Hi Hao, On Wed, Oct 25, 2023 at 02:31:02PM +0200, Hao Sun wrote: > On Tue, Oct 24, 2023 at 2:40 PM Hao Sun <sunhao.th@xxxxxxxxx> 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) Seems like the root cause is a bug with range tracking, before instruction 16, R8_w was R8_w=scalar(smin=-9223372032559808520,umin=4294967288,smin32=-5,smax32=-1,umin32=4294967291,var_off=(0xffff) But after instruction 16 it becomes R8_w=scalar(smin=umin=4294967288,smax=umax=255,smin32=-5,smax32=-1,umin32=4294967291) Where smin_value > smax_value, and umin_value > umax_value (among other things). This should be the main problem. The verifier operates on the assumption that smin_value <= smax_value and umin_value <= umax_value, and if that assumption is not upheld then all kind of things can go wrong. Maybe Andrii may already has this worked out in the range-vs-range that he has mentioned[1] he'll be sending soon. 1: https://lore.kernel.org/bpf/CAEf4BzbJ3hZCSt4nLCZCV4cxV60+kddiSMsy7-9ou_RaQV7B8A@xxxxxxxxxxxxxx/ > > 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 Checking umax_val alone is be enough and we don't need to add a check for u32_max_value, because (when we have correct range tracking) u32_max_value should always be smaller than u32_value. So the fix needed here is to have correct range tracking. > > Best > > Hao Sun
