[PATCH bpf 2/9] bpf: fix incorrect sign extension in check_alu_op()

[Alexei Starovoitov]

From: Jann Horn <ja...@google.com>

Distinguish between
BPF_ALU64|BPF_MOV|BPF_K (load 32-bit immediate, sign-extended to 64-bit)
and BPF_ALU|BPF_MOV|BPF_K (load 32-bit immediate, zero-padded to 64-bit);
only perform sign extension in the first case.

Starting with v4.14, this is exploitable by unprivileged users as long as
the unprivileged_bpf_disabled sysctl isn't set.

Debian assigned CVE-2017-16995 for this issue.

v3:
 - add CVE number (Ben Hutchings)

Fixes: 484611357c19 ("bpf: allow access into map value arrays")
Signed-off-by: Jann Horn <ja...@google.com>
Acked-by: Edward Cree <ec...@solarflare.com>
Signed-off-by: Alexei Starovoitov <a...@kernel.org>
---
 kernel/bpf/verifier.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 625e358ca765..c086010ae51e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2408,7 +2408,13 @@ static int check_alu_op(struct bpf_verifier_env *env, 
struct bpf_insn *insn)
                         * remember the value we stored into this reg
                         */
                        regs[insn->dst_reg].type = SCALAR_VALUE;
-                       __mark_reg_known(regs + insn->dst_reg, insn->imm);
+                       if (BPF_CLASS(insn->code) == BPF_ALU64) {
+                               __mark_reg_known(regs + insn->dst_reg,
+                                                insn->imm);
+                       } else {
+                               __mark_reg_known(regs + insn->dst_reg,
+                                                (u32)insn->imm);
+                       }
                }
 
        } else if (opcode > BPF_END) {
-- 
2.9.5