On 12/10/20 4:10 PM, Daniel Borkmann wrote:
On 12/10/20 2:33 AM, Yonghong Song wrote:
Currently, when checking stack memory accessed by helper calls,
for spills, only PTR_TO_BTF_ID and SCALAR_VALUE are
allowed.
Song discovered an issue where the below bpf program
int dump_task(struct bpf_iter__task *ctx)
{
struct seq_file *seq = ctx->meta->seq;
static char[] info = "abc";
BPF_SEQ_PRINTF(seq, "%s\n", info);
return 0;
}
may cause a verifier failure.
The verifier output looks like:
; struct seq_file *seq = ctx->meta->seq;
1: (79) r1 = *(u64 *)(r1 +0)
; BPF_SEQ_PRINTF(seq, "%s\n", info);
2: (18) r2 = 0xffff9054400f6000
4: (7b) *(u64 *)(r10 -8) = r2
5: (bf) r4 = r10
;
6: (07) r4 += -8
; BPF_SEQ_PRINTF(seq, "%s\n", info);
7: (18) r2 = 0xffff9054400fe000
9: (b4) w3 = 4
10: (b4) w5 = 8
11: (85) call bpf_seq_printf#126
R1_w=ptr_seq_file(id=0,off=0,imm=0)
R2_w=map_value(id=0,off=0,ks=4,vs=4,imm=0)
R3_w=inv4 R4_w=fp-8 R5_w=inv8 R10=fp0 fp-8_w=map_value
last_idx 11 first_idx 0
regs=8 stack=0 before 10: (b4) w5 = 8
regs=8 stack=0 before 9: (b4) w3 = 4
invalid indirect read from stack off -8+0 size 8
Basically, the verifier complains the map_value pointer at "fp-8"
location.
To fix the issue, if env->allow_ptr_leaks is true, let us also permit
pointers on the stack to be accessible by the helper.
Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Reported-by: Song Liu <songliubraving@xxxxxx>
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
kernel/bpf/verifier.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 93def76cf32b..9159c9822ede 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3769,7 +3769,8 @@ static int check_stack_boundary(struct
bpf_verifier_env *env, int regno,
goto mark;
if (state->stack[spi].slot_type[0] == STACK_SPILL &&
- state->stack[spi].spilled_ptr.type == SCALAR_VALUE) {
+ (state->stack[spi].spilled_ptr.type == SCALAR_VALUE ||
+ env->allow_ptr_leaks)) {
Afaik, in check_stack_write() we mark some of the spilled_ptr.type as
NOT_INIT,
shouldn't we at least avoid an implicit transition of NOT_INIT into
SCALAR_VALUE?
Make sense! here we check env->allow_ptr_leaks and we should the
spilled_ptr.type for allow_ptr_leaks should be a pointer (!= NOT_INIT).
Will send v3 soon.
__mark_reg_unknown(env, &state->stack[spi].spilled_ptr);
for (j = 0; j < BPF_REG_SIZE; j++)
state->stack[spi].slot_type[j] = STACK_MISC;
