Re: [syzbot] [bpf?] KMSAN: uninit-value in ieee802154_subif_start_xmit
From: Yonghong Song
Date: Mon Aug 07 2023 - 14:09:51 EST
On 8/7/23 7:45 AM, Eduard Zingerman wrote:
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 { ...
Right, this is indeed better for unprivileged prog run.
I have submitted a patch to fix the issue reported by syzbot.
Please help review. Thanks!
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
[...]