[PATCH bpf-next v2] arm32, bpf: Reject BPF-to-BPF calls and callbacks in the JIT

[Puranjay Mohan]

The ARM32 BPF JIT does not support BPF-to-BPF function calls
(BPF_PSEUDO_CALL) or callbacks (BPF_PSEUDO_FUNC), but it does
not reject them either.

When a program with subprograms is loaded (e.g. libxdp's XDP
dispatcher uses __noinline__ subprograms, or any program using
callbacks like bpf_loop or bpf_for_each_map_elem), the verifier
invokes bpf_jit_subprogs() which calls bpf_int_jit_compile()
for each subprogram.

For BPF_PSEUDO_CALL, since ARM32 does not reject it, the JIT
silently emits code using the wrong address computation:

    func = __bpf_call_base + imm

where imm is a pc-relative subprogram offset, producing a bogus
function pointer.

For BPF_PSEUDO_FUNC, the ldimm64 handler ignores src_reg and
loads the immediate as a normal 64-bit value without error.

In both cases, build_body() reports success and a JIT image is
allocated. ARM32 lacks the jit_data/extra_pass mechanism needed
for the second JIT pass in bpf_jit_subprogs(). On the second
pass, bpf_int_jit_compile() performs a full fresh compilation,
allocating a new JIT binary and overwriting prog->bpf_func. The
first allocation is never freed. bpf_jit_subprogs() then detects
the function pointer changed and aborts with -ENOTSUPP, but the
original JIT binary has already been leaked. Each program
load/unload cycle leaks one JIT binary allocation, as reported
by kmemleak:

    unreferenced object 0xbf0a1000 (size 4096):
      backtrace:
        bpf_jit_binary_alloc+0x64/0xfc
        bpf_int_jit_compile+0x14c/0x348
        bpf_jit_subprogs+0x4fc/0xa60

Fix this by rejecting both BPF_PSEUDO_CALL in the BPF_CALL
handler and BPF_PSEUDO_FUNC in the BPF_LD_IMM64 handler, falling
through to the existing 'notyet' path. This causes build_body()
to fail before any JIT binary is allocated, so
bpf_int_jit_compile() returns the original program unjitted.
bpf_jit_subprogs() then sees !prog->jited and cleanly falls
back to the interpreter with no leak.

Acked-by: Daniel Borkmann <daniel at iogearbox.net>
Fixes: 1c2a088a6626 ("bpf: x64: add JIT support for multi-function programs")
Reported-by: Jonas Rebmann <jre at pengutronix.de>
Closes: https://lore.kernel.org/bpf/b63e9174-7a3d-4e22-8294-16df07a4af89@pengutronix.de
Tested-by: Jonas Rebmann <jre at pengutronix.de>
Signed-off-by: Puranjay Mohan <puranjay at kernel.org>
---

Changelog:
v1: https://lore.kernel.org/all/20260417103004.3552500-1-puranjay@kernel.org/
Changes in v2:
- Add Acked-by: Daniel Borkmann <daniel at iogearbox.net>
- Reject BPF_PSEUDO_FUNC in the BPF_LD | BPF_IMM | BPF_DW handler
- Move code below declarations

---
 arch/arm/net/bpf_jit_32.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index deeb8f292454..a900aa973885 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -1852,6 +1852,9 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
 	{
 		u64 val = (u32)imm | (u64)insn[1].imm << 32;
 
+		if (insn->src_reg == BPF_PSEUDO_FUNC)
+			goto notyet;
+
 		emit_a32_mov_i64(dst, val, ctx);
 
 		return 1;
@@ -2055,6 +2058,9 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
 		const s8 *r5 = bpf2a32[BPF_REG_5];
 		const u32 func = (u32)__bpf_call_base + (u32)imm;
 
+		if (insn->src_reg == BPF_PSEUDO_CALL)
+			goto notyet;
+
 		emit_a32_mov_r64(true, r0, r1, ctx);
 		emit_a32_mov_r64(true, r1, r2, ctx);
 		emit_push_r64(r5, ctx);

base-commit: 1f5ffc672165ff851063a5fd044b727ab2517ae3
-- 
2.52.0