Re: [PATCH bpf v3 3/5] bpf: Fix NULL-ptr-deref in btf_var_show()

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 8/31/26 4:01 AM, Jiayuan Chen wrote:
> btf_var_show() calls btf_type_id_resolve() unconditionally, which
> dereferences btf->resolved_ids. That is NULL for a base BTF - e.g. the
> vmlinux BTF that bpf_snprintf_btf() renders against - since base BTF is
> not resolved during parsing. btf_modifier_show() guards this with
> 'if (btf->resolved_ids)', but btf_var_show() does not.
> 
> A BPF program that passes the type_id of a BTF_KIND_VAR from the vmlinux
> BTF to bpf_snprintf_btf() thus NULL-derefs:
> 
> KASAN: probably user-memory-access in range [0x46638-0x4663f]
> RIP: 0010:btf_var_show (kernel/bpf/btf.c:2929)
> Call Trace:
>  <TASK>
>  btf_type_show (kernel/bpf/btf.c:8259)
>  btf_type_snprintf_show (kernel/bpf/btf.c:8329)
>  bpf_snprintf_btf (kernel/trace/bpf_trace.c:1047)
>  bpf_prog_test_run_raw_tp (net/bpf/test_run.c:829)
>  __sys_bpf (kernel/bpf/syscall.c:4804)
>  do_syscall_64 (arch/x86/entry/syscall_64.c:84)
>  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
>  </TASK>
> 
> Resolve the var's type directly with btf_type_skip_modifiers() when
> resolved_ids is NULL, mirroring btf_modifier_show().
> 
> Fixes: c4d0bfb45068 ("bpf: Add bpf_snprintf_btf helper")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  kernel/bpf/btf.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 6e267c4c4e1f..3cf35ee49c10 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -2926,7 +2926,15 @@ static void btf_var_show(const struct btf *btf, const struct btf_type *t,
>  			 u32 type_id, void *data, u8 bits_offset,
>  			 struct btf_show *show)
>  {
> -	t = btf_type_id_resolve(btf, &type_id);
> +	/*
> +	 * btf_type_id_resolve() dereferences btf->resolved_ids, which is NULL
> +	 * for a base BTF (e.g. the vmlinux BTF that bpf_snprintf_btf() uses).
> +	 * Resolve the var's type directly in that case.
> +	 */
> +	if (btf->resolved_ids)
> +		t = btf_type_id_resolve(btf, &type_id);
> +	else
> +		t = btf_type_skip_modifiers(btf, t->type, &type_id);

Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>

>  
>  	btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
>  }