Re: [PATCH bpf v1 1/2] bpf: Reject non-scalar bpf_loop iteration counts

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Missing negative bounds checks for enum array indices in memcg BPF kfuncs (bpf_mem_cgroup_memory_events and bpf_mem_cgroup_vm_events).
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commit 3f8d69cede626ae022617ccb30f54b8f41f90edd
Author: Kumar Kartikeya Dwivedi <memxor@gmail.com>

bpf: Reject non-scalar bpf_loop iteration counts

This commit fixes a verifier warning and subsequent panic triggered when
pointer values are passed to the bpf_loop() nr_loops argument. It
introduces ARG_SCALAR to ensure generic helper argument validation
rejects pointers before loop inlining and precision processing.

Link: https://lore.kernel.org/bpf/6a9ad24c.b5d4176b.238c3e.0001.GAE@google.com/

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 3a7eb2185c354..591dfab3e706c 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -894,6 +894,7 @@ enum bpf_arg_type {
>  
>  	ARG_PTR_TO_CTX,		/* pointer to context */
>  	ARG_ANYTHING,		/* any (initialized) argument is ok */
> +	ARG_SCALAR,		/* scalar argument */

[Severity: High]
This isn't a bug introduced by this patch, but while reviewing the addition
of ARG_SCALAR and how the verifier handles scalar arguments, a pre-existing
missing bounds check was noticed in the memory cgroup kfuncs.

Could negative enum values passed by untrusted BPF programs cause an
out-of-bounds read in the memcg BPF kfuncs?

Looking at mm/bpf_memcontrol.c:bpf_mem_cgroup_memory_events():

	if (unlikely(event >= MEMCG_NR_MEMORY_EVENTS))
		return (unsigned long)-1;

	return atomic_long_read(&memcg->memory_events[event]);

And similarly in mm/bpf_memcontrol.c:bpf_mem_cgroup_vm_events(), which
exposes validation logic from mm/memcontrol.c:memcg_vm_event_item_valid():

	if (idx >= NR_VM_EVENT_ITEMS)
		return false;

	return !BAD_STAT_IDX(memcg_events_index(idx));

Because the BPF verifier enforces scalar types but does not validate enum
ranges, a BPF program can pass a negative integer as the event parameter.
The bounds checks use signed comparisons without a lower bound (< 0) check,
so negative values will bypass the validation (e.g., -1 >= 4 evaluates to
false).

Can this lead to an out-of-bounds array access on kernel memory when the
negative value is used as an index into memcg->memory_events?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260905014735.1452988-1-memxor@gmail.com?part=1