vmm, virtio-devices: allow mremap for consoles

SerialBuffer uses VecDeque::extend, which calls realloc, which a
maximum buffer size of 1 MiB.  Starting at allocation sizes of
128 KiB, musl's mallocng allocator will use mremap for the allocation.
Since this was not permitted by the seccomp rules, heavy write load
could crash cloud-hypervisor with a seccomp failure.  (Encountered
using virtio-console, but I don't see any reason it wouldn't happen
for the legacy serial device too.)

Signed-off-by: Alyssa Ross <hi@alyssa.is>

virtio-devices/src/seccomp_filters.rs

@@ -259,6 +259,7 @@ fn virtio_thread_common() -> Vec<(i64, Vec<SeccompRule>)> {
         (libc::SYS_madvise, vec![]),
         (libc::SYS_mmap, vec![]),
         (libc::SYS_mprotect, vec![]),
+        (libc::SYS_mremap, vec![]),
         (libc::SYS_munmap, vec![]),
         (libc::SYS_openat, vec![]),
         (libc::SYS_read, vec![]),

vmm/src/seccomp_filters.rs

@@ -715,6 +715,7 @@ fn vcpu_thread_rules(
         (libc::SYS_madvise, vec![]),
         (libc::SYS_mmap, vec![]),
         (libc::SYS_mprotect, vec![]),
+        (libc::SYS_mremap, vec![]),
         (libc::SYS_munmap, vec![]),
         (libc::SYS_nanosleep, vec![]),
         (libc::SYS_newfstatat, vec![]),