Date:   Wed, 5 Apr 2023 13:45:19 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org,
        Alexander Halbuer <halbuer@....uni-hannover.de>
Subject: Re: [PATCH] mm, page_alloc: reduce page alloc/free sanity checks

On Thu, Feb 16, 2023 at 10:51:31AM +0100, Vlastimil Babka wrote:
> Historically, we have performed sanity checks on all struct pages being
> allocated or freed, making sure they have no unexpected page flags or
> certain field values. This can detect insufficient cleanup and some
> cases of use-after-free, although on its own it can't always identify
> the culprit. The result is a warning and the "bad page" being leaked.
> 
> The checks do need some cpu cycles, so in 4.7 with commits 479f854a207c
> ("mm, page_alloc: defer debugging checks of pages allocated from the
> PCP") and 4db7548ccbd9 ("mm, page_alloc: defer debugging checks of freed
> pages until a PCP drain") they were no longer performed in the hot paths
> when allocating and freeing from pcplists, but only when pcplists are
> bypassed, refilled or drained. For debugging purposes, with
> CONFIG_DEBUG_VM enabled the checks were instead still done in the
> hot paths and not when refilling or draining pcplists.
> 
> With 4462b32c9285 ("mm, page_alloc: more extensive free page checking
> with debug_pagealloc"), enabling debug_pagealloc also moved the sanity
> checks back to hot pahs. When both debug_pagealloc and CONFIG_DEBUG_VM
> are enabled, the checks are done both in hotpaths and pcplist
> refill/drain.
> 
> Even though the non-debug default today might seem to be a sensible
> tradeoff between overhead and ability to detect bad pages, on closer
> look it's arguably not. As most allocations go through the pcplists,
> catching any bad pages when refilling or draining pcplists has only a
> small chance, insufficient for debugging or serious hardening purposes.
> On the other hand the cost of the checks is concentrated in the already
> expensive drain/refill batching operations, and those are done under the
> often contended zone lock. That was recently identified as an issue for
> page allocation and the zone lock contention reduced by moving the
> checks outside of the locked section with a patch "mm: reduce lock
> contention of pcp buffer refill", but the cost of the checks is still
> visible compared to their removal [1]. In the pcplist draining path
> free_pcppages_bulk() the checks are still done under zone->lock.
> 
> Thus, remove the checks from pcplist refill and drain paths completely.
> Introduce a static key check_pages_enabled to control checks during page
> allocation a freeing (whether pcplist is used or bypassed). The static
> key is enabled if either is true:
> - kernel is built with CONFIG_DEBUG_VM=y (debugging)
> - debug_pagealloc or page poisoning is boot-time enabled (debugging)
> - init_on_alloc or init_on_free is boot-time enabled (hardening)
> 
> The resulting user visible changes:
> - no checks when draining/refilling pcplists - less overhead, with
>   likely no practical reduction of ability to catch bad pages
> - no checks when bypassing pcplists in default config (no
>   debugging/hardening) - less overhead etc. as above
> - on typical hardened kernels [2], checks are now performed on each page
>   allocation/free (previously only when bypassing/draining/refilling
>   pcplists) - the init_on_alloc/init_on_free enabled should be sufficient
>   indication for preferring more costly alloc/free operations for
>   hardening purposes and we shouldn't need to introduce another toggle
> - code (various wrappers) removal and simplification
> 
> [1] https://lore.kernel.org/all/68ba44d8-6899-c018-dcb3-36f3a96e6bea@sra.uni-hannover.de/
> [2] https://lore.kernel.org/all/63ebc499.a70a0220.9ac51.29ea@mx.google.com/
> 
> Reported-by: Alexander Halbuer <halbuer@....uni-hannover.de>
> Reported-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

Some minor comments below

> @@ -1432,9 +1448,11 @@ static __always_inline bool free_pages_prepare(struct page *page,
>  		for (i = 1; i < (1 << order); i++) {
>  			if (compound)
>  				bad += free_tail_pages_check(page, page + i);

free_tail_pages_check is also a function that only does something useful
when CONFIG_DEBUG_VM is set. While it might be outside the scope of the
patch, it might also benefit from check_pages_enabled checks?

> -			if (unlikely(free_page_is_bad(page + i))) {
> -				bad++;
> -				continue;
> +			if (static_branch_unlikely(&check_pages_enabled)) {
> +				if (unlikely(free_page_is_bad(page + i))) {
> +					bad++;
> +					continue;
> +				}
>  			}
>  			(page + i)->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
>  		}

The unlikely() within a static_branch_unlikely probably adds very little
given the block is so tiny. 

> @@ -2392,56 +2369,20 @@ static inline int check_new_page(struct page *page)
>  	return 1;
>  }
>  
> -static bool check_new_pages(struct page *page, unsigned int order)
> +static inline bool check_new_pages(struct page *page, unsigned int order)
>  {
> -	int i;
> -	for (i = 0; i < (1 << order); i++) {
> -		struct page *p = page + i;
> +	if (static_branch_unlikely(&check_pages_enabled)) {
> +		for (int i = 0; i < (1 << order); i++) {
> +			struct page *p = page + i;
>  
> -		if (unlikely(check_new_page(p)))
> -			return true;
> +			if (unlikely(check_new_page(p)))
> +				return true;
> +		}
>  	}
>  

unlikely() within static_branch_unlikely probably adds very little.

Otherwise, looks good. I agree that with changes over time that the ability
of the checks to detect anything is reduced and it's probably at the point
where it can only detect a very specific bit corruption instead of broken
code. Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be
stored on the per-cpu lists") also likely reduced the ability of the checks
to find anything.

-- 
Mel Gorman
SUSE Labs

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