Re: [PATCH] setup: avoid unconditional open with write flags

["Christian Göttsche" <cgzones@googlemail.com>]

> But it's still no better than the patch here in the happy case, since we
> still have to perform three fcntl() checks to figure out that all three
> descriptors are initialized as-expected (versus just one open() and
> close()).

An alternative to performing three syscalls fot the check one could call
open(2) with O_RDONLY (O_PATH would also work, but seems not
yet to be used in the git source) on a common path ("/", "/dev/null", ...)
and skip the sanitization if the returned descriptor is greater than 2.
This would lead to two (open + close) syscalls in the common case,
same as current.

> If Christian can tighten
> the environment into somewhat unnatural "opening writable FD is a
> failure" way, I suspect such a jail can be augmented to further to
> allow opening /dev/null and other "selected" files writable, so I
> wouldn't worry too much if we dropped this patch entirely.

The seccomp filter only gets the address of the memory where the path
is stored, so simple allow-listing paths is not possible.  And even on
inspection of the path one would need to avoid toctou attacks (the
filter seeing a different memory content at check time than the kernel
at use time).