From: Alexei Starovoitov ast@fb.com

commit c131187db2d3fa2f8bf32fdf4e9a4ef805168467 upstream.

when the verifier detects that register contains a runtime constant and it's compared with another constant it will prune exploration of the branch that is guaranteed not to be taken at runtime. This is all correct, but malicious program may be constructed in such a way that it always has a constant comparison and the other branch is never taken under any conditions. In this case such path through the program will not be explored by the verifier. It won't be taken at run-time either, but since all instructions are JITed the malicious program may cause JITs to complain about using reserved fields, etc. To fix the issue we have to track the instructions explored by the verifier and sanitize instructions that are dead at run time with NOPs. We cannot reject such dead code, since llvm generates it for valid C code, since it doesn't do as much data flow analysis as the verifier does.

Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)") Signed-off-by: Alexei Starovoitov ast@kernel.org Acked-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Daniel Borkmann daniel@iogearbox.net [bwh: Backported to 4.4: - s/bpf_verifier_env/verifier_env/ - Adjust context] Signed-off-by: Ben Hutchings ben.hutchings@codethink.co.uk --- v2: Restore Alexei as author

kernel/bpf/verifier.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 014c2d759916..a62679711de0 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -191,6 +191,7 @@ struct bpf_insn_aux_data { enum bpf_reg_type ptr_type; /* pointer type for load/store insns */ struct bpf_map *map_ptr; /* pointer for call insn into lookup_elem */ }; + bool seen; /* this insn was processed by the verifier */ };

#define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */ @@ -1793,6 +1794,7 @@ static int do_check(struct verifier_env *env) print_bpf_insn(env, insn); }

+ env->insn_aux_data[insn_idx].seen = true; if (class == BPF_ALU || class == BPF_ALU64) { err = check_alu_op(env, insn); if (err) @@ -1988,6 +1990,7 @@ process_bpf_exit: return err;

insn_idx++; + env->insn_aux_data[insn_idx].seen = true; } else { verbose("invalid BPF_LD mode\n"); return -EINVAL; @@ -2125,6 +2128,7 @@ static int adjust_insn_aux_data(struct verifier_env *env, u32 prog_len, u32 off, u32 cnt) { struct bpf_insn_aux_data *new_data, *old_data = env->insn_aux_data; + int i;

if (cnt == 1) return 0; @@ -2134,6 +2138,8 @@ static int adjust_insn_aux_data(struct verifier_env *env, u32 prog_len, memcpy(new_data, old_data, sizeof(struct bpf_insn_aux_data) * off); memcpy(new_data + off + cnt - 1, old_data + off, sizeof(struct bpf_insn_aux_data) * (prog_len - off - cnt + 1)); + for (i = off; i < off + cnt - 1; i++) + new_data[i].seen = true; env->insn_aux_data = new_data; vfree(old_data); return 0; @@ -2152,6 +2158,25 @@ static struct bpf_prog *bpf_patch_insn_data(struct verifier_env *env, u32 off, return new_prog; }

+/* The verifier does more data flow analysis than llvm and will not explore + * branches that are dead at run time. Malicious programs can have dead code + * too. Therefore replace all dead at-run-time code with nops. + */ +static void sanitize_dead_code(struct verifier_env *env) +{ + struct bpf_insn_aux_data *aux_data = env->insn_aux_data; + struct bpf_insn nop = BPF_MOV64_REG(BPF_REG_0, BPF_REG_0); + struct bpf_insn *insn = env->prog->insnsi; + const int insn_cnt = env->prog->len; + int i; + + for (i = 0; i < insn_cnt; i++) { + if (aux_data[i].seen) + continue; + memcpy(insn + i, &nop, sizeof(nop)); + } +} + /* convert load instructions that access fields of 'struct __sk_buff' * into sequence of instructions that access fields of 'struct sk_buff' */ @@ -2370,6 +2395,9 @@ skip_full_check: while (pop_stack(env, NULL) >= 0); free_states(env);

+ if (ret == 0) + sanitize_dead_code(env); + if (ret == 0) /* program is valid, convert *(u32*)(ctx + off) accesses */ ret = convert_ctx_accesses(env);