[PATCH bpf-next v3 14/21] bpf: handle spilled PTR_TO_BTF_ID properly when checking stack_boundary

Yonghong Song
2020-05-07T05:40:19Z

This specifically to handle the case like below:
// ptr below is a socket ptr identified by PTR_TO_BTF_ID
u64 param[2] = { ptr, val };
bpf_seq_printf(seq, fmt, sizeof(fmt), param, sizeof(param));

In this case, the 16 bytes stack for "param" contains:
8 bytes for ptr with spilled PTR_TO_BTF_ID
8 bytes for val as STACK_MISC

The current verifier will complain the ptr should not be visible
to the helper.
...
16: (7b) *(u64 *)(r10 -64) = r2
18: (7b) *(u64 *)(r10 -56) = r1
19: (bf) r4 = r10
;
20: (07) r4 += -64
; BPF_SEQ_PRINTF(seq, fmt1, (long)s, s->sk_protocol);
21: (bf) r1 = r6
22: (18) r2 = 0xffffa8d00018605a
24: (b4) w3 = 10
25: (b4) w5 = 16
26: (85) call bpf_seq_printf#125
R0=inv(id=0) R1_w=ptr_seq_file(id=0,off=0,imm=0)
R2_w=map_value(id=0,off=90,ks=4,vs=144,imm=0) R3_w=inv10
R4_w=fp-64 R5_w=inv16 R6=ptr_seq_file(id=0,off=0,imm=0)
R7=ptr_netlink_sock(id=0,off=0,imm=0) R10=fp0 fp-56_w=mmmmmmmm
fp-64_w=ptr_
last_idx 26 first_idx 13
regs=8 stack=0 before 25: (b4) w5 = 16
regs=8 stack=0 before 24: (b4) w3 = 10
invalid indirect read from stack off -64+0 size 16

Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
kernel/bpf/verifier.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 36b2a38a06fe..2a1826c76bb6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3494,6 +3494,11 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
*stype = STACK_MISC;
goto mark;
}
+
+ if (state->stack[spi].slot_type[0] == STACK_SPILL &&
+ state->stack[spi].spilled_ptr.type == PTR_TO_BTF_ID)
+ goto mark;
+
if (state->stack[spi].slot_type[0] == STACK_SPILL &&
state->stack[spi].spilled_ptr.type == SCALAR_VALUE) {
__mark_reg_unknown(env, &state->stack[spi].spilled_ptr);
--
2.24.1