fixes

On Thu, Sep 06, 2018 at 05:01:25PM +0000, Nadav Amit wrote:
> In addition, there might be a couple of issues with your fix:

It boots on my box ;-)

> 1. __set_pte_vaddr() is not used exclusive by set_fixmap(). This means
> the warning might be wrong, but also means that these code patches (Xen’s
> set_pte_mfn(), CPU-entry-area setup) needs to be checked. And as you said
> before, someone might use this function for other purposes as well.

CEA is fine, that actually needs it too.

The one thing I missed out on earlier is the below chunk, that is no
longer needed now that cea_set_pte() actually does the right thing.

diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index b7b01d762d32..14ad97fa0749 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -293,12 +293,6 @@ static void ds_update_cea(void *cea, void *addr, size_t size, pgprot_t prot)
 	preempt_disable();
 	for (; msz < size; msz += PAGE_SIZE, pa += PAGE_SIZE, cea += PAGE_SIZE)
 		cea_set_pte(cea, pa, prot);
-
-	/*
-	 * This is a cross-CPU update of the cpu_entry_area, we must shoot down
-	 * all TLB entries for it.
-	 */
-	flush_tlb_kernel_range(start, start + size);
 	preempt_enable();
 }
 
@@ -310,8 +304,6 @@ static void ds_clear_cea(void *cea, size_t size)
 	preempt_disable();
 	for (; msz < size; msz += PAGE_SIZE, cea += PAGE_SIZE)
 		cea_set_pte(cea, 0, PAGE_NONE);
-
-	flush_tlb_kernel_range(start, start + size);
 	preempt_enable();
 }
 

> 2. Printing the virtual address can break KASLR.

Local KASLR is a myth.. but sure, we can fix the print.

> 3. The WARN() can introduce some overhead since checking if IRQs are
> disabled takes considerably long time. Perhaps VM_WARN() or something is
> better. I realize this code-path is not on the hot-path though...

Yeah, if it triggers you have bigger problems. We can make it
WARN_ONCE() I suppose.

> 4. I guess flush_tlb_kernel_range() should also have something like
> VM_WARN_ON(irqs_disabled()), just as an additional general sanity check.

It has, it's hidden in kernel/smp.c:smp_call_function_many().

> Let me know if you want me to make these changes and include your patch in
> the set.

The set is no longer needed. text_poke() is fine and correct with this
one patch.