Re: [PATCH ipsec] xfrm: use compat translator only for u64 alignment mismatch

[Steffen Klassert]

On Sun, Jun 07, 2026 at 04:47:34PM +0000, Pradhan, Sanman wrote:
> From: Sanman Pradhan <psanman@xxxxxxxxxxx>
> 
> The XFRM compat layer (CONFIG_XFRM_USER_COMPAT) translates 32-bit xfrm
> netlink and setsockopt messages into the native 64-bit layout. It is
> only needed on architectures where the 32-bit and 64-bit ABIs disagree
> on u64 alignment, which the kernel encodes as COMPAT_FOR_U64_ALIGNMENT.
> 
> That symbol is defined only by arch/x86. XFRM_USER_COMPAT depends on it,
> so the translator can never be built on any other architecture,
> including arm64, which still provides a 32-bit compat ABI (CONFIG_COMPAT)
> for AArch32 EL0 userspace. On arm64 the AArch32 EABI already aligns u64
> to 8 bytes, identical to the AArch64 ABI, so no translation is required
> and the native code path is correct for 32-bit tasks.
> 
> However, xfrm_user_rcv_msg() and xfrm_user_policy() gate on
> in_compat_syscall() alone and then call xfrm_get_translator(), which
> returns NULL when no translator is registered. On arm64 that is always
> the case, so every xfrm netlink message and the XFRM_POLICY setsockopt
> issued by a 32-bit task returns -EOPNOTSUPP. A 32-bit userspace process
> on arm64 (and on any other arch with CONFIG_COMPAT but without
> COMPAT_FOR_U64_ALIGNMENT) therefore cannot configure XFRM state or
> policy through the XFRM_USER netlink API, and cannot use the XFRM_POLICY
> setsockopt path, because both fail before reaching the native parser.
> 
> The translator series replaced the blanket compat rejection with a
> translator lookup. That made the path usable on x86 when the translator
> is available, but left architectures that cannot build the translator
> permanently rejected even when their compat layout already matches the
> native layout. Let those architectures use the native parser instead.
> 
> Gate the translator requirement on COMPAT_FOR_U64_ALIGNMENT instead of
> on in_compat_syscall() alone. Gating on the ABI property rather than on
> CONFIG_XFRM_USER_COMPAT is deliberate: on x86 with IA32_EMULATION=y but
> XFRM_USER_COMPAT=n, a 32-bit task must still be rejected rather than
> routed through the native parser, which would misread genuinely
> 4-byte-aligned x86-32 messages. COMPAT_FOR_U64_ALIGNMENT is the ABI
> property that makes the XFRM translator mandatory.
> 
> Only the receive/input direction needs the guard. The send, dump and
> notification paths already call the translator as "if (xtr) { ... }"
> with no error on NULL, so on arches without a translator they no-op and
> the kernel emits native 64-bit-layout messages, which is what an AArch32
> task expects.
> 
> Tested on Juniper SRX hardware: with the fix, 32-bit IPsec userspace
> netlink and XFRM_POLICY setsockopt operations that previously failed
> with -EOPNOTSUPP now succeed; x86 behaviour is unchanged by inspection.
> 
> Fixes: 5106f4a8acff ("xfrm/compat: Add 32=>64-bit messages translator")
> Fixes: 96392ee5a13b ("xfrm/compat: Translate 32-bit user_policy from sockptr")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sanman Pradhan <psanman@xxxxxxxxxxx>

Patch applied, thanks a lot!