linux-hardening - Re: [RFC PATCH v2 10/19] x86/mm: Use alloc_table() for fill

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Message-ID: <YS3stfkkCLfqROx1@kernel.org>
Date:   Tue, 31 Aug 2021 11:47:49 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     dave.hansen@...el.com, luto@...nel.org, peterz@...radead.org,
        x86@...nel.org, akpm@...ux-foundation.org, keescook@...omium.org,
        shakeelb@...gle.com, vbabka@...e.cz, linux-mm@...ck.org,
        linux-hardening@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, ira.weiny@...el.com,
        dan.j.williams@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 10/19] x86/mm: Use alloc_table() for fill_pte(),
 etc

On Mon, Aug 30, 2021 at 04:59:18PM -0700, Rick Edgecombe wrote:
> fill_pte(), set_pte_vaddr(), etc allocate page tables with
> spp_getpage(). Use alloc_table() for these allocations in order to get
> tables from the cache of protected pages when needed.
 
I can't say I tracked all the users of set_pte_vaddr(), but I don't see a
fundamental reason why spp_getpage() would need GFP_ATOMIC. Even if there
is a caller of set_pte_vaddr() that cannot sleep, it seems that page tables
can be prepopulated so that set_pte_vaddr() will not need to allocate
anything.  

> Opportunistically, fix a stale comment.

Ack for this one :)
 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
>  arch/x86/mm/init_64.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 3c0323ad99da..de5a785ee89f 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -220,16 +220,19 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
>  
>  /*
>   * NOTE: This function is marked __ref because it calls __init function
> - * (alloc_bootmem_pages). It's safe to do it ONLY when after_bootmem == 0.
> + * (memblock_alloc). It's safe to do it ONLY when after_bootmem == 0.
>   */
>  static __ref void *spp_getpage(void)
>  {
>  	void *ptr;
>  
> -	if (after_bootmem)
> -		ptr = (void *) get_zeroed_page(GFP_ATOMIC);
> -	else
> +	if (after_bootmem) {
> +		struct page *page = alloc_table(GFP_ATOMIC | __GFP_ZERO);
> +
> +		ptr = page ? page_address(page) : NULL;
> +	} else {
>  		ptr = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
> +	}
>  
>  	if (!ptr || ((unsigned long)ptr & ~PAGE_MASK)) {
>  		panic("set_pte_phys: cannot allocate page data %s\n",
> -- 
> 2.17.1
> 

-- 
Sincerely yours,
Mike.