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

From: Hao Sun
Date: Thu Nov 02 2023 - 10:17:32 EST

On Wed, Nov 1, 2023 at 11:45 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Nov 1, 2023 at 2:53 AM Hao Sun <sunhao.th@xxxxxxxxx> wrote:
> >
> > On Fri, Oct 27, 2023 at 7:51 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > 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
> >
> > A similar issue can be triggered after your patch for JNE/JEQ.
> >
> > For the following case, the verifier would shift out of bound:
> > // 0: r0 = -2
> > BPF_MOV64_IMM(BPF_REG_0, -2),
> > // 1: r0 /= 1
> > BPF_ALU64_IMM(BPF_DIV, BPF_REG_0, 1),
> > // 2: r8 = r0
> > BPF_MOV64_REG(BPF_REG_8, BPF_REG_0),
> > // 3: if w8 != 0xfffffffe goto+4
> > BPF_JMP32_IMM(BPF_JNE, BPF_REG_8, 0xfffffffe, 4),
> > // 4: if r8 s> 0xd goto+3
> > BPF_JMP_IMM(BPF_JSGT, BPF_REG_8, 0xd, 3),
> > // 5: r4 = 0x2
> > BPF_MOV64_IMM(BPF_REG_4, 0x2),
> > // 6: if r8 s<= r4 goto+1
> > BPF_JMP_REG(BPF_JSLE, BPF_REG_8, BPF_REG_4, 1),
> > // 7: w8 s>>= w0 # shift out of bound here
> > BPF_ALU32_REG(BPF_ARSH, BPF_REG_8, BPF_REG_0),
> > // 8: exit
> > BPF_EXIT_INSN(),
> >
> > -------- Verifier Log --------
> > func#0 @0
> > 0: R1=ctx(off=0,imm=0) 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=-9223372032559808514,smax=9223372036854775806,umin=umin32=4294967294,umax=18446744073709551614,smin32=-2,smax32=-2,
> > umax32=4294967294,var_off=(0xfffffffe; 0xffffffff00000000))
> > 4: (65) if r8 s> 0xd goto pc+3 ;
> > R8_w=scalar(id=1,smin=-9223372032559808514,smax=13,umin=umin32=4294967294,umax=18446744073709551614,smin32=-2,smax32=-2,umax32=4294967294,
> > var_off=(0xfffffffe; 0xffffffff00000000))
> > 5: (b7) r4 = 2 ; R4_w=2
> > 6: (dd) if r8 s<= r4 goto pc+1 ; R4_w=2 R8_w=4294967294
> > 7: (cc) w8 s>>= w0 ; R0=4294967294 R8=4294967295
> > 8: (95) exit
> >
> > Here, after #6, reg range is incorrect, seems to be an issue in JSLE case
> > in is_branch_taken(). Is this issue fixed in your patch series?
>
> I don't know, but you can easily check by applying my patches on top
> of bpf-next and then trying your change.
>

After applying your change, the verifier does not shift out of bound,
but the range
is still not correct. See this verifier log:

-------- Verifier Log --------
func#0 @0
0: R1=ctx(off=0,imm=0) 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=-9223372032559808514,smax=9223372036854775806,umin=umin32=4294967294,umax=18446744073709551614,smin32=-2,smax32=-2,umax32=4294967294,var_off=(0xfffffffe;
0xffffffff00000000))
4: (65) if r8 s> 0xd goto pc+3 ;
R8_w=scalar(id=1,smin=-9223372032559808514,smax=13,umin=umin32=4294967294,umax=18446744073709551614,smin32=-2,smax32=-2,umax32=4294967294,var_off=(0xfffffffe;
0xffffffff00000000))
5: (b7) r4 = 2 ; R4_w=2
6: (dd) if r8 s<= r4 goto pc+1 ; R4_w=2 R8_w=4294967294
7: (cc) w8 s>>= w0 ; R0=4294967294 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

-------- Test Run Log --------
RetVal: 1

At #9, the verifier thinks the value of R0 is 0, but it's 1, because
r0 is -2 all the time before
#8. The test run also shows return value is 1.

This program can reproduce the above: https://pastebin.com/raw/a0WuXaKh