Re: [PATCH v3] arm64: bpf: Force 8-byte alignment for JIT buffer to prevent atomic tearing

[Will Deacon <will@kernel.org>]

On Thu, Feb 26, 2026 at 07:55:25AM +0000, Fuad Tabba wrote:
> struct bpf_plt contains a u64 target field. Currently, the BPF JIT
> allocator requests an alignment of 4 bytes (sizeof(u32)) for the JIT
> buffer.
> 
> Because the base address of the JIT buffer can be 4-byte aligned (e.g.,
> ending in 0x4 or 0xc), the relative padding logic in build_plt() fails
> to ensure that target lands on an 8-byte boundary.
> 
> This leads to two issues:
> 1. UBSAN reports misaligned-access warnings when dereferencing the
>    structure.
> 2. More critically, target is updated concurrently via WRITE_ONCE() in
>    bpf_arch_text_poke() while the JIT'd code executes ldr. On arm64,
>    64-bit loads/stores are only guaranteed to be single-copy atomic if
>    they are 64-bit aligned. A misaligned target risks a torn read,
>    causing the JIT to jump to a corrupted address.
> 
> Fix this by increasing the allocation alignment requirement to 8 bytes
> (sizeof(u64)) in bpf_jit_binary_pack_alloc(). This anchors the base of
> the JIT buffer to an 8-byte boundary, allowing the relative padding math
> in build_plt() to correctly align the target field.
> 
> Fixes: b2ad54e1533e ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  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;

Acked-by: Will Deacon <will@kernel.org>

Will