Re: [syzbot] [bpf?] KMSAN: uninit-value in ieee802154_subif_start_xmit

From: Eduard Zingerman
Date: Mon Aug 07 2023 - 10:46:06 EST

On Mon, 2023-08-07 at 07:40 -0700, Yonghong Song wrote:
>
> On 8/7/23 6:11 AM, Eduard Zingerman wrote:
> > On Sun, 2023-08-06 at 23:40 -0700, Yonghong Song wrote:
> > >
> > > On 8/6/23 4:23 PM, syzbot wrote:
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit: 25ad10658dc1 riscv, bpf: Adapt bpf trampoline to optimized..
> > > > git tree: bpf-next
> > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=147cbb29a80000
> > > > kernel config: https://syzkaller.appspot.com/x/.config?x=8acaeb93ad7c6aaa
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=d61b595e9205573133b3
> > > > compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14d73ccea80000
> > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1276aedea80000
> > > >
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/3d378cc13d42/disk-25ad1065.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/44580fd5d1af/vmlinux-25ad1065.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/840587618b41/bzImage-25ad1065.xz
> > > >
> > > > The issue was bisected to:
> > > >
> > > > commit 8100928c881482a73ed8bd499d602bab0fe55608
> > > > Author: Yonghong Song <yonghong.song@xxxxxxxxx>
> > > > Date: Fri Jul 28 01:12:02 2023 +0000
> > > >
> > > > bpf: Support new sign-extension mov insns
> > >
> > > Thanks for reporting. I will look into this ASAP.
> >
> > Hi Yonghong,
> >
> > I guess it's your night and my morning, so I did some initial assessment.
> > The BPF program being loaded is:
> >
> > 0 : (62) *(u32 *)(r10 -8) = 553656332
> > 1 : (bf) r1 = (s16)r10
> > 2 : (07) r1 += -8
> > 3 : (b7) r2 = 3
> > 4 : (bd) if r2 <= r1 goto pc+0
> > 5 : (85) call bpf_trace_printk#6
> > 6 : (b7) r0 = 0
> > 7 : (95) exit
> >
> > (Note: when using bpftool (prog dump xlated id <some-id>) the disassembly
> > of the instruction #1 is incorrectly printed as "1: (bf) r1 = r10")
> >
> > The error occurs when instruction #5 (call to printk) is executed.
> > An incorrect address for the format string is passed to printk.
> > Disassembly of the jited program looks as follows:
> >
> > $ bpftool prog dump jited id <some-id>
> > bpf_prog_ebeed182d92b487f:
> > 0: nopl (%rax,%rax)
> > 5: nop
> > 7: pushq %rbp
> > 8: movq %rsp, %rbp
> > b: subq $8, %rsp
> > 12: movl $553656332, -8(%rbp)
> > 19: movswq %bp, %rdi ; <---- Note movswq %bp !
> > 1d: addq $-8, %rdi
> > 21: movl $3, %esi
> > 26: cmpq %rdi, %rsi
> > 29: jbe 0x2b
> > 2b: callq 0xffffffffe11c484c
> > 30: xorl %eax, %eax
> > 32: leave
> > 33: retq
> >
> > Note jit instruction #19 corresponding to BPF instruction #1, which
> > loads truncated and sign-extended value of %rbp's first byte as an
> > address of format string.
> >
> > Here is how verifier log looks for (slightly modified) program:
> >
> > func#0 @0
> > 0: R1=ctx(off=0,imm=0) R10=fp0
> > ; asm volatile (" \n\
> > 0: (b7) r1 = 553656332 ; R1_w=553656332
> > 1: (63) *(u32 *)(r10 -8) = r1 ; R1_w=553656332 R10=fp0 fp-8=553656332
> > 2: (bf) r1 = (s16)r10 ; R1_w=fp0 R10=fp0
> > 3: (07) r1 += -8 ; R1_w=fp-8
> > 4: (b7) r2 = 3 ; R2_w=3
> > 5: (bd) if r2 <= r1 goto pc+0 ; R1_w=fp-8 R2_w=3
> > 6: (85) call bpf_trace_printk#6
> > mark_precise: frame0: last_idx 6 first_idx 0 subseq_idx -1
> > ...
> > mark_precise: frame0: falling back to forcing all scalars precise
> > 7: R0=scalar()
> > 7: (b7) r0 = 0 ; R0_w=0
> > 8: (95) exit
> >
> > from 5 to 6: R1_w=fp-8 R2_w=3 R10=fp0 fp-8=553656332
> > 6: (85) call bpf_trace_printk#6
> > mark_precise: frame0: last_idx 6 first_idx 0 subseq_idx -1
> > ...
> > mark_precise: frame0: falling back to forcing all scalars precise
> > 7: safe
> >
> > Note the following line:
> >
> > 2: (bf) r1 = (s16)r10 ; R1_w=fp0 R10=fp0
> >
> > Verifier incorrectly marked r1 as fp0, hence not noticing the problem
> > with address passed to printk.
>
> Thanks, Eduard. Right. I am also able to dump xlated code like
> below:
>
> 0: (62) *(u32 *)(r10 -8) = 553656332
> 1: (bf) r1 = (s16)r10
> 2: (07) r1 += -8
> 3: (b7) r2 = 3
> 4: (bd) if r2 <= r1 goto pc+0
> 5: (85) call bpf_trace_printk#-138320
> 6: (b7) r0 = 0
> 7: (95) exit
>
> Something like below can fix the problem,
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 132f25dab931..db72619551b2 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -13171,6 +13171,7 @@ static int check_alu_op(struct bpf_verifier_env
> *env, struct bpf_insn *insn)
> if (no_sext && need_id)
> src_reg->id =
> ++env->id_gen;
> copy_register_state(dst_reg,
> src_reg);
> + dst_reg->type = SCALAR_VALUE;
> if (!no_sext)
> dst_reg->id = 0;
> coerce_reg_to_size_sx(dst_reg,
> insn->off >> 3);
>
> After insn 1, we need change r1 type to SCALAR_VALUE. Will add
> the the test to selftest and submit the patch to fix the problem
> today.

Should this be an error?
Like in the same function but slightly below, when u32 moves are
processed:

/* R1 = (u32) R2 */
if (is_pointer_value(env, insn->src_reg)) {
verbose(env,
"R%d partial copy of pointer\n",
insn->src_reg);
return -EACCES;
} else { ...

>
> >
> > Thanks,
> > Eduard.
> >
> > > >
> > > > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=17970c5da80000
> > > > final oops: https://syzkaller.appspot.com/x/report.txt?x=14570c5da80000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=10570c5da80000
> > > >
> [...]