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

On Thu, 1 May 2025 at 04:17, Luis Gerhorst [off-list ref] 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 <redacted>
Acked-by: Henriette Herzog <redacted>
Cc: Maximilian Ott <redacted>
Cc: Milan Stephan <redacted>
---

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@gmail.com>