Re: bpf: incorrect value spill in check_stack_write_fixed_off()

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2023-10-25 at 15:14 +0300, Eduard Zingerman wrote:
> On Wed, 2023-10-25 at 11:16 +0200, Hao Sun wrote:
> > 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.
> 
> This bug is on me.
> Thank you for reporting it along with the example program.
> Looks like the patch below is sufficient to fix the issue.
> Have no idea at the moment why I used u32 cast there.
> Let me think a bit more about it and I'll submit an official patch.

Yeap, I see no drawbacks in that patch, imm field is declared as s32,
so it would be correctly sign extended by compiler before cast to u64,
so there is no need for additional casts.
It would be wrong if I submit the fix, because you've done all the work.
Here is a refined test-case to be placed in verifier/bpf_st_mem.c
(be careful with \t, test_verifier uses those as glob marks inside errstr).

{
	"BPF_ST_MEM stack imm sign",
	/* Check if verifier correctly reasons about sign of an
	 * immediate spilled to stack by BPF_ST instruction.
	 *
	 *   fp[-8] = -44;
	 *   r0 = fp[-8];
	 *   if r0 s< 0 goto ret0;
	 *   r0 = -1;
	 *   exit;
	 * ret0:
	 *   r0 = 0;
	 *   exit;
	 */
	.insns = {
	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, -44),
	BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),
	BPF_JMP_IMM(BPF_JSLT, BPF_REG_0, 0, 2),
	BPF_MOV64_IMM(BPF_REG_0, -1),
	BPF_EXIT_INSN(),
	BPF_MOV64_IMM(BPF_REG_0, 0),
	BPF_EXIT_INSN(),
	},
	/* Use prog type that requires return value in range [0, 1] */
	.prog_type = BPF_PROG_TYPE_SK_LOOKUP,
	.expected_attach_type = BPF_SK_LOOKUP,
	.result = VERBOSE_ACCEPT,
	.runs = -1,
	.errstr = "0: (7a) *(u64 *)(r10 -8) = -44        ; R10=fp0 fp-8_w=-44\
	2: (c5) if r0 s< 0x0 goto pc+2\
	2: R0_w=-44",
},