Re: [PATCH v2 0/6] x86/alternatives: text_poke() fixes

[Nadav Amit <namit@vmware.com>]

at 3:16 AM, Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Sep 06, 2018 at 10:13:00AM +0200, Peter Zijlstra wrote:
>> No, you got it the first time. There are in fact more fixmap abusers;
>> see drivers/acpi/apei/ghes.c.  Also, as long as set_fixmap() allows
>> overwriting a _PAGE_PRESENT pte and has that dodgy
>> __flush_tlb_one_kernel() in it, the broken remains (and can return).
>> 
>> So we need to fix fixmap, to either disallow overwriting a _PAGE_PRESENT
>> pte, or to issue a full TLB invalidate.
>> 
>> Either fix will terminally break GHES, but that's OK, they've known
>> about this issue since 2015 and haven't cared, so I can't be bothered
>> about them.
> 
> In fact, the below seems to cure all woes..
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index b9d5e7c9ef43..00f1c9e4f0a3 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -709,22 +709,25 @@ void *text_poke(void *addr, const void *opcode, size_t len)
> 		pages[1] = virt_to_page(addr + PAGE_SIZE);
> 	}
> 	BUG_ON(!pages[0]);
> -	local_irq_save(flags);
> +
> 	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
> 	if (pages[1])
> 		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
> +
> +	local_irq_save(flags);
> 	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
> 	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
> -	clear_fixmap(FIX_TEXT_POKE0);
> -	if (pages[1])
> -		clear_fixmap(FIX_TEXT_POKE1);
> -	local_flush_tlb();
> 	sync_core();
> 	/* Could also do a CLFLUSH here to speed up CPU recovery; but
> 	   that causes hangs on some VIA CPUs. */
> 	for (i = 0; i < len; i++)
> 		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
> 	local_irq_restore(flags);
> +
> +	clear_fixmap(FIX_TEXT_POKE0);
> +	if (pages[1])
> +		clear_fixmap(FIX_TEXT_POKE1);
> +
> 	return addr;
> }
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index dd519f372169..fd6af66bc400 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -260,14 +260,28 @@ static void __set_pte_vaddr(pud_t *pud, unsigned long vaddr, pte_t new_pte)
> {
> 	pmd_t *pmd = fill_pmd(pud, vaddr);
> 	pte_t *pte = fill_pte(pmd, vaddr);
> +	pte_t old_pte = *pte;
> 
> 	set_pte(pte, new_pte);
> 
> 	/*
> -	 * It's enough to flush this one mapping.
> -	 * (PGE mappings get flushed as well)
> +	 * If it was present and changes, we need to invalidate TLBs.
> 	 */
> -	__flush_tlb_one_kernel(vaddr);
> +	if (!(pte_present(old_pte) && !pte_same(old_pte, new_pte)))
> +		return;
> +
> +	if (WARN(irqs_disabled(), "broken set_fixmap(%d, %lx), was: %lx",
> +				(int)__virt_to_fix(vaddr),
> +				pte_val(new_pte), pte_val(old_pte))) {
> +		/*
> +		 * It is _NOT_ enough to flush just the local mapping;
> +		 * it might mostly work, but there is no guarantee a remote
> +		 * CPU didn't load this entry into its TLB.
> +		 */
> +		__flush_tlb_one_kernel(vaddr);
> +	} else {
> +		flush_tlb_kernel_range(vaddr, vaddr + PAGE_SIZE);
> +	}
> }
> 
> void set_pte_vaddr_p4d(p4d_t *p4d_page, unsigned long vaddr, pte_t new_pte)
> diff --git a/drivers/acpi/apei/Kconfig b/drivers/acpi/apei/Kconfig
> index 52ae5438edeb..e3c415bdbfbf 100644
> --- a/drivers/acpi/apei/Kconfig
> +++ b/drivers/acpi/apei/Kconfig
> @@ -19,6 +19,7 @@ config ACPI_APEI
> 
> config ACPI_APEI_GHES
> 	bool "APEI Generic Hardware Error Source"
> +	depends on BROKEN if X86
> 	depends on ACPI_APEI
> 	select ACPI_HED
> 	select IRQ_WORK

Indeed, I missed these other instances that use the fixmap.

I’ll give your patch a try once my server goes back online. I was (and still
am) worried that interrupts would be disabled when __set_pte_vaddr() is
called, which would make the fix more complicated.

In addition, there might be a couple of issues with your fix:

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.

2. Printing the virtual address can break KASLR.

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...

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

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

Regards,
Nadav