Re: [RFC PATCH 3/3] luo: memfd: Move to feature flags instead of compatibility strings

[sashiko-bot@kernel.org]

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

Pre-existing issues:
- [High] Permanent memory leak of preserved KHO vmalloc block and physical folios when memfd_luo_retrieve() fails on early error paths.
--

commit 5ce2719ae4caecc0b1ca1429438f4dfe06609564
Author: Logan Odell <loganodell@google.com>

luo: memfd: Move to feature flags instead of compatibility strings

This commit updates the memfd liveupdate serialization structure to use
feature flags instead of compatibility strings. It introduces granular
compatibility checking by validating required features and active flags
during deserialization to prevent unsupported states from being restored.

> diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
> index 59de210bee5f9..36ee503672a23 100644
> --- a/mm/memfd_luo.c
> +++ b/mm/memfd_luo.c
[ ... ]
> @@ -526,12 +531,21 @@ static int memfd_luo_retrieve(struct liveupdate_file_op_args *args)
>  	if (!ser)
>  		return -EINVAL;
>  
> -	/* Make sure the file only has seals supported by this version. */
> -	if (ser->seals & ~MEMFD_LUO_ALL_SEALS) {
> +	if (ser->features.req & ~MEMFD_LUO_FEATURES_SUPP) {
> +		pr_err("Unsupported required memfd feature (req: 0x%llx, supp: 0x%llx)\n",
> +		       ser->features.req, (u64)MEMFD_LUO_FEATURES_SUPP);
>  		err = -EOPNOTSUPP;
>  		goto free_ser;
>  	}
>  
> +	if (LUO_FEATURE_IS_ACTIVE(ser, MEMFD_LUO_FEATURE_SEALS)) {
> +		/* Make sure the file only has seals supported by this version. */
> +		if (ser->seals & ~MEMFD_LUO_ALL_SEALS) {
> +			err = -EOPNOTSUPP;
> +			goto free_ser;
> +		}
> +	}
> +

[Severity: High]
This is a pre-existing issue, but does this early error path permanently leak
the preserved KHO vmalloc block and physical folios?

When memfd_luo_retrieve() in mm/memfd_luo.c fails here and jumps to the
free_ser label:

free_ser:
	kho_restore_free(ser);
	return err;

It abruptly frees the serialization struct. Bypassing the restoration of the
vmalloc array and physical folios leaves them locked in memblock indefinitely.

Furthermore, any fallback cleanup is skipped because the finalizer
memfd_luo_finish() returns immediately if args->retrieve_status is set:

mm/memfd_luo.c:memfd_luo_finish() {
    ...
	if (args->retrieve_status)
		return;
    ...
}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260903023452.721732-1-loganodell@google.com?part=3