[PATCH v2] fork: check exit_signal passed in clone3() call

[Eugene Syromiatnikov]

Previously, higher 32 bits of exit_signal fields were lost when
copied to the kernel args structure (that uses int as a type for the
respective field).  Moreover, as Oleg has noted[1], exit_signal is used
unchecked, so it has to be checked for sanity before use; for the legacy
syscalls, applying CSIGNAL mask guarantees that it is at least non-negative;
however, there's no such thing is done in clone3() code path, and that can
break at least thread_group_leader.

Checking user-passed exit_signal against ~CSIGNAL mask solves both
of these problems.

[1] https://lkml.org/lkml/2019/9/10/467

* kernel/fork.c (copy_clone_args_from_user): Fail with -EINVAL if
args.exit_signal has bits set outside CSIGNAL mask.
(_do_fork): Note that exit_signal is expected to be checked for the
sanity by the caller.

Fixes: 7f192e3cd316 ("fork: add clone3")
Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Eugene Syromiatnikov <esyr@xxxxxxxxxx>
---
 kernel/fork.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index 2852d0e..9dee2ab 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2338,6 +2338,8 @@ struct mm_struct *copy_init_mm(void)
  *
  * It copies the process, and if successful kick-starts
  * it and waits for it to finish using the VM if required.
+ *
+ * args->exit_signal is expected to be checked for sanity by the caller.
  */
 long _do_fork(struct kernel_clone_args *args)
 {
@@ -2562,6 +2564,16 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 	if (copy_from_user(&args, uargs, size))
 		return -EFAULT;
 
+	/*
+	 * exit_signal is confined to CSIGNAL mask in legacy syscalls,
+	 * so it is used unchecked deeper in syscall handling routines;
+	 * moreover, copying to struct kernel_clone_args.exit_signals
+	 * trims higher 32 bits, so it is has to be checked that they
+	 * are zero.
+	 */
+	if (unlikely(args.exit_signal & ~((u64)CSIGNAL)))
+		return -EINVAL;
+
 	*kargs = (struct kernel_clone_args){
 		.flags		= args.flags,
 		.pidfd		= u64_to_user_ptr(args.pidfd),
-- 
2.1.4