netdev - [PATCH bpf 1/2] bpf: mark registers as safe or unknown in all frames

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Date:   Sat, 20 Apr 2019 14:39:55 +0200
From:   Paul Chaignon <paul.chaignon@...nge.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     Xiao Han <xiao.han@...nge.com>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: [PATCH bpf 1/2] bpf: mark registers as safe or unknown in all frames

In case of a null check on a pointer inside a subprog, we should mark all
registers with this pointer as either safe or unknown, in both the current
and previous frames.  Currently, only spilled registers and registers in
the current frame are marked.  This patch also marks registers in previous
frames.

A good reproducer looks as follow:

1: ptr = bpf_map_lookup_elem(map, &key);
2: ret = subprog(ptr) {
3:   return ptr != NULL;
4: }
5: if (ret)
6:   value = *ptr;

With the above, the verifier will complain on line 6 because it sees ptr
as map_value_or_null despite the null check in subprog 1.

Note that this patch fixes another resulting bug when using
bpf_sk_release():

1: sk = bpf_sk_lookup_tcp();
2: subprog(sk) {
3:   if (sk)
4:     bpf_sk_release(sk, 0);
5: }
6: if (!sk)
7:   return 0;
8: return sk;

In the above, mark_ptr_or_null_regs will warn on line 6 because it will
try to free the reference state, even though it was already freed on
line 3.

Fixes: f4d7e40a5b71 ("bpf: introduce function calls (verification)")
Signed-off-by: Paul Chaignon <paul.chaignon@...nge.com>
---
 kernel/bpf/verifier.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index db301e9b5295..777970d8c245 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4876,11 +4876,11 @@ static void mark_ptr_or_null_regs(struct bpf_verifier_state *vstate, u32 regno,
 		 */
 		WARN_ON_ONCE(release_reference_state(state, id));
 
-	for (i = 0; i < MAX_BPF_REG; i++)
-		mark_ptr_or_null_reg(state, &regs[i], id, is_null);
-
 	for (j = 0; j <= vstate->curframe; j++) {
 		state = vstate->frame[j];
+		for (i = 0; i < MAX_BPF_REG; i++)
+			mark_ptr_or_null_reg(state, &state->regs[i], id,
+					     is_null);
 		bpf_for_each_spilled_reg(i, state, reg) {
 			if (!reg)
 				continue;
-- 
2.17.1