netdev - [PATCH net] bpf: fix states equal logic for varlen access

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Date:   Mon, 28 Nov 2016 14:44:10 -0500
From:   Josef Bacik <jbacik@...com>
To:     <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <daniel@...earbox.net>, <ast@...nel.org>, <jannh@...gle.com>
Subject: [PATCH net] bpf: fix states equal logic for varlen access

If we have a branch that looks something like this

int foo = map->value;
if (condition) {
  foo += blah;
} else {
  foo = bar;
}
map->array[foo] = baz;

We will incorrectly assume that the !condition branch is equal to the condition
branch as the register for foo will be UNKNOWN_VALUE in both cases.  We need to
adjust this logic to only do this if we didn't do a varlen access after we
processed the !condition branch, otherwise we have different ranges and need to
check the other branch as well.

Signed-off-by: Josef Bacik <jbacik@...com>
---
 kernel/bpf/verifier.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 89f787c..2c8a688 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2478,6 +2478,7 @@ static bool states_equal(struct bpf_verifier_env *env,
 {
 	struct bpf_reg_state *rold, *rcur;
 	int i;
+	bool map_access = env->varlen_map_value_access;
 
 	for (i = 0; i < MAX_BPF_REG; i++) {
 		rold = &old->regs[i];
@@ -2489,12 +2490,17 @@ static bool states_equal(struct bpf_verifier_env *env,
 		/* If the ranges were not the same, but everything else was and
 		 * we didn't do a variable access into a map then we are a-ok.
 		 */
-		if (!env->varlen_map_value_access &&
+		if (!map_access &&
 		    rold->type == rcur->type && rold->imm == rcur->imm)
 			continue;
 
+		/* If we didn't map access then again we don't care about the
+		 * mismatched range values and it's ok if our old type was
+		 * UNKNOWN and we didn't go to a NOT_INIT'ed reg.
+		 */
 		if (rold->type == NOT_INIT ||
-		    (rold->type == UNKNOWN_VALUE && rcur->type != NOT_INIT))
+		    (!map_access && (rold->type == UNKNOWN_VALUE &&
+				     rcur->type != NOT_INIT)))
 			continue;
 
 		if (rold->type == PTR_TO_PACKET && rcur->type == PTR_TO_PACKET &&
-- 
2.7.4