From: Jann Horn <jannh@xxxxxxxxxx>
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@xxxxxxxxxx>
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
kernel/bpf/verifier.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2f19c94..b03af36 100644
--- 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 &&
--
2.9.5