On Thu, Oct 26, 2023 at 05:13:10PM +0200, Hao Sun wrote:
In check_stack_write_fixed_off(), imm value is cast to u32 before being spilled to the stack. Therefore, the sign information is lost, and the range information is incorrect when load from the stack again.
For the following prog: 0: r2 = r10 1: *(u64*)(r2 -40) = -44 2: r0 = *(u64*)(r2 - 40) 3: if r0 s<= 0xa goto +2 4: r0 = 1 5: exit 6: r0 = 0 7: exit
The verifier gives: 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
So remove the incorrect cast, since imm field is declared as s32, and __mark_reg_known() takes u64, so imm would be correctly sign extended by compiler.
Signed-off-by: Hao Sun sunhao.th@gmail.com
Acked-by: Shung-Hsi Yu shung-hsi.yu@suse.com
The acked-by applies to future version of the patchset as well.
FWIW I think we'd also need the same treatment for the (BPF_ALU | BPF_MOV | BPF_K) case in check_alu_op().
kernel/bpf/verifier.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 857d76694517..44af69ce1301 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -4674,7 +4674,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env, insn->imm != 0 && env->bpf_capable) { struct bpf_reg_state fake_reg = {};
__mark_reg_known(&fake_reg, (u32)insn->imm);
__mark_reg_known(&fake_reg, insn->imm);-- 2.34.1