On Tue, Jun 16, 2020 at 12:49 AM Kees Cook [off-list ref] 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