From: Daniel Borkmann daniel@iogearbox.net

mainline inclusion from mainline-v5.13-rc7 commit 9183671af6dbf60a1219371d4ed73e23f43b49db category: bugfix bugzilla: NA CVE: CVE-2021-33624

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The verifier only enumerates valid control-flow paths and skips paths that are unreachable in the non-speculative domain. And so it can miss issues under speculative execution on mispredicted branches.

For example, a type confusion has been demonstrated with the following crafted program:

// r0 = pointer to a map array entry // r6 = pointer to readable stack slot // r9 = scalar controlled by attacker 1: r0 = *(u64 *)(r0) // cache miss 2: if r0 != 0x0 goto line 4 3: r6 = r9 4: if r0 != 0x1 goto line 6 5: r9 = *(u8 *)(r6) 6: // leak r9

Since line 3 runs iff r0 == 0 and line 5 runs iff r0 == 1, the verifier concludes that the pointer dereference on line 5 is safe. But: if the attacker trains both the branches to fall-through, such that the following is speculatively executed ...

r6 = r9 r9 = *(u8 *)(r6) // leak r9

... then the program will dereference an attacker-controlled value and could leak its content under speculative execution via side-channel. This requires to mistrain the branch predictor, which can be rather tricky, because the branches are mutually exclusive. However such training can be done at congruent addresses in user space using different branches that are not mutually exclusive. That is, by training branches in user space ...

A: if r0 != 0x0 goto line C B: ... C: if r0 != 0x0 goto line D D: ...

... such that addresses A and C collide to the same CPU branch prediction entries in the PHT (pattern history table) as those of the BPF program's lines 2 and 4, respectively. A non-privileged attacker could simply brute force such collisions in the PHT until observing the attack succeeding.

Alternative methods to mistrain the branch predictor are also possible that avoid brute forcing the collisions in the PHT. A reliable attack has been demonstrated, for example, using the following crafted program:

// r0 = pointer to a [control] map array entry // r7 = *(u64 *)(r0 + 0), training/attack phase // r8 = *(u64 *)(r0 + 8), oob address // [...] // r0 = pointer to a [data] map array entry 1: if r7 == 0x3 goto line 3 2: r8 = r0 // crafted sequence of conditional jumps to separate the conditional // branch in line 193 from the current execution flow 3: if r0 != 0x0 goto line 5 4: if r0 == 0x0 goto exit 5: if r0 != 0x0 goto line 7 6: if r0 == 0x0 goto exit [...] 187: if r0 != 0x0 goto line 189 188: if r0 == 0x0 goto exit // load any slowly-loaded value (due to cache miss in phase 3) ... 189: r3 = *(u64 *)(r0 + 0x1200) // ... and turn it into known zero for verifier, while preserving slowly- // loaded dependency when executing: 190: r3 &= 1 191: r3 &= 2 // speculatively bypassed phase dependency 192: r7 += r3 193: if r7 == 0x3 goto exit 194: r4 = *(u8 *)(r8 + 0) // leak r4

As can be seen, in training phase (phase != 0x3), the condition in line 1 turns into false and therefore r8 with the oob address is overridden with the valid map value address, which in line 194 we can read out without issues. However, in attack phase, line 2 is skipped, and due to the cache miss in line 189 where the map value is (zeroed and later) added to the phase register, the condition in line 193 takes the fall-through path due to prior branch predictor training, where under speculation, it'll load the byte at oob address r8 (unknown scalar type at that point) which could then be leaked via side-channel.

One way to mitigate these is to 'branch off' an unreachable path, meaning, the current verification path keeps following the is_branch_taken() path and we push the other branch to the verification stack. Given this is unreachable from the non-speculative domain, this branch's vstate is explicitly marked as speculative. This is needed for two reasons: i) if this path is solely seen from speculative execution, then we later on still want the dead code elimination to kick in in order to sanitize these instructions with jmp-1s, and ii) to ensure that paths walked in the non-speculative domain are not pruned from earlier walks of paths walked in the speculative domain. Additionally, for robustness, we mark the registers which have been part of the conditional as unknown in the speculative path given there should be no assumptions made on their content.

The fix in here mitigates type confusion attacks described earlier due to i) all code paths in the BPF program being explored and ii) existing verifier logic already ensuring that given memory access instruction references one specific data structure.

