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

[Alexei Starovoitov]

On Mon, Jan 9, 2023 at 7:07 AM Luis Gerhorst <gerhorst@xxxxxxxxx> wrote:
>
> 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")

All makes sense to me.
Daniel,

please take a look.

> Signed-off-by: Luis Gerhorst <gerhorst@xxxxxxxxx>
> Acked-by: Henriette Hofmeier <henriette.hofmeier@xxxxxx>
> ---
>  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
>