bpf: reject out-of-bounds stack pointer calculation

From: Jann Horn <jannh@google.com>

Reject programs that compute wildly out-of-bounds stack pointers.
Otherwise, pointers can be computed with an offset that doesn't fit into an
`int`, causing security issues in the stack memory access check (as well as
signed integer overflow during offset addition).

This is a fix specifically for the v4.9 stable tree because the mainline
code looks very different at this point.

Fixes: 7bca0a9702edf ("bpf: enhance verifier to understand stack pointer arithmetic")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-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 2f19c94..b03af36 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1861,10 +1861,28 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
                           ((BPF_SRC(insn->code) == BPF_X &&
                             regs[insn->src_reg].type == CONST_IMM) ||
                            BPF_SRC(insn->code) == BPF_K)) {
-                       if (BPF_SRC(insn->code) == BPF_X)
+                       if (BPF_SRC(insn->code) == BPF_X) {
+                               /* check in case the register contains a big
+                                * 64-bit value
+                                */
+                               if (regs[insn->src_reg].imm < -MAX_BPF_STACK ||
+                                   regs[insn->src_reg].imm > MAX_BPF_STACK) {
+                                       verbose("R%d value too big in R%d pointer arithmetic\n",
+                                               insn->src_reg, insn->dst_reg);
+                                       return -EACCES;
+                               }
                                dst_reg->imm += regs[insn->src_reg].imm;
-                       else
+                       } else {
+                               /* safe against overflow: addition of 32-bit
+                                * numbers in 64-bit representation
+                                */
                                dst_reg->imm += insn->imm;
+                       }
+                       if (dst_reg->imm > 0 || dst_reg->imm < -MAX_BPF_STACK) {
+                               verbose("R%d out-of-bounds pointer arithmetic\n",
+                                       insn->dst_reg);
+                               return -EACCES;
+                       }
                        return 0;
                } else if (opcode == BPF_ADD &&
                           BPF_CLASS(insn->code) == BPF_ALU64 &&