netdev - [PATCH net 1/2] bpf/verifier: fix bounds calculation on BPF

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Message-ID: <f5041e0a-85d9-6b14-a052-59bae94a5f63@solarflare.com>
Date:   Tue, 5 Dec 2017 19:15:18 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     davem <davem@...emloft.net>
CC:     <netdev@...r.kernel.org>, Jann Horn <jannh@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH net 1/2] bpf/verifier: fix bounds calculation on BPF_RSH

Incorrect signed bounds were being computed, although this had no effect
 since the propagation in __reg_deduce_bounds() happened to overwrite them.

Fixes: b03c9f9fdc37 ("bpf/verifier: track signed and unsigned min/max values")
Reported-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Edward Cree <ecree@...arflare.com>
---
 kernel/bpf/verifier.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d4593571c404..5bed7f773c87 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2184,20 +2184,22 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 			mark_reg_unknown(env, regs, insn->dst_reg);
 			break;
 		}
-		/* BPF_RSH is an unsigned shift, so make the appropriate casts */
-		if (dst_reg->smin_value < 0) {
-			if (umin_val) {
-				/* Sign bit will be cleared */
-				dst_reg->smin_value = 0;
-			} else {
-				/* Lost sign bit information */
-				dst_reg->smin_value = S64_MIN;
-				dst_reg->smax_value = S64_MAX;
-			}
-		} else {
-			dst_reg->smin_value =
-				(u64)(dst_reg->smin_value) >> umax_val;
-		}
+		/* BPF_RSH is an unsigned shift.  If the value in dst_reg might
+		 * be negative, then either:
+		 * 1) src_reg might be zero, so the sign bit of the result is
+		 *    unknown, so we lose our signed bounds
+		 * 2) it's known negative, thus the unsigned bounds capture the
+		 *    signed bounds
+		 * 3) the signed bounds cross zero, so they tell us nothing
+		 *    about the result
+		 * If the value in dst_reg is known nonnegative, then again the
+		 * unsigned bounts capture the signed bounds.
+		 * Thus, in all cases it suffices to blow away our signed bounds
+		 * and rely on inferring new ones from the unsigned bounds and
+		 * var_off of the result.
+		 */
+		dst_reg->smin_value = S64_MIN;
+		dst_reg->smax_value = S64_MAX;
 		if (src_known)
 			dst_reg->var_off = tnum_rshift(dst_reg->var_off,
 						       umin_val);