lxc_clone: pass non-stack allocated stack to clone

There are two problems with this code:

1. The math is wrong. We allocate a char *foo[__LXC_STACK_SIZE]; which
   means it's really sizeof(char *) * __LXC_STACK_SIZE, instead of just
   __LXC_STACK SIZE.

2. We can't actually allocate it on our stack. When we use CLONE_VM (which
   we do in the shared ns case) that means that the new thread is just
   running one page lower on the stack, but anything that allocates a page
   on the stack may clobber data. This is a pretty short race window since
   we just do the shared ns stuff and then do a clone without CLONE_VM.

However, it does point out an interesting possible privilege escalation if
things aren't configured correctly: do_share_ns() sets up namespaces while
it shares the address space of the task that spawned it; once it enters the
pid ns of the thing it's sharing with, the thing it's sharing with can
ptrace it and write stuff into the host's address space. Since the function
that does the clone() is lxc_spawn(), it has a struct cgroup_ops* on the
stack, which itself has function pointers called later in the function, so
it's possible to allocate shellcode in the address space of the host and
run it fairly easily.

ASLR doesn't mitigate this since we know exactly the stack offsets; however
this patch has the kernel allocate a new stack, which will help. Of course,
the attacker could just check /proc/pid/maps to find the location of the
stack, but they'd still have to guess where to write stuff in.

The thing that does prevent this is the default configuration of apparmor.
Since the apparmor profile is set in the second clone, and apparmor
prevents ptracing things under a different profile, attackers confined by
apparmor can't do this. However, if users are using a custom configuration
with shared namespaces, care must be taken to avoid this race.

Shared namespaces aren't widely used now, so perhaps this isn't a problem,
but with the advent of crio-lxc for k8s, this functionality will be used
more.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>

diff --git a/src/lxc/namespace.c b/src/lxc/namespace.c
index 59fba412dd20f6d40a4338bf074af99788d7f363..bc14fb52c91aaba4c03c6fb4396fca301fcf2a17 100644 (file)
--- a/src/lxc/namespace.c
+++ b/src/lxc/namespace.c
@@ -56,19 +56,23 @@ static int do_clone(void *arg)
 #define __LXC_STACK_SIZE 4096
 pid_t lxc_clone(int (*fn)(void *), void *arg, int flags, int *pidfd)
 {
-       size_t stack_size;
        pid_t ret;
        struct clone_arg clone_arg = {
            .fn = fn,
            .arg = arg,
        };
-       char *stack[__LXC_STACK_SIZE] = {0};
-       stack_size = __LXC_STACK_SIZE;
+       void *stack;
+
+       stack = malloc(__LXC_STACK_SIZE);
+       if (!stack) {
+               SYSERROR("Failed to allocate clone stack");
+               return -ENOMEM;
+       }
 
 #ifdef __ia64__
-       ret = __clone2(do_clone, stack, stack_size, flags | SIGCHLD, &clone_arg, pidfd);
+       ret = __clone2(fn, stack, __LXC_STACK_SIZE, flags | SIGCHLD, &clone_arg, pidfd);
 #else
-       ret = clone(do_clone, stack + stack_size, flags | SIGCHLD, &clone_arg, pidfd);
+       ret = clone(fn, stack + __LXC_STACK_SIZE, flags | SIGCHLD, &clone_arg, pidfd);
 #endif
        if (ret < 0)
                SYSERROR("Failed to clone (%#x)", flags);