From: Edward Cree <ecree@solarflare.com>
Date: Mon, 18 Dec 2017 20:11:53 -0800
Subject: bpf/verifier: fix bounds calculation on BPF_RSH
Patch-mainline: v4.15-rc5
Git-commit: 4374f256ce8182019353c0c639bb8d0695b4c941
References: CVE-2017-17853 bsc#1056787 bsc#1073928

Incorrect signed bounds were being computed.
If the old upper signed bound was positive and the old lower signed bound was
negative, this could cause the new upper signed bound to be too low,
leading to security issues.

Fixes: b03c9f9fdc37 ("bpf/verifier: track signed and unsigned min/max values")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
[jannh@google.com: changed description to reflect bug impact]
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>

---
 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 4320caac53e8..0b835c64eb54 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2197,20 +2197,22 @@ 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);
-- 
2.15.1