netdev - [PATCH bpf 7/9] bpf: don't prune branches when a scalar is replaced with a pointer

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Date:   Mon, 18 Dec 2017 20:11:59 -0800
From:   Alexei Starovoitov <ast@...nel.org>
To:     "David S . Miller" <davem@...emloft.net>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        Jann Horn <jannh@...gle.com>,
        Edward Cree <ecree@...arflare.com>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: [PATCH bpf 7/9] bpf: don't prune branches when a scalar is replaced with a pointer

From: Jann Horn <jannh@...gle.com>

This could be made safe by passing through a reference to env and checking
for env->allow_ptr_leaks, but it would only work one way and is probably
not worth the hassle - not doing it will not directly lead to program
rejection.

Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
---
 kernel/bpf/verifier.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 102c519836f6..982bd9ec721a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3467,15 +3467,14 @@ static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
 			return range_within(rold, rcur) &&
 			       tnum_in(rold->var_off, rcur->var_off);
 		} else {
-			/* if we knew anything about the old value, we're not
-			 * equal, because we can't know anything about the
-			 * scalar value of the pointer in the new value.
+			/* We're trying to use a pointer in place of a scalar.
+			 * Even if the scalar was unbounded, this could lead to
+			 * pointer leaks because scalars are allowed to leak
+			 * while pointers are not. We could make this safe in
+			 * special cases if root is calling us, but it's
+			 * probably not worth the hassle.
 			 */
-			return rold->umin_value == 0 &&
-			       rold->umax_value == U64_MAX &&
-			       rold->smin_value == S64_MIN &&
-			       rold->smax_value == S64_MAX &&
-			       tnum_is_unknown(rold->var_off);
+			return false;
 		}
 	case PTR_TO_MAP_VALUE:
 		/* If the new min/max/var_off satisfy the old ones and
-- 
2.9.5