Re: [PATCH bpf-next] Detect jumping to reserved code during check_cfg()
From: Daniel Borkmann
Date: Wed Oct 11 2023 - 10:50:16 EST
On 10/11/23 8:46 AM, Hao Sun wrote:
On Wed, Oct 11, 2023 at 4:42 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
On Tue, Oct 10, 2023 at 1:33 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
On 10/10/23 9:02 AM, John Fastabend wrote:
Hao Sun wrote:
Currently, we don't check if the branch-taken of a jump is reserved code of
ld_imm64. Instead, such a issue is captured in check_ld_imm(). The verifier
gives the following log in such case:
func#0 @0
0: R1=ctx(off=0,imm=0) R10=fp0
0: (18) r4 = 0xffff888103436000 ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
2: (18) r1 = 0x1d ; R1_w=29
4: (55) if r4 != 0x0 goto pc+4 ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
5: (1c) w1 -= w1 ; R1_w=0
6: (18) r5 = 0x32 ; R5_w=50
8: (56) if w5 != 0xfffffff4 goto pc-2
mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx -1
mark_precise: frame0: regs=r5 stack= before 6: (18) r5 = 0x32
7: R5_w=50
7: BUG_ld_00
invalid BPF_LD_IMM insn
Here the verifier rejects the program because it thinks insn at 7 is an
invalid BPF_LD_IMM, but such a error log is not accurate since the issue
is jumping to reserved code not because the program contains invalid insn.
Therefore, make the verifier check the jump target during check_cfg(). For
the same program, the verifier reports the following log:
I think we at least would want a test case for this. Also how did you create
this case? Is it just something you did manually and noticed a strange error?
Curious as well.
We do have test cases which try to jump into the middle of a double insn as can
be seen that this patch breaks BPF CI with regards to log mismatch below (which
still needs to be adapted, too). Either way, it probably doesn't hurt to also add
the above snippet as a test.
Hao, as I understand, the patch here is an usability improvement (not a fix per se)
where we reject such cases earlier during cfg check rather than at a later point
where we validate ld_imm instruction. Or are there cases you found which were not
yet captured via current check_ld_imm()?
test_verifier failure log :
#458/u test1 ld_imm64 FAIL
Unexpected verifier log!
EXP: R1 pointer comparison
RES:
FAIL
Unexpected error message!
EXP: R1 pointer comparison
RES: jump to reserved code from insn 0 to 2
verification time 22 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
jump to reserved code from insn 0 to 2
verification time 22 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
#458/p test1 ld_imm64 FAIL
Unexpected verifier log!
EXP: invalid BPF_LD_IMM insn
RES:
FAIL
Unexpected error message!
EXP: invalid BPF_LD_IMM insn
RES: jump to reserved code from insn 0 to 2
verification time 9 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
jump to reserved code from insn 0 to 2
verification time 9 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
#459/u test2 ld_imm64 FAIL
Unexpected verifier log!
EXP: R1 pointer comparison
RES:
FAIL
Unexpected error message!
EXP: R1 pointer comparison
RES: jump to reserved code from insn 0 to 2
verification time 11 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
jump to reserved code from insn 0 to 2
verification time 11 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
#459/p test2 ld_imm64 FAIL
Unexpected verifier log!
EXP: invalid BPF_LD_IMM insn
RES:
FAIL
Unexpected error message!
EXP: invalid BPF_LD_IMM insn
RES: jump to reserved code from insn 0 to 2
verification time 8 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
jump to reserved code from insn 0 to 2
verification time 8 usec
stack depth 0
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
#460/u test3 ld_imm64 OK
func#0 @0
jump to reserved code from insn 8 to 7
Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
nit: This needs to be before the "---" line.
---
kernel/bpf/verifier.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index eed7350e15f4..725ac0b464cf 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14980,6 +14980,7 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
{
int *insn_stack = env->cfg.insn_stack;
int *insn_state = env->cfg.insn_state;
+ struct bpf_insn *insns = env->prog->insnsi;
if (e == FALLTHROUGH && insn_state[t] >= (DISCOVERED | FALLTHROUGH))
return DONE_EXPLORING;
@@ -14993,6 +14994,12 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
return -EINVAL;
}
+ if (e == BRANCH && insns[w].code == 0) {
+ verbose_linfo(env, t, "%d", t);
+ verbose(env, "jump to reserved code from insn %d to %d\n", t, w);
+ return -EINVAL;
+ }
Other than that, lgtm.
We do rely quite a lot on verifier not complaining eagerly about some
potentially invalid instructions if it's provable that some portion of
the code won't ever be reached (think using .rodata variables for
feature gating, poisoning intructions due to failed CO-RE relocation,
which libbpf does actively, except it's using a call to non-existing
helper). As such, check_cfg() is a wrong place to do such validity
checks because some of the branches might never be run and validated
in practice.
Don't really agree. Jump to the middle of ld_imm64 is just like jumping
out of bounds, both break the CFG integrity immediately. For those
apparently incorrect jumps, rejecting early makes everything simple;
otherwise, we probably need some rewrite in the end.
Could you elaborate on the 'breaking CFG integrity immediately'? This was
what I was trying to gather earlier with log improvement vs actual fix.
Do you mean /potentially/ breaking CFG integrity, if, say, we had a double
insn jump in future and there is a back-jump to the 2nd part of the insn?
Also, as you mentioned, libbpf relies on non-existing helpers, not jump
to the middle of ld_imm64. It seems better and easier to not leave this
hole.
Thanks,
Daniel