[PATCH 3/5] um: Do a double clone to disable rseq

benjamin
2024-05-28T08:54:43Z

From: Benjamin Berg <benjamin.berg@intel.com>

Newer glibc versions are enabling rseq support by default. This remains
enabled in the cloned child process, potentially causing the host kernel
to write/read memory in the child.

It appears that this was purely not an issue because the used memory
area happened to be above TASK_SIZE and remains mapped.

Note that a better approach would be to exec a small static binary that
does not link with other libraries. Using a memfd and execveat the
binary could be embedded into UML itself and it would result in an
entirely clean execution environment for userspace.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
---
arch/um/os-Linux/skas/process.c | 54 ++++++++++++++++++++++++++++++---
1 file changed, 50 insertions(+), 4 deletions(-)

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 41a288dcfc34..ee332a2aeea6 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -255,6 +255,31 @@ static int userspace_tramp(void *stack)
int userspace_pid[NR_CPUS];
int kill_userspace_mm[NR_CPUS];

+struct tramp_data {
+ int pid;
+ void *clone_sp;
+ void *stack;
+};
+
+static int userspace_tramp_clone_vm(void *data)
+{
+ struct tramp_data *tramp_data = data;
+
+ /*
+ * This helper exist to do a double-clone. First with CLONE_VM which
+ * effectively disables things like rseq, and then the second one to
+ * get a new memory space.
+ */
+
+ tramp_data->pid = clone(userspace_tramp, tramp_data->clone_sp,
+ CLONE_PARENT | CLONE_FILES | SIGCHLD,
+ tramp_data->stack);
+ if (tramp_data->pid < 0)
+ tramp_data->pid = -errno;
+
+ exit(0);
+}
+
/**
* start_userspace() - prepare a new userspace process
* @stub_stack: pointer to the stub stack.
@@ -268,9 +293,10 @@ int kill_userspace_mm[NR_CPUS];
*/
int start_userspace(unsigned long stub_stack)
{
+ struct tramp_data tramp_data;
void *stack;
unsigned long sp;
- int pid, status, n, flags, err;
+ int pid, status, n, err;

/* setup a temporary stack page */
stack = mmap(NULL, UM_KERN_PAGE_SIZE,
@@ -286,10 +312,13 @@ int start_userspace(unsigned long stub_stack)
/* set stack pointer to the end of the stack page, so it can grow downwards */
sp = (unsigned long)stack + UM_KERN_PAGE_SIZE;

- flags = CLONE_FILES | SIGCHLD;
+ tramp_data.stack = (void *) stub_stack;
+ tramp_data.clone_sp = (void *) sp;
+ tramp_data.pid = -EINVAL;

/* clone into new userspace process */
- pid = clone(userspace_tramp, (void *) sp, flags, (void *) stub_stack);
+ pid = clone(userspace_tramp_clone_vm, (void *) sp,
+ CLONE_VM | CLONE_FILES | SIGCHLD, &tramp_data);
if (pid < 0) {
err = -errno;
printk(UM_KERN_ERR "%s : clone failed, errno = %d\n",
@@ -305,7 +334,24 @@ int start_userspace(unsigned long stub_stack)
__func__, errno);
goto out_kill;
}
- } while (WIFSTOPPED(status) && (WSTOPSIG(status) == SIGALRM));
+ } while (!WIFEXITED(status));
+
+ pid = tramp_data.pid;
+ if (pid < 0) {
+ printk(UM_KERN_ERR "%s : second clone failed, errno = %d\n",
+ __func__, -pid);
+ return pid;
+ }
+
+ do {
+ CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED | __WALL));
+ if (n < 0) {
+ err = -errno;
+ printk(UM_KERN_ERR "%s : wait failed, errno = %d\n",
+ __func__, errno);
+ goto out_kill;
+ }
+ } while (WIFEXITED(status) && (WSTOPSIG(status) == SIGALRM));

if (!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGSTOP)) {
err = -EINVAL;
--
2.45.1