Re: [PATCH v3 6/7] x86/alternatives: use temporary mm for text poking

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 02, 2018 at 04:29:45PM -0700, Nadav Amit wrote:
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 9ceae28db1af..1a40df4db450 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c

> @@ -699,41 +700,110 @@ __ro_after_init unsigned long poking_addr;
>   */
>  void *text_poke(void *addr, const void *opcode, size_t len)
>  {
> +	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
> +	temporary_mm_state_t prev;
>  	struct page *pages[2];
> +	unsigned long flags;
> +	pte_t pte, *ptep;
> +	spinlock_t *ptl;
>  
>  	/*
> +	 * While boot memory allocator is running we cannot use struct pages as
> +	 * they are not yet initialized.
>  	 */
>  	BUG_ON(!after_bootmem);
>  
>  	if (!core_kernel_text((unsigned long)addr)) {
>  		pages[0] = vmalloc_to_page(addr);
> +		if (cross_page_boundary)
> +			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
>  	} else {
>  		pages[0] = virt_to_page(addr);
>  		WARN_ON(!PageReserved(pages[0]));
> +		if (cross_page_boundary)
> +			pages[1] = virt_to_page(addr + PAGE_SIZE);
>  	}
> +
> +	/* TODO: let the caller deal with a failure and fail gracefully. */
>  	BUG_ON(!pages[0]);
> +	BUG_ON(cross_page_boundary && !pages[1]);
>  	local_irq_save(flags);
> +
> +	/*
> +	 * The lock is not really needed, but this allows to avoid open-coding.
> +	 */
> +	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
> +
> +	/*
> +	 * If we failed to allocate a PTE, fail silently. The caller (text_poke)

we _are_ text_poke()..

> +	 * will detect that the write failed when it compares the memory with
> +	 * the new opcode.
> +	 */
> +	if (unlikely(!ptep))
> +		goto out;

This is the one site I'm a little uncomfortable with; OTOH it really
never should happen, since we explicitily instantiate these page-tables
earlier.

Can't we simply assume ptep will not be zero here? Like with so many
boot time memory allocations, we mostly assume they'll work.

> +	pte = mk_pte(pages[0], PAGE_KERNEL);
> +	set_pte_at(poking_mm, poking_addr, ptep, pte);
> +
> +	if (cross_page_boundary) {
> +		pte = mk_pte(pages[1], PAGE_KERNEL);
> +		set_pte_at(poking_mm, poking_addr + PAGE_SIZE, ptep + 1, pte);
> +	}
> +
> +	/*
> +	 * Loading the temporary mm behaves as a compiler barrier, which
> +	 * guarantees that the PTE will be set at the time memcpy() is done.
> +	 */
> +	prev = use_temporary_mm(poking_mm);
> +
> +	kasan_disable_current();
> +	memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
> +	kasan_enable_current();
> +
> +	/*
> +	 * Ensure that the PTE is only cleared after the instructions of memcpy
> +	 * were issued by using a compiler barrier.
> +	 */
> +	barrier();
> +
> +	pte_clear(poking_mm, poking_addr, ptep);
> +
> +	/*
> +	 * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
> +	 * as it also flushes the corresponding "user" address spaces, which
> +	 * does not exist.
> +	 *
> +	 * Poking, however, is already very inefficient since it does not try to
> +	 * batch updates, so we ignore this problem for the time being.
> +	 *
> +	 * Since the PTEs do not exist in other kernel address-spaces, we do
> +	 * not use __flush_tlb_one_kernel(), which when PTI is on would cause
> +	 * more unwarranted TLB flushes.
> +	 *
> +	 * There is a slight anomaly here: the PTE is a supervisor-only and
> +	 * (potentially) global and we use __flush_tlb_one_user() but this
> +	 * should be fine.
> +	 */
> +	__flush_tlb_one_user(poking_addr);
> +	if (cross_page_boundary) {
> +		pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
> +		__flush_tlb_one_user(poking_addr + PAGE_SIZE);
> +	}
> +
> +	/*
> +	 * Loading the previous page-table hierarchy requires a serializing
> +	 * instruction that already allows the core to see the updated version.
> +	 * Xen-PV is assumed to serialize execution in a similar manner.
> +	 */
> +	unuse_temporary_mm(prev);
> +
> +	pte_unmap_unlock(ptep, ptl);
> +out:
> +	/*
> +	 * TODO: allow the callers to deal with potential failures and do not
> +	 * panic so easily.
> +	 */
> +	BUG_ON(memcmp(addr, opcode, len));
>  	local_irq_restore(flags);
>  	return addr;
>  }