seccomp: add SECCOMP_RET_ACK for non-fatal SIGSYS
Tracing processes for syscall usage can be done one step at a time with
SECCOMP_RET_TRAP, but this will block the syscall. Alternatively, using
a ptrace manager to handle SECCOMP_RET_TRACE returns can be used but is
heavy weight and depends on the ptrace infrastructure. A light-weight
method to learn syscalls is needed, which can reuse the existing delivery
of SIGSYS but without skipping the syscall. This is implemented as
SECCOMP_RET_ACK which is as permissive as SECCOMP_RET_ALLOW but delivers
SIGSYS after syscall completion, as long as the SECCOMP_RET_DATA is
non-zero. A signal handler can install a new rule for each syscall as
they are signaled with SECCOMP_RET_DATA set to 0 to disable reporting
for that syscall in the future (which is required for restarting syscalls
that are signal-sensitive like nanosleep).
Registers from the signal will reflect registers after the syscall returns
rather than before. Signal-sensitive syscalls will trigger EINTR, so they
must be whitelisted before they are resumed. Not allowing the sigreturn
syscall (and likely prctl to whitelist) will make using SECCOMP_RET_ACK
useless.
Signed-off-by: Kees Cook <keescook@chromium.org>
3 files changed
