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

Puranjay Mohan
2026-04-17T16:16:24Z

On Fri, Apr 17, 2026 at 4:48 PM Emil Tsalapatis <emil@etsalapatis.com> wrote:
>
> On Fri Apr 17, 2026 at 10:33 AM EDT, Puranjay Mohan wrote:
> > 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.
>
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
>
> The Fixes tag is a bit unrelated since it's for x64 but the original
> commit that adds the file (ddecdfcea0ae8 ?) is so far back it probably
> doesn't matter.

That fixes tag commit has verifier changes too:

-- 8< --

+ }
+ }
+ for (i = 0; i <= env->subprog_cnt; i++) {
+ old_bpf_func = func[i]->bpf_func;
+ tmp = bpf_int_jit_compile(func[i]);
+ if (tmp != func[i] || func[i]->bpf_func != old_bpf_func) {
+ verbose(env, "JIT doesn't support bpf-to-bpf calls\n");
+ err = -EFAULT;
+ goto out_free;
+ }
+ cond_resched();
+ }
+

-- >8 --

This call to bpf_int_jit_compile() is where the memory leak was
introduced, before this commit, there was no memory leak.