Re: [PATCH 5.10.y] bpf: Enforce expected_attach_type for tailcall compatibility

[sashiko-bot@kernel.org]

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

Pre-existing issues:
- [High] Incomplete fix for tailcall expected_attach_type bypass: BPF_PROG_TYPE_EXT programs bypass the check.
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commit d87774423a605def68b6715a3edf5dc1feea5da7
Author: Daniel Borkmann <daniel@iogearbox.net>

bpf: Enforce expected_attach_type for tailcall compatibility

This patch aims to prevent context confusion by enforcing that programs
inserted into a tailcall map share the same expected_attach_type.
This restricts tailcalls from violating attach type constraints.

Link: https://lore.kernel.org/r/20250926171201.188490-1-daniel@iogearbox.net

> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
[ ... ]
> @@ -1795,10 +1795,14 @@ bool bpf_prog_array_compatible(struct bpf_array *array,
[ ... ]
>  	} else {
>  		ret = array->aux->owner.type  == fp->type &&
>  		      array->aux->owner.jited == fp->jited;
> +		if (ret &&
> +		    array->aux->owner.expected_attach_type != fp->expected_attach_type)
> +			ret = false;

[Severity: High]
This is a pre-existing issue, but does this check effectively become 0 != 0
for all BPF_PROG_TYPE_EXT programs, allowing arbitrary mixing of EXT programs
with different underlying target contexts?

It appears that for BPF_PROG_TYPE_EXT programs, expected_attach_type is
reset to 0 at the end of verification in bpf_check().

If userspace loads two BPF_PROG_TYPE_EXT programs targeting entirely
different functions (e.g., XDP_DEVMAP vs XDP_CPUMAP) and inserts them both
into the same BPF_MAP_TYPE_PROG_ARRAY, couldn't a tailcall between them
bypass this expected_attach_type check?

Can this allow a program verified for one target context to execute in a
different context, potentially leading to out-of-bounds accesses or NULL
pointer dereferences?

>  	}
>  	spin_unlock(&array->aux->owner.lock);
>  	return ret;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908-cve-2025-40123-v1-1-be62451196fb@bootlin.com?part=1