[PATCH 6.6.y] bpf, arm64: Force 8-byte alignment for JIT buffer to prevent atomic tearing

[PATCH 6.6.y] bpf, arm64: Force 8-byte alignment for JIT buffer to prevent atomic tearing

[Artem Dinaburg]

From: Fuad Tabba <tabba@google.com>

[ Upstream commit ef06fd16d48704eac868441d98d4ef083d8f3d07 ]

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>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20260226075525.233321-1-tabba@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Assisted-by: LLM
[ Backport to 6.6.y: Request eight-byte image alignment from
  bpf_jit_binary_alloc(); this target does not use the upstream pack
  allocator. ]
Signed-off-by: Artem Dinaburg <artem@trailofbits.com>
---
Hi,

Please consider this backport of the fix for CVE-2026-23383 to 6.6.y.

The backport requests the same eight-byte JIT image alignment from the
older allocator. See the bracketed backport note above.

The complete fix is already present in 6.12.y, 6.18.y and 7.2.y.

Build-tested only as part of a cumulative 6.6.y series: the ARM64 JIT
object with BPF_SYSCALL=y, BPF_JIT=y and WERROR=y; no runtime testing.

Thanks,
Artem Dinaburg

 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 9fd0b658e4602bec2be466665d880a355e5bac1a..c5f7276ad1af7e32b69eb9bb5c0f14c3feaa74e8 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -1648,7 +1648,7 @@ 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;
 	header = bpf_jit_binary_alloc(image_size, &image_ptr,
-				      sizeof(u32), jit_fill_hole);
+				      sizeof(u64), jit_fill_hole);
 	if (header == NULL) {
 		prog = orig_prog;
 		goto out_off;