linux-kernel - Re: [PATCH v2] bpf: fix NULL pointer dereference in print_reg

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <cdv7vgob4ulsmmgmyklgoi5ttzyhby6zmlr2s2kjq6m2dxrnpi@7c2umi7pfhyg>
Date: Fri, 26 Sep 2025 07:26:14 +0530
From: Brahmajit Das <listout@...tout.xyz>
To: KaFai Wan <kafai.wan@...ux.dev>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, 
	syzbot+d36d5ae81e1b0a53ef58@...kaller.appspotmail.com, Andrii Nakryiko <andrii@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Eduard <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>, 
	John Fastabend <john.fastabend@...il.com>, Jiri Olsa <jolsa@...nel.org>, KP Singh <kpsingh@...nel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Stanislav Fomichev <sdf@...ichev.me>, Song Liu <song@...nel.org>, 
	syzkaller-bugs <syzkaller-bugs@...glegroups.com>, Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH v2] bpf: fix NULL pointer dereference in print_reg_state()

On 26.09.2025 06:34, Brahmajit Das wrote:
> On 25.09.2025 23:31, KaFai Wan wrote:
> > On Wed, 2025-09-24 at 23:58 +0530, Brahmajit Das wrote:
> > > On 25.09.2025 01:38, KaFai Wan wrote:
> > > > On Wed, 2025-09-24 at 21:10 +0530, Brahmajit Das wrote:
> > > > > On 24.09.2025 09:32, Alexei Starovoitov wrote:
> > > > > > On Wed, Sep 24, 2025 at 1:43 AM Brahmajit Das
> > > > > > <listout@...tout.xyz>
> > > > > > wrote:
> > > > > > > 
> > > > > > > Syzkaller reported a general protection fault due to a NULL
> > > > > > > pointer
> > > > > > > dereference in print_reg_state() when accessing reg->map_ptr
> > > > > > > without
> > > > > > > checking if it is NULL.
> > > > > > > 
> ...snip...
> > > > 
> > > > Looks like we're getting somewhere.
> > > > It seems the verifier is not clearing reg->type.
> > > > adjust_scalar_min_max_vals() should be called on scalar types only.
> > > 
> > > Right, there is a check in check_alu_op
> > > 
> > > 		if (is_pointer_value(env, insn->dst_reg)) {
> > > 			verbose(env, "R%d pointer arithmetic
> > > prohibited\n",
> > > 				insn->dst_reg);
> > > 			return -EACCES;
> > > 		}
> > > 
> > > is_pointer_value calls __is_pointer_value which takes bool
> > > allow_ptr_leaks as the first argument. Now for some reason in this
> > > case
> > > allow_ptr_leaks is being passed as true, as a result
> > > __is_pointer_value
> > > (and in turn is_pointer_value) returns false when even when register
> > > type is CONST_PTR_TO_MAP.
> > > 
> > 
> > IIUC, `env->allow_ptr_leaks` set true means privileged mode (
> > CAP_PERFMON or CAP_SYS_ADMIN ), false for unprivileged mode. 
> > 
> > 
> > We can use __is_pointer_value to check if the register type is a
> > pointer. For pointers, we check as before (before checking BPF_NEG
> > separately), and for scalars, it remains unchanged. Perhaps this way we
> > can fix the error.
> > 
> > if (opcode == BPF_NEG) {
> > 	if (__is_pointer_value(false, &regs[insn->dst_reg])) {
> > 		err = check_reg_arg(env, insn->dst_reg, DST_OP);
> > 	} else {
> > 		err = check_reg_arg(env, insn->dst_reg,
> > DST_OP_NO_MARK);
> > 		err = err ?: adjust_scalar_min_max_vals(env, insn,
> > 						&regs[insn->dst_reg],
> > 						regs[insn->dst_reg]);
> > 	}
> > } else {
> > 
> > 
> > -- 
> > Thanks,
> > KaFai
> 
> Yep, that works.
> 
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -15505,10 +15505,17 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> 
>                 /* check dest operand */
>                 if (opcode == BPF_NEG) {
> -                       err = check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK);
> -                       err = err ?: adjust_scalar_min_max_vals(env, insn,
> -                                                        &regs[insn->dst_reg],
> -                                                        regs[insn->dst_reg]);
> +                       if (__is_pointer_value(false, &regs[insn->dst_reg])) {
> +                               err = check_reg_arg(env, insn->dst_reg, DST_OP);
> +                       } else {
> +                               err = check_reg_arg(env, insn->dst_reg,
> +                                                   DST_OP_NO_MARK);
> +                               err = err   ?:
> +                                             adjust_scalar_min_max_vals(
> +                                                     env, insn,
> +                                                     &regs[insn->dst_reg],
> +                                                     regs[insn->dst_reg]);
> +                       }
>                 } else {
>                         err = check_reg_arg(env, insn->dst_reg, DST_OP);
>                 }
> 
> I'll just wait for other developer or Alexei, in case they have any
> feedback before sending a v3.

Just my 2 cents, thought this looked cleaner

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15497,7 +15497,8 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
                if (err)
                        return err;

-               if (is_pointer_value(env, insn->dst_reg)) {
+               if (is_pointer_value(env, insn->dst_reg) ||
+                   __is_pointer_value(false, &regs[insn->dst_reg])) {
                        verbose(env, "R%d pointer arithmetic prohibited\n",
                                insn->dst_reg);
                        return -EACCES;

-- 
Regards,
listout