[PATCH bpf-next v3 10/11] bpf: Allow nospec-protected var-offset stack access

[Kumar Kartikeya Dwivedi]

On Thu, 1 May 2025 at 04:17, Luis Gerhorst <luis.gerhorst at fau.de> wrote:
>
> Insert a nospec before the access to prevent it from ever using an index
> that is subject to speculative scalar-confusion.
>
> The access itself can either happen directly in the BPF program (reads
> only, check_stack_read_var_off()) or in a helper (read/write,
> check_helper_mem_access()).
>
> This relies on the fact that the speculative scalar confusion that leads
> to the variable-stack access going OOBs must stem from a prior
> speculative store or branch bypass. Adding a nospec before the
> variable-stack access will force all previously bypassed stores/branches
> to complete and cause the stack access to only ever go to the stack slot
> that is accessed architecturally.
>
> Alternatively, the variable-offset stack access might be a write that
> can itself be subject to speculative store bypass (this can happen in
> theory even if this code adds a nospec /before/ the variable-offset
> write). Only indirect writes by helpers might be affected here (e.g.,
> those taking ARG_PTR_TO_MAP_VALUE). (Because check_stack_write_var_off()
> does not use check_stack_range_initialized(), in-program variable-offset
> writes are not affected.) If the in-helper write can be subject to
> Spectre v4 and the helper writes/overwrites pointers on the BPF stack,
> they are already a problem for fixed-offset stack accesses and should be
> subject to Spectre v4 sanitization.
>
> Signed-off-by: Luis Gerhorst <luis.gerhorst at fau.de>
> Acked-by: Henriette Herzog <henriette.herzog at rub.de>
> Cc: Maximilian Ott <ott at cs.fau.de>
> Cc: Milan Stephan <milan.stephan at fau.de>
> ---

Please also address sanitize_check_bounds, it's probably prevented by
retrieve_ptr_limit rejecting other types but it'd be better to add a
default statement for clarity.

Acked-by: Kumar Kartikeya Dwivedi <memxor at gmail.com>