bpf: don't prune branches when a scalar is replaced with a pointer

From: Jann Horn <jannh@google.com>

[ Upstream commit 179d1c5602997fef5a940c6ddcf31212cbfebd14 ]

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@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8c35355..5a30eda 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3337,15 +3337,14 @@ 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