Re: [PATCH 2/2] mm/page_alloc: don't spin_trylock() when disallowed in free_one_page()

Re: [PATCH 2/2] mm/page_alloc: don't spin_trylock() when disallowed in free_one_page()

[Vlastimil Babka (SUSE)]

On 7/15/26 11:03, Brendan Jackman wrote:
> __free_frozen_pages() checks can_spin_trylock() before entering into the
> main free_frozen_page_commit()/free_one_page() path, but before this it
> can get to free_one_page() via the !pcp_allowed_order() and
> MIGRATE_ISOLATE paths.
> 
> The !pcp_allowed_order() path depends on usage by callers so might not
> be possible in practice. The MIGRATE_ISOLATE path probably means kernel
> crashes and privilege escalation if anyone ever did memory hotplug and
> BPF tracing on a PREEMPT_RT or !SMP build.
> 
> Cc: stable@vger.kernel.org
> Fixes: 8c57b687e8331 ("mm, bpf: Introduce free_pages_nolock()")
> Reported-by: sashiko-bot@kernel.org
> Link: https://sashiko.dev/#/patchset/20260710-spin-trylock-followup-v1-0-affb5fe5ed00%40google.com?part=2

Closes: also

> Signed-off-by: Brendan Jackman <jackmanb@google.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5f9873dfccc5a..46e5ea59c71df 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1559,7 +1559,7 @@ static void free_one_page(struct zone *zone, struct page *page,
>  	unsigned long flags;
>  
>  	if (unlikely(fpi_flags & FPI_TRYLOCK)) {
> -		if (!spin_trylock_irqsave(&zone->lock, flags)) {
> +		if (!can_spin_trylock() || !spin_trylock_irqsave(&zone->lock, flags)) {
>  			add_page_to_zone_llist(zone, page, order);
>  			return;
>  		}
> 

Re: [PATCH 2/2] mm/page_alloc: don't spin_trylock() when disallowed in free_one_page()

[Harry Yoo]

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On 7/15/26 6:03 PM, Brendan Jackman wrote:
> __free_frozen_pages() checks can_spin_trylock() before entering into the
> main free_frozen_page_commit()/free_one_page() path, but before this it
> can get to free_one_page() via the !pcp_allowed_order() and
> MIGRATE_ISOLATE paths.
> 
> The !pcp_allowed_order() path depends on usage by callers so might not
> be possible in practice. The MIGRATE_ISOLATE path probably means kernel
> crashes and privilege escalation if anyone ever did memory hotplug and
> BPF tracing on a PREEMPT_RT or !SMP build.
> 
> Cc: stable@vger.kernel.org
> Fixes: 8c57b687e8331 ("mm, bpf: Introduce free_pages_nolock()")
> Reported-by: sashiko-bot@kernel.org
> Link: https://sashiko.dev/#/patchset/20260710-spin-trylock-followup-v1-0-affb5fe5ed00%40google.com?part=2
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---

Looks good to me, so:
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>

-- 
Cheers,
Harry / Hyeonggon

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Re: [PATCH 0/2] mm/page_alloc: fixes for free_pages_nolock() on RT/UP

[Andrew Morton]

On Wed, 15 Jul 2026 09:03:57 +0000 Brendan Jackman <jackmanb@google.com> wrote:

> First patch was originally part of a different series but Andrew asked
> for it to be separate:
> https://lore.kernel.org/all/20260710170311.e22bfd21c658e8357ceddeec@linux-foundation.org/
> 
> Pre-existing bugs found by Sashiko during review of this other series:
> https://lore.kernel.org/all/20260703-alloc-trylock-v5-0-c87b714e19d3@google.com/
> 
> I have not reproduced these bugs, and I suspect there is no real-world
> user that is affected by them.

Right, thanks.  I'll stage these for the next merge window - no need to
rush these fixes into mainline.

And I staged them ahead of your "mm/page_alloc: rename FPI_TRYLOCK ->
FPI_NOLOCK", to lessen backporting hassles.

Re: [PATCH 1/2] mm/page_alloc: don't spin_trylock() in NMI on UP

[Vlastimil Babka (SUSE)]

On 7/15/26 11:03, Brendan Jackman wrote:
> As noted in can_spin_trylock(), using this is unsafe in this context.
> commit 620b46ed6ae17 ("mm/page_alloc: return NULL early from
> alloc_frozen_pages_nolock() in NMI on UP") fixed this on the alloc side
> but missed the free side.
> 
> Impact: If BPF programs using these features in NMI (probably tracing)
> are present on non-SMP builds this might crash the kernel and is
> probably exploitable by local attackers for privilege escalation.
> 
> Reported-by: sashiko-bot@kernel.org
> Link: https://sashiko.dev/#/patchset/20260703-alloc-trylock-v5-0-c87b714e19d3%40google.com?part=18

Should be "Closes:"?
(Andrew can fix up locally)

> Cc: stable@vger.kernel.org
> Fixes: 8c57b687e8331 ("mm, bpf: Introduce free_pages_nolock()")
> Signed-off-by: Brendan Jackman <jackmanb@google.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  mm/page_alloc.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index af63558391345..5f9873dfccc5a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2979,8 +2979,7 @@ static void __free_frozen_pages(struct page *page, unsigned int order,
>  		migratetype = MIGRATE_MOVABLE;
>  	}
>  
> -	if (unlikely((fpi_flags & FPI_TRYLOCK) && IS_ENABLED(CONFIG_PREEMPT_RT)
> -		     && (in_nmi() || in_hardirq()))) {
> +	if (unlikely((fpi_flags & FPI_TRYLOCK) && !can_spin_trylock())) {
>  		add_page_to_zone_llist(zone, page, order);
>  		return;
>  	}
> 

Re: [PATCH 1/2] mm/page_alloc: don't spin_trylock() in NMI on UP

[Harry Yoo]

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On 7/15/26 6:03 PM, Brendan Jackman wrote:
> As noted in can_spin_trylock(), using this is unsafe in this context.
> commit 620b46ed6ae17 ("mm/page_alloc: return NULL early from
> alloc_frozen_pages_nolock() in NMI on UP") fixed this on the alloc side
> but missed the free side.
> 
> Impact: If BPF programs using these features in NMI (probably tracing)
> are present on non-SMP builds this might crash the kernel and is
> probably exploitable by local attackers for privilege escalation.
> 
> Reported-by: sashiko-bot@kernel.org
> Link: https://sashiko.dev/#/patchset/20260703-alloc-trylock-v5-0-c87b714e19d3%40google.com?part=18
> Cc: stable@vger.kernel.org
> Fixes: 8c57b687e8331 ("mm, bpf: Introduce free_pages_nolock()")
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---

Looks good to me, so:
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>

-- 
Cheers,
Harry / Hyeonggon

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