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 <o...@redhat.com> Signed-off-by: Eugene Syromiatnikov <e...@redhat.com> --- 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
