Re: bpf: shift-out-of-bounds in tnum_rshift()

From: Andrii Nakryiko
Date: Fri Oct 27 2023 - 13:51:37 EST

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In reply to: Eduard Zingerman: "Re: bpf: shift-out-of-bounds in tnum_rshift()"
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On Wed, Oct 25, 2023 at 10:34 AM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> On Tue, 2023-10-24 at 14:40 +0200, Hao Sun 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 is a simplified example:
>
> SEC("?tp")
> __success __retval(0)
> __naked void large_shifts(void)
> {
> asm volatile (" \
> call %[bpf_get_prandom_u32]; \n\
> r8 = r0; \n\
> r6 = r0; \n\
> r6 &= 0xf; \n\
> if w8 < 0xffffffff goto +2; \n\
> if r8 != r6 goto +1; \n\
> w0 >>= w8; /* shift-out-bounds here */ \n\
> exit; \n\
> " :
> : __imm(bpf_get_prandom_u32)
> : __clobber_all);
> }
>

With my changes the verifier does correctly derive that r8 != r6 will
always happen, and thus skips w0 >>= w8. But the test itself with
__retval(0) is not a valid test, so it would be good to construct
something that will correctly return 0 at runtime (or use some other
check). So I won't put this test into my patch set and will live it as
a follow up for someone. But here's the log for anyone curious:

VERIFIER LOG:
=============
func#0 @0
0: R1=ctx(off=0,imm=0) R10=fp0
; asm volatile (" \
0: (85) call bpf_get_prandom_u32#7 ; R0_w=scalar()
1: (bf) r8 = r0 ; R0_w=scalar(id=1) R8_w=scalar(id=1)
2: (bf) r6 = r0 ; R0_w=scalar(id=1) R6_w=scalar(id=1)
3: (57) r6 &= 15 ;
R6_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=15,var_off=(0x0;
0xf))
4: (a6) if w8 < 0xffffffff goto pc+2 ;
R8_w=scalar(id=1,smin=-9223372032559808513,umin=umin32=4294967295,smin32=-1,smax32=-1,var_off=(0xffffffff;
0xffffffff00000000))
5: (5d) if r8 != r6 goto pc+1
mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1
mark_precise: frame0: regs=r0,r8 stack= before 4: (a6) if w8 <
0xffffffff goto pc+2
mark_precise: frame0: regs=r0,r8 stack= before 3: (57) r6 &= 15
mark_precise: frame0: regs=r0,r8 stack= before 2: (bf) r6 = r0
mark_precise: frame0: regs=r0,r8 stack= before 1: (bf) r8 = r0
mark_precise: frame0: regs=r0 stack= before 0: (85) call bpf_get_prandom_u32#7
mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1
mark_precise: frame0: regs=r6 stack= before 4: (a6) if w8 < 0xffffffff goto pc+2
mark_precise: frame0: regs=r6 stack= before 3: (57) r6 &= 15
mark_precise: frame0: regs=r6 stack= before 2: (bf) r6 = r0
mark_precise: frame0: regs=r0 stack= before 1: (bf) r8 = r0
mark_precise: frame0: regs=r0 stack= before 0: (85) call bpf_get_prandom_u32#7
5: R6_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=15,var_off=(0x0;
0xf)) R8_w=scalar(id=1,smin=-9223372032559808513,umin=umin32=4294967295,smin32=-1,smax32=-1,var_off=(0xffffffff;
0xffffffff00000000))
7: (95) exit

from 4 to 7: R0=scalar(id=1,smax=9223372036854775806,umax=18446744073709551614,umax32=4294967294)
R6=scalar(smin=smin32=0,smax=umax=smax32=umax32=15,var_off=(0x0; 0xf))
R8=scalar(id=1,smax=9223372036854775806,umax=18446744073709551614,umax32=4294967294)
R10=fp0
7: R0=scalar(id=1,smax=9223372036854775806,umax=18446744073709551614,umax32=4294967294)
R6=scalar(smin=smin32=0,smax=umax=smax32=umax32=15,var_off=(0x0; 0xf))
R8=scalar(id=1,smax=9223372036854775806,umax=18446744073709551614,umax32=4294967294)
R10=fp0
7: (95) exit
processed 8 insns (limit 1000000) max_states_per_insn 0 total_states 1
peak_states 1 mark_read 1
=============

at insn #4, simulating a FALSE condition, verifier knows that r6 is
[0, 15], while w8 is exactly 0xffffffff, so at insn #5 it can tell
that 0xffffffff can never be equal to a value in [0, 15] range, and
thus skips the shift instruction.

> The issue is caused by an invalid range assigned to R8 after R8 != R6
> check, here is GDB log:
>
> (gdb) bt
> #0 scalar32_min_max_rsh ... at kernel/bpf/verifier.c:13368
> #1 0xffffffff81295236 in adjust_scalar_min_max_vals ... at kernel/bpf/verifier.c:13592
> #2 adjust_reg_min_max_vals .... at kernel/bpf/verifier.c:13706
> #3 0xffffffff8128701f in check_alu_op ... at kernel/bpf/verifier.c:13938
> #4 do_check ... at kernel/bpf/verifier.c:17327
> (gdb) p *src_reg
> $2 = {
> type = SCALAR_VALUE,
> ...
> smin_value = 4294967295,
> smax_value = 15,
> umin_value = 4294967295,
> umax_value = 15,
> s32_min_value = -1,
> s32_max_value = -1,
> u32_min_value = 4294967295,
> u32_max_value = 4294967295,
> ...
> }
>
> The invalid range is assigned within reg_combine_min_max() function in
> BPF_JNE branch. The following diff removes the error:
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 857d76694517..3d140bf85282 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14485,7 +14485,7 @@ static void reg_combine_min_max(struct bpf_reg_state *true_src,
> __reg_combine_min_max(true_src, true_dst);
> break;
> case BPF_JNE:
> - __reg_combine_min_max(false_src, false_dst);
> + //__reg_combine_min_max(false_src, false_dst);
> break;
> }
> }
>
> I do not understand what BPF_JNE branch logically means in
> reg_combine_min_max(), does anyone has any insight?

Not equal check? When you have either `if r1 == r2 goto ` or `if r1 !=
r2`, verifier simulates both TRUE and FALSE conditions, so basically
both BPF_JEQ and BPF_JNE, depending on the branch.

>
> > 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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