Re: [PATCH v2] net: ipv4: fix ARM64 alignment fault in multipath hash seed

[Sam Su]

Hi Eric,

Got it! Thank you so much for the clarification and all your help with
the review.

Best regards,
Yung Chih.

Eric Dumazet <edumazet@xxxxxxxxxx> 於 2026年3月2日週一 下午2:12寫道：

>
> On Mon, Mar 2, 2026 at 7:03 AM Yung Chih Su <yuuchihsu@xxxxxxxxx> wrote:
> >
> > `struct sysctl_fib_multipath_hash_seed` contains two u32 fields
> > (user_seed and mp_seed), making it an 8-byte structure with a 4-byte
> > alignment requirement.
> >
> > In `fib_multipath_hash_from_keys()`, the code evaluates the entire
> > struct atomically via `READ_ONCE()`:
> >
> >     mp_seed = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_seed).mp_seed;
> >
> > While this silently works on GCC by falling back to unaligned regular
> > loads which the ARM64 kernel tolerates, it causes a fatal kernel panic
> > when compiled with Clang and LTO enabled.
> >
> > Commit e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire
> > when CONFIG_LTO=y") strengthens `READ_ONCE()` to use Load-Acquire
> > instructions (`ldar` / `ldapr`) to prevent compiler reordering bugs
> > under Clang LTO. Since the macro evaluates the full 8-byte struct,
> > Clang emits a 64-bit `ldar` instruction. ARM64 architecture strictly
> > requires `ldar` to be naturally aligned, thus executing it on a 4-byte
> > aligned address triggers a strict Alignment Fault (FSC = 0x21).
> >
> > Fix the read side by moving the `READ_ONCE()` directly to the `u32`
> > member, which emits a safe 32-bit `ldar Wn`.
> >
> > Furthermore, Eric Dumazet pointed out that `WRITE_ONCE()` on the entire
> > struct in `proc_fib_multipath_hash_set_seed()` is also flawed. Analysis
> > shows that Clang splits this 8-byte write into two separate 32-bit
> > `str` instructions. While this avoids an alignment fault, it destroys
> > atomicity and exposes a tear-write vulnerability. Fix this by
> > explicitly splitting the write into two 32-bit `WRITE_ONCE()`
> > operations.
> >
> > Finally, add the missing `READ_ONCE()` when reading `user_seed` in
> > `proc_fib_multipath_hash_seed()` to ensure proper pairing and
> > concurrency safety.
> >
> > Fixes: 4ee2a8cace3f ("net: ipv4: Add a sysctl to set multipath hash seed")
> > Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> Nit: Use of Suggested-by: implies I made the original suggestion for
> this patch (before the V1)
> I simply gave a feedback on your V1. The idea of the patch came from you :)
>
> So next time, do not add a "Suggested-by:", as the expected step is
> that I add a Reviewed-by tag when I am happy with a new version,
> if I see it in time before it is merged.
>
> No need for a V3, this is simply a reminder for your next patches.
>
> > Signed-off-by: Yung Chih Su <yuuchihsu@xxxxxxxxx>
> > ---
>
> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> Thanks !