On Thu, Feb 05, 2015 at 04:09:06PM -0800, Andy Lutomirski wrote:

On Thu, Feb 5, 2015 at 3:49 PM, Kees Cook [off-list ref] wrote:

quoted

On Thu, Feb 5, 2015 at 3:39 PM, Dmitry V. Levin [off-list ref] wrote:

[...]

quoted

quoted

There is a clear difference: before these changes, SECCOMP_RET_ERRNO used
to keep the syscall number unchanged and suppress syscall-exit-stop event,
which was awful because userspace cannot distinguish syscall-enter-stop
from syscall-exit-stop and therefore relies on the kernel that
syscall-enter-stop is followed by syscall-exit-stop (or tracee's death, etc.).

After these changes, SECCOMP_RET_ERRNO no longer causes syscall-exit-stop
events to be suppressed, but now the syscall number is lost.

Ah-ha! Okay, thanks, I understand now. I think this means seccomp
phase1 should not treat RET_ERRNO as a "skip" event. Andy, what do you
think here?

I still don't quite see how this change caused this.

I have a test for this at
http://sourceforge.net/p/strace/code/ci/HEAD/~/tree/test/seccomp.c

I can play with
it a bit more.  But RET_ERRNO *has* to be some kind of skip event,
because it needs to skip the syscall.

We could change this by treating RET_ERRNO as an instruction to enter
phase 2 and then asking for a skip in phase 2 without changing
orig_ax, but IMO this is pretty ugly.

I think this all kind of sucks.  We're trying to run ptrace after
seccomp, so ptrace is seeing the syscalls as transformed by seccomp.
That means that if we use RET_TRAP, then ptrace will see the
possibly-modified syscall, if we use RET_ERRNO, then ptrace is (IMO
correctly given the current design) showing syscall -1, and if we use
RET_KILL, then ptrace just sees the process mysteriously die.

Userspace is usually not prepared to see syscall -1.
For example, strace had to be patched, otherwise it just skipped such
syscalls as "not a syscall" events or did other improper things:
http://sourceforge.net/p/strace/code/ci/c3948327717c29b10b5e00a436dc138b4ab1a486
http://sourceforge.net/p/strace/code/ci/8e398b6c4020fb2d33a5b3e40271ebf63199b891

A slightly different but related story: userspace is also not prepared
to handle large errno values produced by seccomp filters like this:
BPF_STMT(BPF_RET, SECCOMP_RET_ERRNO | SECCOMP_RET_DATA)

For example, glibc assumes that syscalls do not return errno values greater than 0xfff:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86_64/sysdep.h#l55
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86_64/syscall.S#l20

If it isn't too late, I'd recommend changing SECCOMP_RET_DATA mask
applied in SECCOMP_RET_ERRNO case from current 0xffff to 0xfff.


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ldv