Re: [PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Xu Kuohai]

On 2/25/2026 5:08 PM, Fuad Tabba wrote:
> Hi Xu,
>
> On Wed, 25 Feb 2026 at 01:43, Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx> wrote:
> > On 2/24/2026 5:31 PM, Fuad Tabba wrote:
> > > struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
> > > was using an alignment of 4 bytes (sizeof(u32)), which could lead
> > > to the 'target' field being misaligned in the JIT buffer.
> > >
> > > Increase the alignment requirement to 8 bytes (sizeof(u64)) in
> > > bpf_jit_binary_pack_alloc() to guarantee proper alignment for
> > > struct bpf_plt.
> > >
> > > Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> > > Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> > > ---
> > >    arch/arm64/net/bpf_jit_comp.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > > index 356d33c7a4ae..adf84962d579 100644
> > > --- a/arch/arm64/net/bpf_jit_comp.c
> > > +++ b/arch/arm64/net/bpf_jit_comp.c
> > > @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > >        extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
> > >        image_size = extable_offset + extable_size;
> > >        ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
> > > -                                           sizeof(u32), &header, &image_ptr,
> > > +                                           sizeof(u64), &header, &image_ptr,
> > >                                              jit_fill_hole);
> > >        if (!ro_header) {
> > >                prog = orig_prog;
> >
> > Good catch. build_plt pads NOP instructions to ensure a 64-bit relative offset for
> > plt target, but it misses the alignment check for image base itself.
> >
> > Acked-by: Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx>
> >
> > nit: Add check for base alignment in build_plt, or a comment to clarify?
>
> Thanks for the having a look and for the Ack.
>
> You're right that build_plt() assumes 64-bit alignment when
> calculating the NOP padding. However, Will pointed out, over-aligning
> the entire JIT buffer just to satisfy the C standard is somewhat
> heavy-handed. I didn't actually run into a functional bug. The issue
> is that UBSAN complains because we violate the standard's alignment
> rules.
> I'll dropping the allocator change in favor of marking struct bpf_plt
> as __packed.

Interesting, I think the plt target should be 64-bit aligned to ensure
atomic reading on arm64. It can be updated concurrently by WRITE_ONCE
in the bpf_arch_text_poke function while the ldr instruction in the plt is
executed. If it is not aligned correctly, the ldr may read a half-old
half-new value, causing the plt to jump to an invalid destination.

To avoid over-aligning the entire buffer, how about fixing the padding
method in build_plt to just make the plt target aligned correctly?


> Thanks again,
> /fiad