Re: [REGRESSION] Discussion on "xfrm: Duplicate SPI Handling"

Nathan Harold
2026-02-09T19:06:01Z

Unfortunately, Android's requirements significantly limit our ability
to implement compatibility fixes between the OS and kernel due to
interface freezes and cross-version compatibility requirements, so
thank you for taking us into consideration! Generally, without a
userspace mechanism to opt in to new behavior though, it's tough. For
now, the Android kernel is just reverting these patches, but that's
not a long-term strategy.

One approach, which seems generally sound to me for this specific
issue (and perhaps more broadly), would be to leverage the relatively
new XFRM_SA_DIR attribute to distinguish between legacy behavior and
stricter 4301 compliance. Much of the optimization is
direction-specific, so we can infer that an implementation using the
newer API and opting in to direction-specific behavior wants 4301/new
feature support--also, Android doesn't yet use that attribute :-) .
Or, as Steffen suggested, there could be a new toggle that indicates a
standards-based target behavior (like 4301), which sounds great if the
SA_DIR flag is deemed insufficient. Thoughts?

-Nathan

On Sun, Feb 8, 2026 at 7:03 PM Yan Yan <evitayan@google.com> wrote:
>
> Thank you all for the reply! Please see my response inline below
>
> > This is meant for multicast SAs, we don't support multicast SAs.
> I reread the RFC and that makes sense to me. Thanks for the explanation.
>
> > To clarify, how are larval outbound SAs created in Android?
> > Are they created with zero or non-zero SPIs? Multiple zero SPIs (acquire states) are allowed.
>
> In Android, larval outbound SAs are created with non-zero SPIs because
> Android userspace uses XFRM_MSG_ALLOCSPI to manually reserve a unique
> value before the SA is fully finalized. These are not kernel-generated
> "acquire" states.
>
> Regarding the default behavior:
>
> I'm fine with defaulting to RFC 4301 for the upstream kernel to align
> with the modern standard. As long as a toggle exists to fallback to
> the legacy RFC 2401 behavior, Android’s compatibility requirements are
> met.
>
>
>
>
> On Mon, Feb 2, 2026 at 10:27 PM Antony Antony <antony.antony@secunet.com> wrote:
> >
> > Hi Yan,
> >
> > On Wed, Jan 28, 2026 at 16:43:06 +0800, Yan Yan wrote:
> > > Hi all,
> > >
> > > I am writing because unfortunately commit: 94f39804d891 ("xfrm:
> > > Duplicate SPI Handling") has caused a regression in the Android OS, so
> > > we would like to gain some context to help determine next steps. The
> > > issue is caused by the new requirement for global SPI uniqueness
> > > during allocation. Based on our review of RFC 4301 and the previous
> > > review history, we would like to highlight a few concerns:
> > >
> > > 1. Regression on Android:
> > > This change breaks production behavior in Android, which uses
> > > XFRM_MSG_ALLOCSPI to create larval SAs for both directions. Since RFC
> > > 4301 allows duplicate outbound SPIs, and larval SAs are often created
> >
> > To clarify, how are larval outbound SAs created in Android?
> > Are they created with zero or non-zero SPIs? Multiple zero SPIs (acquire states) are allowed. However, I guess there is no user API for managing these acquire states. Only the kernel can create them and handle expiration or deletion via SA updates with acq_req?
> >
> > A user API (UAPI) for creating and deleting acquire states might be a possible solution.
> >
> > I haven’t been able to consistently reproduce the issue Marvell reported,
> > but I suspect the bug could also affect outbound SAs with non-zero SPIs.
> > Also when one peer is behind NAT.
> >
> > I wonder wouldn't duplicate SPI when behind NAT cause issues for output SAs?
> > Because the triplet is SPI, Protocol, Daddr. There is no dport in it.
> >
> > > before direction or selectors are finalized, the allocator must remain
> > > permissive (at least in our current design).
> > > This also aligns with a concern Herbert Xu raised during the initial
> > > review regarding compatibility:
> > > > "It's also dangerous to unilaterally do this since existing deployments
> > > > could rely on the old behaviour. You'd need to add a toggle for
> > > > compatibility."
> > >
> > > 2. Inbound SPI uniqueness should not be a hard requirement:
> > > The justification for enforcing global SPI uniqueness often cites the
> > > statement in RFC 4301, Section 4.1, that for unicast traffic, the SPI
> > > "by itself suffices to specify an SA." However, we don’t think this
> > > means inbound SPI uniqueness is a hard requirement because of the two
> > > following reasons:
> > >
> > > – Another statement implies that SPI uniqueness is just an
> > > implementation choice:
> > > > "Each entry in the SA Database (SAD) MUST
> > > > indicate whether the SA lookup makes use of the destination IP address, or the
> > > > destination and source IP addresses, in addition to the SPI."
> > >
> > > – There is a "Longest Match" mandate which makes SPI uniqueness unnecessary:
> > > > "For each inbound, IPsec-protected packet, an implementation MUST
> > > > conduct its search of the SAD such that it finds the entry that
> > > > matches the 'longest' SA identifier. In this context, if two or more
> > > > SAD entries match based on the SPI value, then the entry that also
> > > > matches based on destination address... is the 'longest' match."
> > >
> > > 3. Further clarification on the specific problem being addressed would
> > > be helpful. The "real-world" problem this commit intends to fix
> > > remains unclear. The patch mentions:
> > > > "This behavior causes inconsistencies during SPI lookups for inbound packets.
> > > > Since the lookup may return an arbitrary SA among those with the same SPI,
> > > > packet processing can fail, resulting in packet drops."
> > >
> > > However, Linux kernel lookups using the triplet (SPI, Protocol, Daddr)
> > > are deterministic. The lookup will not return an "arbitrary" SA
> > > because the destination address is used to disambiguate the state.
> > >
> > > As Antony suggested, this change may cater to SPI-only hardware
> > > indexing. If that is the case, we are concerned about applying such
> > > hardware-specific limits to the software stack, especially if the
> > > behavior is not opt-in, as it appears to require an overly-narrow
> > > reading of the RFC 4301.
> >
> > I agree with your suggestion that making the behavior opt-in.
> > I would prefer the Default : to allow duplicate.
> >
> > > Given these concerns, would it be possible to discuss a revert or,
> > > alternatively, could further context be provided regarding the
> > > specific real-world problem this commit was intended to address? Once
> > > the underlying issue is clearly defined, we can work together to find
> > > a backward-compatible solution that satisfies all requirements.
> > >
> > > Review threads are attached for easy reference:
> > > https://lore.kernel.org/netdev/aDQhZ_ikHEt_pLn_@gondor.apana.org.au/T/#r45c1786651ce5af730f757aca7438474d494a323
> > > https://lore.kernel.org/netdev/20250616100621.837472-1-saakashkumar@marvell.com/T/#u
> >
> > -antony
>
>
>
> --
> --
> Best,
> Yan