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Date:   Wed, 25 Oct 2023 14:31:02 +0200
From:   Hao Sun <sunhao.th@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        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>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>
Cc:     bpf <bpf@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: bpf: shift-out-of-bounds in tnum_rshift()

On Tue, Oct 24, 2023 at 2:40 PM Hao Sun <sunhao.th@...il.com> wrote:
>
> Hi,
>
> The following program can trigger a shift-out-of-bounds in
> tnum_rshift(), called by scalar32_min_max_rsh():
>
> 0: (bc) w0 = w1
> 1: (bf) r2 = r0
> 2: (18) r3 = 0xd
> 4: (bc) w4 = w0
> 5: (bf) r5 = r0
> 6: (bf) r7 = r3
> 7: (bf) r8 = r4
> 8: (2f) r8 *= r5
> 9: (cf) r5 s>>= r5
> 10: (a6) if w8 < 0xfffffffb goto pc+10
> 11: (1f) r7 -= r5
> 12: (71) r6 = *(u8 *)(r1 +17)
> 13: (5f) r3 &= r8
> 14: (74) w2 >>= 30
> 15: (1f) r7 -= r5
> 16: (5d) if r8 != r6 goto pc+4
> 17: (c7) r8 s>>= 5
> 18: (cf) r0 s>>= r0
> 19: (7f) r0 >>= r0
> 20: (7c) w5 >>= w8         # shift-out-bounds here
> 21: exit
>

Here are the c macros for the above program in case anyone needs this:

        // 0: (bc) w0 = w1
        BPF_MOV32_REG(BPF_REG_0, BPF_REG_1),
        // 1: (bf) r2 = r0
        BPF_MOV64_REG(BPF_REG_2, BPF_REG_0),
        // 2: (18) r3 = 0xd
        BPF_LD_IMM64(BPF_REG_3, 0xd),
        // 4: (bc) w4 = w0
        BPF_MOV32_REG(BPF_REG_4, BPF_REG_0),
        // 5: (bf) r5 = r0
        BPF_MOV64_REG(BPF_REG_5, BPF_REG_0),
        // 6: (bf) r7 = r3
        BPF_MOV64_REG(BPF_REG_7, BPF_REG_3),
        // 7: (bf) r8 = r4
        BPF_MOV64_REG(BPF_REG_8, BPF_REG_4),
        // 8: (2f) r8 *= r5
        BPF_ALU64_REG(BPF_MUL, BPF_REG_8, BPF_REG_5),
        // 9: (cf) r5 s>>= r5
        BPF_ALU64_REG(BPF_ARSH, BPF_REG_5, BPF_REG_5),
        // 10: (a6) if w8 < 0xfffffffb goto pc+10
        BPF_JMP32_IMM(BPF_JLT, BPF_REG_8, 0xfffffffb, 10),
        // 11: (1f) r7 -= r5
        BPF_ALU64_REG(BPF_SUB, BPF_REG_7, BPF_REG_5),
        // 12: (71) r6 = *(u8 *)(r1 +17)
        BPF_LDX_MEM(BPF_B, BPF_REG_6, BPF_REG_1, 17),
        // 13: (5f) r3 &= r8
        BPF_ALU64_REG(BPF_AND, BPF_REG_3, BPF_REG_8),
        // 14: (74) w2 >>= 30
        BPF_ALU32_IMM(BPF_RSH, BPF_REG_2, 30),
        // 15: (1f) r7 -= r5
        BPF_ALU64_REG(BPF_SUB, BPF_REG_7, BPF_REG_5),
        // 16: (5d) if r8 != r6 goto pc+4
        BPF_JMP_REG(BPF_JNE, BPF_REG_8, BPF_REG_6, 4),
        // 17: (c7) r8 s>>= 5
        BPF_ALU64_IMM(BPF_ARSH, BPF_REG_8, 5),
        // 18: (cf) r0 s>>= r0
        BPF_ALU64_REG(BPF_ARSH, BPF_REG_0, BPF_REG_0),
        // 19: (7f) r0 >>= r0
        BPF_ALU64_REG(BPF_RSH, BPF_REG_0, BPF_REG_0),
        // 20: (7c) w5 >>= w8
        BPF_ALU32_REG(BPF_RSH, BPF_REG_5, BPF_REG_8),
        BPF_EXIT_INSN()

