linux-kernel - [PATCH] bpf: Fix pointer-leak due to insufficient speculative store bypass mitigation

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Message-Id: <20230109150544.41465-1-gerhorst@cs.fau.de>
Date:   Mon,  9 Jan 2023 16:05:46 +0100
From:   Luis Gerhorst <gerhorst@...fau.de>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Piotr Krysiuk <piotras@...il.com>,
        Benedict Schlueter <benedict.schlueter@....de>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Cc:     Luis Gerhorst <gerhorst@...fau.de>,
        stefan.saecherl@....startmail.com,
        Henriette Hofmeier <henriette.hofmeier@....de>
Subject: [PATCH] bpf: Fix pointer-leak due to insufficient speculative store bypass mitigation

To mitigate Spectre v4, 2039f26f3aca ("bpf: Fix leakage due to
insufficient speculative store bypass mitigation") inserts lfence
instructions after 1) initializing a stack slot and 2) spilling a
pointer to the stack.

However, this does not cover cases where a stack slot is first
initialized with a pointer (subject to sanitization) but then
overwritten with a scalar (not subject to sanitization because the slot
was already initialized). In this case, the second write may be subject
to speculative store bypass (SSB) creating a speculative
pointer-as-scalar type confusion. This allows the program to
subsequently leak the numerical pointer value using, for example, a
branch-based cache side channel.

To fix this, also sanitize scalars if they write a stack slot that
previously contained a pointer. Assuming that pointer-spills are only
generated by LLVM on register-pressure, the performance impact on most
real-world BPF programs should be small.

The following unprivileged BPF bytecode drafts a minimal exploit and the
mitigation:

  [...]
  // r6 = 0 or 1 (skalar, unknown user input)
  // r7 = accessible ptr for side channel
  // r10 = frame pointer (fp), to be leaked
  //
  r9 = r10 # fp alias to encourage ssb
  *(u64 *)(r9 - 8) = r10 // fp[-8] = ptr, to be leaked
  // lfence added here because of pointer spill to stack.
  //
  // Ommitted: Dummy bpf_ringbuf_output() here to train alias predictor
  // for no r9-r10 dependency.
  //
  *(u64 *)(r10 - 8) = r6 // fp[-8] = scalar, overwrites ptr
  // 2039f26f3aca: no lfence added because stack slot was not STACK_INVALID,
  // store may be subject to SSB
  //
  // fix: also add an lfence when the slot contained a ptr
  //
  r8 = *(u64 *)(r9 - 8)
  // r8 = architecturally a scalar, speculatively a ptr
  //
  // leak ptr using branch-based cache side channel:
  r8 &= 1 // choose bit to leak
  if r8 == 0 goto SLOW // no mispredict
  // architecturally dead code if input r6 is 0,
  // only executes speculatively iff ptr bit is 1
  r8 = *(u64 *)(r7 + 0) # encode bit in cache (0: slow, 1: fast)
SLOW:
  [...]

After running this, the program can time the access to *(r7 + 0) to
determine whether the chosen pointer bit was 0 or 1. Repeat this 64
times to recover the whole address on amd64.

In summary, sanitization can only be skipped if one scalar is
overwritten with another scalar. Scalar-confusion due to speculative
store bypass can not lead to invalid accesses because the pointer bounds
deducted during verification are enforced using branchless logic. See
979d63d50c0c ("bpf: prevent out of bounds speculation on pointer
arithmetic") for details.

Do not make the mitigation depend on
!env->allow_{uninit_stack,ptr_leaks} because speculative leaks are
likely unexpected if these were enabled. For example, leaking the
address to a protected log file may be acceptable while disabling the
mitigation might unintentionally leak the address into the cached-state
of a map that is accessible to unprivileged processes.

Fixes: 2039f26f3aca ("bpf: Fix leakage due to insufficient speculative store bypass mitigation")
Signed-off-by: Luis Gerhorst <gerhorst@...fau.de>
Acked-by: Henriette Hofmeier <henriette.hofmeier@....de>
---
 kernel/bpf/verifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a5255a0dcbb6..5e3aa4a75bd6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3287,7 +3287,8 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
 		bool sanitize = reg && is_spillable_regtype(reg->type);

 		for (i = 0; i < size; i++) {
-			if (state->stack[spi].slot_type[i] == STACK_INVALID) {
+			u8 type = state->stack[spi].slot_type[i];
+			if (type != STACK_MISC && type != STACK_ZERO) {
 				sanitize = true;
 				break;
 			}
-- 
2.34.1