An alternative to this fix that has also been looked at in this scope was to mark aux->alu_state at the jump instruction with a BPF_JMP_TAKEN state as well as direction encoding (always-goto, always-fallthrough, unknown), such that mixing of different always-* directions themselves as well as mixing of always-* with unknown directions would cause a program rejection by the verifier, e.g. programs with constructs like 'if ([...]) { x = 0; } else { x = 1; }' with subsequent 'if (x == 1) { [...] }'. For unprivileged, this would result in only single direction always-* taken paths, and unknown taken paths being allowed, such that the former could be patched from a conditional jump to an unconditional jump (ja). Compared to this approach here, it would have two downsides: i) valid programs that otherwise are not performing any pointer arithmetic, etc, would potentially be rejected/broken, and ii) we are required to turn off path pruning for unprivileged, where both can be avoided in this work through pushing the invalid branch to the verification stack.

The issue was originally discovered by Adam and Ofek, and later independently discovered and reported as a result of Benedict and Piotr's research work.

Fixes: b2157399cc98 ("bpf: prevent out-of-bounds speculation") Reported-by: Adam Morrison mad@cs.tau.ac.il Reported-by: Ofek Kirzner ofekkir@gmail.com Reported-by: Benedict Schlueter benedict.schlueter@rub.de Reported-by: Piotr Krysiuk piotras@gmail.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Reviewed-by: John Fastabend john.fastabend@gmail.com Reviewed-by: Benedict Schlueter benedict.schlueter@rub.de Reviewed-by: Piotr Krysiuk piotras@gmail.com Acked-by: Alexei Starovoitov ast@kernel.org

onflicts: kernel/bpf/verifier.c [yyl: bypass_spec_v1 is not introduced in kernel-4.19, use allow_ptr_leaks instead]

Signed-off-by: Yang Yingliang yangyingliang@huawei.com Signed-off-by: He Fengqinghefengqing@huawei.com Reviewed-by: Kuohai Xu xukuohai@huawei.com Reviewed-by: Xiu Jianfeng xiujianfeng@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- kernel/bpf/verifier.c | 46 +++++++++++++++++++++++++++++++++++++++---- 1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 38b4a013c0ccc..f6d092809ec3f 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2763,6 +2763,27 @@ struct bpf_sanitize_info { bool mask_to_left; };

+static struct bpf_verifier_state * +sanitize_speculative_path(struct bpf_verifier_env *env, + const struct bpf_insn *insn, + u32 next_idx, u32 curr_idx) +{ + struct bpf_verifier_state *branch; + struct bpf_reg_state *regs; + + branch = push_stack(env, next_idx, curr_idx, true); + if (branch && insn) { + regs = branch->frame[branch->curframe]->regs; + if (BPF_SRC(insn->code) == BPF_K) { + mark_reg_unknown(env, regs, insn->dst_reg); + } else if (BPF_SRC(insn->code) == BPF_X) { + mark_reg_unknown(env, regs, insn->dst_reg); + mark_reg_unknown(env, regs, insn->src_reg); + } + } + return branch; +} + static int sanitize_ptr_alu(struct bpf_verifier_env *env, struct bpf_insn *insn, const struct bpf_reg_state *ptr_reg, @@ -2846,7 +2867,8 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env, tmp = *dst_reg; *dst_reg = *ptr_reg; } - ret = push_stack(env, env->insn_idx + 1, env->insn_idx, true); + ret = sanitize_speculative_path(env, NULL, env->insn_idx + 1, + env->insn_idx); if (!ptr_is_dst_reg && ret) *dst_reg = tmp; return !ret ? REASON_STACK : 0; @@ -4405,14 +4427,30 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, tnum_is_const(src_reg->var_off)) pred = is_branch_taken(dst_reg, src_reg->var_off.value, opcode, is_jmp32); + if (pred == 1) { - /* only follow the goto, ignore fall-through */ + /* Only follow the goto, ignore fall-through. If needed, push + * the fall-through branch for simulation under speculative + * execution. + */ + if (!env->allow_ptr_leaks && + !sanitize_speculative_path(env, insn, *insn_idx + 1, + *insn_idx)) + return -EFAULT; + *insn_idx += insn->off; return 0; } else if (pred == 0) { - /* only follow fall-through branch, since - * that's where the program will go + /* Only follow the fall-through branch, since that's where the + * program will go. If needed, push the goto branch for + * simulation under speculative execution. */ + if (!env->allow_ptr_leaks && + !sanitize_speculative_path(env, insn, + *insn_idx + insn->off + 1, + *insn_idx)) + return -EFAULT; + return 0; }