bpf: incorrect value spill in check_stack_write_fixed_off()

[Hao Sun]

Hi,

In check_stack_write_fixed_off(), the verifier creates a fake reg to store the
imm in a BPF_ST_MEM:
...
else if (!reg && !(off % BPF_REG_SIZE) && is_bpf_st_mem(insn) &&
insn->imm != 0 && env->bpf_capable) {
        struct bpf_reg_state fake_reg = {};

        __mark_reg_known(&fake_reg, (u32)insn->imm);
        fake_reg.type = SCALAR_VALUE;
        save_register_state(state, spi, &fake_reg, size);

Here, insn->imm is cast to u32, and used to mark fake_reg, which is incorrect
and may lose sign information. Consider the following program:

r2 = r10
*(u64*)(r2 -40) = -44
r0 = *(u64*)(r2 - 40)
if r0 s<= 0xa goto +2
r0 = 0
exit
r0  = 1
exit

The verifier gives the following log:

-------- Verifier Log --------
func#0 @0
0: R1=ctx(off=0,imm=0) R10=fp0
0: (bf) r2 = r10                      ; R2_w=fp0 R10=fp0
1: (7a) *(u64 *)(r2 -40) = -44        ; R2_w=fp0 fp-40_w=4294967252
2: (79) r0 = *(u64 *)(r2 -40)         ; R0_w=4294967252 R2_w=fp0
fp-40_w=4294967252
3: (c5) if r0 s< 0xa goto pc+2
mark_precise: frame0: last_idx 3 first_idx 0 subseq_idx -1
mark_precise: frame0: regs=r0 stack= before 2: (79) r0 = *(u64 *)(r2 -40)
3: R0_w=4294967252
4: (b7) r0 = 1                        ; R0_w=1
5: (95) exit
verification time 7971 usec
stack depth 40
processed 6 insns (limit 1000000) max_states_per_insn 0 total_states 0
peak_states 0 mark_read 0

Here, the verifier incorrectly thinks R0 is 0xffffffd4, which should
be 0xffffffffffffffd4,
due to the u32 cast in check_stack_write_fixed_off(). This makes the verifier
collect incorrect reg scalar range.

Since insn->imm is i32, we should cast it to the signed integer with
correct size
according to BPF_MEM, then promoting the imm to u64 to mark fake reg as
known, right?

Best
Hao