[PATCH 06/12] bpf: restrict unknown scalars of mixed signed bounds for unprivileged

[Tyler Hicks]

From: Daniel Borkmann <daniel at iogearbox.net>

For unknown scalars of mixed signed bounds, meaning their smin_value is
negative and their smax_value is positive, we need to reject arithmetic
with pointer to map value. For unprivileged the goal is to mask every
map pointer arithmetic and this cannot reliably be done when it is
unknown at verification time whether the scalar value is negative or
positive. Given this is a corner case, the likelihood of breaking should
be very small.

Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Acked-by: Alexei Starovoitov <ast at kernel.org>
Signed-off-by: Alexei Starovoitov <ast at kernel.org>

CVE-2019-7308

(backported from commit 9d7eceede769f90b66cfa06ad5b357140d5141ed)
[tyhicks: adjust_ptr_min_max_vals() uses if statements, not a switch]
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 kernel/bpf/verifier.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b85a29a6953c..24bf85f2f276 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2680,8 +2680,8 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	    smin_ptr = ptr_reg->smin_value, smax_ptr = ptr_reg->smax_value;
 	u64 umin_val = off_reg->umin_value, umax_val = off_reg->umax_value,
 	    umin_ptr = ptr_reg->umin_value, umax_ptr = ptr_reg->umax_value;
+	u32 dst = insn->dst_reg, src = insn->src_reg;
 	u8 opcode = BPF_OP(insn->code);
-	u32 dst = insn->dst_reg;
 
 	dst_reg = &regs[dst];
 
@@ -2717,6 +2717,13 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 			dst);
 		return -EACCES;
 	}
+	if (ptr_reg->type == PTR_TO_MAP_VALUE) {
+		if (!env->allow_ptr_leaks && !known && (smin_val < 0) != (smax_val < 0)) {
+			verbose(env, "R%d has unknown scalar with mixed signed bounds, pointer arithmetic with it prohibited for !root\n",
+				off_reg == dst_reg ? dst : src);
+			return -EACCES;
+		}
+	}
 
 	/* In case of 'scalar += pointer', dst_reg inherits pointer type and id.
 	 * The id may be overwritten later if we create a new variable offset.
-- 
2.7.4