netdev - [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers

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Message-Id: <20190301210529.8218-1-daniel@iogearbox.net>
Date:   Fri,  1 Mar 2019 22:05:29 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     ast@...com
Cc:     afabre@...udflare.com, marek@...udflare.com, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH bpf] bpf: fix sanitation rewrite in case of non-pointers

Marek reported that he saw an issue with the below snippet in that
timing measurements where off when loaded as unpriv while results
were reasonable when loaded as privileged:

    [...]
    uint64_t a = bpf_ktime_get_ns();
    uint64_t b = bpf_ktime_get_ns();
    uint64_t delta = b - a;
    if ((int64_t)delta > 0) {
    [...]

Turns out there is a bug where a corner case is missing in the fix
d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar
type from different paths"), namely fixup_bpf_calls() only checks
whether aux has a non-zero alu_state, but it also needs to test for
the case of BPF_ALU_NON_POINTER since in both occasions we need to
skip the masking rewrite (as there is nothing to mask).

Fixes: d3bd7413e0ca ("bpf: fix sanitation of alu op with pointer / scalar type from different paths")
Reported-by: Marek Majkowski <marek@...udflare.com>
Reported-by: Arthur Fabre <afabre@...udflare.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Link: https://lore.kernel.org/netdev/CAJPywTJqP34cK20iLM5YmUMz9KXQOdu1-+BZrGMAGgLuBWz7fg@mail.gmail.com/T/
---
 [ Test case will be routed via bpf-next to avoid useless merge churn
   due to test_verifier rework in bpf-next. ]

 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 8f295b790297..5fcce2f4209d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6920,7 +6920,8 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			u32 off_reg;
 
 			aux = &env->insn_aux_data[i + delta];
-			if (!aux->alu_state)
+			if (!aux->alu_state ||
+			    aux->alu_state == BPF_ALU_NON_POINTER)
 				continue;
 
 			isneg = aux->alu_state & BPF_ALU_NEG_VALUE;
-- 
2.17.1