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Luis Gerhorst authored
For every BPF_ADD/SUB involving a pointer, adjust_ptr_min_max_vals() ensures that the resulting pointer has a constant offset if bypass_spec_v1 is false. This is ensured by calling sanitize_check_bounds() which in turn calls check_stack_access_for_ptr_arithmetic(). There, -EACCESS is returned if the register's offset is not constant, thereby rejecting the program. In summary, an unprivileged user must never be able to create stack pointers with a variable offset. That is also the case, because a respective check in check_stack_write() is missing. If they were able to create a variable-offset pointer, users could still use it in a stack-write operation to trigger unsafe speculative behavior [1]. Because unprivileged users must already be prevented from creating variable-offset stack pointers, viable options are to either remove this check (replacing it with a clarifying comment), or to turn it into a "verifier BUG"-message, also adding a similar check in check_stack_write() (for consistency, as a second-level defense). This patch implements the first option to reduce verifier bloat. This check was introduced by commit 01f810ac ("bpf: Allow variable-offset stack access") which correctly notes that "variable-offset reads and writes are disallowed (they were already disallowed for the indirect access case) because the speculative execution checking code doesn't support them". However, it does not further discuss why the check in check_stack_read() is necessary. The code which made this check obsolete was also introduced in this commit. I have compiled ~650 programs from the Linux selftests, Linux samples, Cilium, and libbpf/examples projects and confirmed that none of these trigger the check in check_stack_read() [2]. Instead, all of these programs are, as expected, already rejected when constructing the variable-offset pointers. Note that the check in check_stack_access_for_ptr_arithmetic() also prints "off=%d" while the code removed by this patch does not (the error removed does not appear in the "verification_error" values). For reproducibility, the repository linked includes the raw data and scripts used to create the plot. [1] https://arxiv.org/pdf/1807.03757.pdf [2] https://gitlab.cs.fau.de/un65esoq/bpf-spectre/-/raw/53dc19fcf459c186613b1156a81504b39c8d49db/data/plots/23-02-26_23-56_bpftool/bpftool/0004-errors.pdf?inline=false Fixes: 01f810ac ("bpf: Allow variable-offset stack access") Signed-off-by: Luis Gerhorst <gerhorst@cs.fau.de> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20230315165358.23701-1-gerhorst@cs.fau.de
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