[RFC][PATCH 0/8] seccomp: Implement constant action bitmaps

[Andy Lutomirski]

On Tue, Jun 16, 2020 at 12:49 AM Kees Cook <keescook at chromium.org> wrote:
>
> Hi,
>

> In order to build this mapping at filter attach time, each filter is
> executed for every syscall (under each possible architecture), and
> checked for any accesses of struct seccomp_data that are not the "arch"
> nor "nr" (syscall) members. If only "arch" and "nr" are examined, then
> there is a constant mapping for that syscall, and bitmaps can be updated
> accordingly. If any accesses happen outside of those struct members,
> seccomp must not bypass filter execution for that syscall, since program
> state will be used to determine filter action result.

>
> During syscall action probing, in order to determine whether other members
> of struct seccomp_data are being accessed during a filter execution,
> the struct is placed across a page boundary with the "arch" and "nr"
> members in the first page, and everything else in the second page. The
> "page accessed" flag is cleared in the second page's PTE, and the filter
> is run. If the "page accessed" flag appears as set after running the
> filter, we can determine that the filter looked beyond the "arch" and
> "nr" members, and exclude that syscall from the constant action bitmaps.

This is... evil.  I don't know how I feel about it.  It's also
potentially quite slow.

I don't suppose you could, instead, instrument the BPF code to get at
this without TLB hackery?  Or maybe try to do some real symbolic
execution of the BPF code?

--Andy