Daan De Meyer
f80d935027
Many io_uring operations can do the work of a system call: IORING_OP_OPENAT opens a file like openat(), IORING_OP_SOCKET creates a socket like socket(), and so on. SystemCallFilter= doesn't apply to io_uring operations. A service that can use io_uring can thus do things that its system call filter denies. On kernels with io_uring BPF filters, let's map each operation to the system calls that it can do the work of, and deny the operation if the filter denies any of them. IORING_OP_URING_CMD maps to ioctl() and also to setsockopt(), getsockopt(), and getsockname(), because it runs those socket calls on sockets. We don't deny operations that we don't know about, even for an allow list. A denied request fails with EACCES, while the kernel keeps reporting the operation as supported, so programs couldn't fall back. Our seccomp allow lists return ENOSYS for unknown system calls for the same reason. We skip system calls that are denied with an error number of 0. A denied io_uring request fails with EACCES and can't pretend to succeed. We also don't register any io_uring filters if the system call filter denies io_uring_setup(), because the service can't create rings then.
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