> After load:
> ================================================================================
> UBSAN: shift-out-of-bounds in kernel/bpf/tnum.c:44:9
> shift exponent 255 is too large for 64-bit type 'long long unsigned int'
> CPU: 2 PID: 8574 Comm: bpf-test Not tainted
> 6.6.0-rc5-01400-g7c2f6c9fb91f-dirty #21
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0x8e/0xb0 lib/dump_stack.c:106
>  ubsan_epilogue lib/ubsan.c:217 [inline]
>  __ubsan_handle_shift_out_of_bounds+0x15a/0x2f0 lib/ubsan.c:387
>  tnum_rshift.cold+0x17/0x32 kernel/bpf/tnum.c:44
>  scalar32_min_max_rsh kernel/bpf/verifier.c:12999 [inline]
>  adjust_scalar_min_max_vals kernel/bpf/verifier.c:13224 [inline]
>  adjust_reg_min_max_vals+0x1936/0x5d50 kernel/bpf/verifier.c:13338
>  do_check kernel/bpf/verifier.c:16890 [inline]
>  do_check_common+0x2f64/0xbb80 kernel/bpf/verifier.c:19563
>  do_check_main kernel/bpf/verifier.c:19626 [inline]
>  bpf_check+0x65cf/0xa9e0 kernel/bpf/verifier.c:20263
>  bpf_prog_load+0x110e/0x1b20 kernel/bpf/syscall.c:2717
>  __sys_bpf+0xfcf/0x4380 kernel/bpf/syscall.c:5365
>  __do_sys_bpf kernel/bpf/syscall.c:5469 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:5467 [inline]
>  __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:5467
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x5610511e23cd
> Code: 24 80 00 00 00 48 0f 42 d0 48 89 94 24 68 0c 00 00 b8 41 01 00
> 00 bf 05 00 00 00 ba 90 00 00 00 48 8d b44
> RSP: 002b:00007f5357fc7820 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> RAX: ffffffffffffffda RBX: 0000000000000095 RCX: 00005610511e23cd
> RDX: 0000000000000090 RSI: 00007f5357fc8410 RDI: 0000000000000005
> RBP: 0000000000000000 R08: 00007f5357fca458 R09: 00007f5350005520
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000002b
> R13: 0000000d00000000 R14: 000000000000002b R15: 000000000000002b
>  </TASK>
>
> If remove insn #20, the verifier gives:
>  -------- Verifier Log --------
>  func#0 @0
>  0: R1=ctx(off=0,imm=0) R10=fp0
>  0: (bc) w0 = w1                       ;
> R0_w=scalar(smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
> R1=ctx(off=0,
>  imm=0)
>  1: (bf) r2 = r0                       ;
> R0_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0;
> 0xffffffff))
>  R2_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
>  2: (18) r3 = 0xd                      ; R3_w=13
>  4: (bc) w4 = w0                       ;
> R0_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0;
> 0xffffffff))
>  R4_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
>  5: (bf) r5 = r0                       ;
> R0_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0;
> 0xffffffff))
>  R5_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
>  6: (bf) r7 = r3                       ; R3_w=13 R7_w=13
>  7: (bf) r8 = r4                       ;
> R4_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0;
> 0xffffffff))
>  R8_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
>  8: (2f) r8 *= r5                      ;
> R5_w=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0;
> 0xffffffff))
>  R8_w=scalar()
>  9: (cf) r5 s>>= r5                    ; R5_w=scalar()
>  10: (a6) if w8 < 0xfffffffb goto pc+9         ;
> R8_w=scalar(smin=-9223372032559808520,umin=4294967288,smin32=-5,smax32=-1,
>  umin32=4294967291,var_off=(0xfffffff8; 0xffffffff00000007))
>  11: (1f) r7 -= r5                     ; R5_w=scalar() R7_w=scalar()
>  12: (71) r6 = *(u8 *)(r1 +17)         ; R1=ctx(off=0,imm=0)
> R6_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=255,
>  var_off=(0x0; 0xff))
>  13: (5f) r3 &= r8                     ;
> R3_w=scalar(smin=umin=smin32=umin32=8,smax=umax=smax32=umax32=13,var_off=(0x8;
>  0x5)) R8_w=scalar(smin=-9223372032559808520,umin=4294967288,smin32=-5,smax32=-1,umin32=4294967291,var_off=(0xffff)
>  14: (74) w2 >>= 30                    ;
> R2_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=3,var_off=(0x0;
> 0x3))
>  15: (1f) r7 -= r5                     ; R5_w=scalar() R7_w=scalar()
>  16: (5d) if r8 != r6 goto pc+3        ;
> R6_w=scalar(smin=umin=umin32=4294967288,smax=umax=umax32=255,smin32=-8,smax32=-1,
>  var_off=(0xfffffff8; 0x7))
> R8_w=scalar(smin=umin=4294967288,smax=umax=255,smin32=-5,smax32=-1,umin32=4294967291)
>  17: (c7) r8 s>>= 5                    ; R8_w=134217727
>  18: (cf) r0 s>>= r0                   ; R0_w=scalar()
>  19: (7f) r0 >>= r0                    ; R0=scalar()
>  20: (95) exit
>
>  from 16 to 20: safe
>
>  from 10 to 20: safe
>  processed 22 insns (limit 1000000) max_states_per_insn 0 total_states
> 1 peak_states 1 mark_read 1
> -------- End of Verifier Log --------
>
> In adjust_scalar_min_max_vals(), src_reg.umax_value is 7, thus pass
> the check here:
>          if (umax_val >= insn_bitness) {
>              /* Shifts greater than 31 or 63 are undefined.
>               * This includes shifts by a negative number.
>               */
>              mark_reg_unknown(env, regs, insn->dst_reg);
>              break;
>          }
>
> However in scalar32_min_max_rsh(), both src_reg->u32_min_value and
> src_reg->u32_max_value is 134217727, causing tnum_rsh() shit by 255.
>
> Should we check if(src_reg->u32_max_value < insn_bitness) before calling
> scalar32_min_max_rsh(), rather than only checking umax_val? Or, is it
> because issues somewhere else, incorrectly setting u32_min_value to
> 34217727
>
> Best
> Hao Sun

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