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

[Daniel Borkmann <daniel@iogearbox.net>]

On 1/9/23 4:05 PM, Luis Gerhorst 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")
> Signed-off-by: Luis Gerhorst <gerhorst@cs.fau.de>
> Acked-by: Henriette Hofmeier <henriette.hofmeier@rub.de>

This looks good to me, thank you for the research on this topic! Applied
to bpf tree. (I've also added a link tag to your other mail.)

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=e4f4db47794c9f474b184ee1418f42e6a07412b6

Thanks,
Daniel