linux-kernel - [Bug Report] bpf: reg invariant voilation after JSLE

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Date:   Tue, 21 Nov 2023 16:07:58 +0100
From:   Hao Sun <sunhao.th@...il.com>
To:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     bpf <bpf@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [Bug Report] bpf: reg invariant voilation after JSLE

Hi,

The following program (reduced) breaks reg invariant:

C Repro: https://pastebin.com/raw/SRQJYx91

-------- Verifier Log --------
func#0 @0
0: R1=ctx() R10=fp0
0: (b7) r0 = -2                       ; R0_w=-2
1: (37) r0 /= 1                       ; R0_w=scalar()
2: (bf) r8 = r0                       ; R0_w=scalar(id=1) R8_w=scalar(id=1)
3: (56) if w8 != 0xfffffffe goto pc+4         ;
R8_w=scalar(id=1,smin=0x80000000fffffffe,smax=0x7ffffffffffffffe,umin=umin32=0xfffffffe,umax=0xfffffffffffffffe,smin32=-2,smax32=-2,umax32=0xfffffffe,var_off=(0xfffffffe;
0xffffffff00000000))
4: (65) if r8 s> 0xd goto pc+3        ;
R8_w=scalar(id=1,smin=0x80000000fffffffe,smax=13,umin=umin32=0xfffffffe,umax=0xfffffffffffffffe,smin32=-2,smax32=-2,umax32=0xfffffffe,var_off=(0xfffffffe;
0xffffffff00000000))
5: (b7) r4 = 2                        ; R4_w=2
6: (dd) if r8 s<= r4 goto pc+1
REG INVARIANTS VIOLATION (false_reg1): range bounds violation
u64=[0xfffffffe, 0xd] s64=[0xfffffffe, 0xd] u32=[0xfffffffe, 0xd]
s32=[0x3, 0xfffffffe] var_off=(0xfffffffe, 0x0)
6: R4_w=2 R8_w=0xfffffffe
7: (cc) w8 s>>= w0                    ; R0=0xfffffffe R8=scalar()
8: (77) r0 >>= 32                     ; R0_w=0
9: (57) r0 &= 1                       ; R0_w=0
10: (95) exit

from 6 to 8: safe

from 4 to 8: safe

from 3 to 8: safe
processed 14 insns (limit 1000000) max_states_per_insn 0 total_states
1 peak_states 1 mark_read 1


Besides, the verifier enforces the return value of some prog types to
be zero, the bug may lead to programs with arbitrary values loaded.

Best
Hao Sun