Re: [PATCH bpf-next v2 1/4] bpf: Add a sleepable page allocator for map memory

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:

New issues:
- [High] Using __GFP_ZERO to zero pages intended for user-space mapping skips required dcache flushes on cache-aliasing architectures.
- [Medium] bpf_map_alloc_page_sleepable() and __bpf_alloc_page() silently bypass task/VMA NUMA mempolicy when the requested node is NUMA_NO_NODE.

Pre-existing issues:
- [High] The fallback allocation path in __bpf_alloc_page() omits __GFP_ZERO, returning uninitialized kernel memory that can be exposed to user space.
--

--- Patch [1]: [PATCH bpf-next v2 1/4] bpf: Add a sleepable page allocator for map memory ---
commit 2a4210c942ca919834ce20218ab8bedc6cbb40a8
Author: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>

bpf: Add a sleepable page allocator for map memory

This patch adds bpf_map_alloc_page_sleepable() to provide a blocking page
allocator for sleepable contexts. This avoids using the non-blocking
allocator, which never reclaims or engages the OOM machinery, for contexts
that can actually sleep.

> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 8d111da886553..67d8157c6623e 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -602,15 +602,14 @@ static bool can_alloc_pages(void)
>  		!IS_ENABLED(CONFIG_PREEMPT_RT);
>  }
>  
> +#define BPF_PAGE_GFP (GFP_KERNEL | __GFP_ZERO | __GFP_ACCOUNT | __GFP_NOWARN)
> +
>  static struct page *__bpf_alloc_page(int nid)
>  {
>  	if (!can_alloc_pages())
>  		return alloc_pages_nolock(__GFP_ACCOUNT, nid, 0);

[Severity: High]
This is a pre-existing issue, but does this fallback path leak uninitialized
kernel memory? 

The call to alloc_pages_nolock() passes only __GFP_ACCOUNT and completely
omits __GFP_ZERO. If a BPF map is allocated on PREEMPT_RT where
can_alloc_pages() returns false, this could expose uninitialized pages to
user space.

>  
> -	return alloc_pages_node(nid,
> -				GFP_KERNEL | __GFP_ZERO | __GFP_ACCOUNT
> -				| __GFP_NOWARN,
> -				0);
> +	return alloc_pages_node(nid, BPF_PAGE_GFP, 0);

[Severity: Medium]
When nid is NUMA_NO_NODE, does alloc_pages_node() bypass the task's NUMA
mempolicy?

Since alloc_pages_node() automatically resolves NUMA_NO_NODE to the local
CPU node rather than defaulting to the task's mempolicy, this could cause
pages to land on the wrong nodes for user applications.

>  }
>  
[ ... ]
> @@ -636,6 +635,20 @@ int bpf_map_alloc_pages(const struct bpf_map *map, int nid,
>  	return ret;
>  }
>  
> +/*
> + * For callers that know they run in a sleepable context, e.g. a user page
> + * fault handler. can_alloc_pages() is a conservative guess made for BPF
> + * program context - notably it is always false on PREEMPT_RT - so going
> + * through bpf_map_alloc_pages() there would needlessly pick the
> + * non-blocking allocator, which never reclaims and never engages the OOM
> + * machinery.
> + */
> +struct page *bpf_map_alloc_page_sleepable(const struct bpf_map *map)
> +{
> +	might_sleep();
> +	return alloc_pages_node(map->numa_node, BPF_PAGE_GFP, 0);

[Severity: High]
Does using __GFP_ZERO (via BPF_PAGE_GFP) skip required dcache flushes on
cache-aliasing architectures? 

When allocating pages destined for user-space mapping (like the arena page
fault handler), __GFP_ZERO uses clear_page() which leaves zeros in the
kernel's dcache. Could user processes reading from these pages see stale,
uninitialized physical memory instead of zeros?

[Severity: Medium]
Similar to the issue in __bpf_alloc_page(), does using alloc_pages_node()
here bypass the task mempolicy when map->numa_node is NUMA_NO_NODE?

> +}
> +

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260805091720.139924-1-jiayuan.chen@xxxxxxxxx?part=1