From 09e7827e785729f391c8d46dc71becce70d296ab Mon Sep 17 00:00:00 2001 From: Deepanshu Kartikey Date: Mon, 16 Mar 2026 20:49:56 +0530 Subject: kernel/fork: validate exit_signal in kernel_clone() When a child process exits, it sends exit_signal to its parent via do_notify_parent(). The clone() syscall constructs exit_signal as: (lower_32_bits(clone_flags) & CSIGNAL) CSIGNAL is 0xff, so values in the range 65-255 are possible. However, valid_signal() only accepts signals up to _NSIG (64 on x86_64). A non-zero non-valid exit_signal acts the same as exit_signal == 0: the parent process is not signaled when the child terminates. The syzkaller reproducer triggers this by calling clone() with flags=0x80, resulting in exit_signal = (0x80 & CSIGNAL) = 128, which exceeds _NSIG and is not a valid signal. The v1 of this patch added the check only in the clone() syscall handler, which is incomplete. kernel_clone() has other callers such as sys_ia32_clone() which would remain unprotected. Move the check to kernel_clone() to cover all callers. Since the valid_signal() check is now in kernel_clone() and covers all callers including clone3(), the same check in copy_clone_args_from_user() becomes redundant and is removed. The higher 32bits check for clone3() is kept as it is clone3() specific. Note that this is a user-visible change: previously, passing an invalid exit_signal to clone() was silently accepted. The man page for clone() does not document any defined behavior for invalid exit_signal values, so rejecting them with -EINVAL is the correct behavior. It is unlikely that any sane application relies on passing an invalid exit_signal. [oleg@redhat.com: the comment above kernel_clone() should be updated] Link: https://lore.kernel.org/abwvgU17W8wuW2-J@redhat.com Link: https://lore.kernel.org/20260316151956.563558-1-kartikey406@gmail.com Fixes: 3f2c788a1314 ("fork: prevent accidental access to clone3 features") Signed-off-by: Deepanshu Kartikey Signed-off-by: Oleg Nesterov Reported-by: syzbot+bbe6b99feefc3a0842de@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=bbe6b99feefc3a0842de Tested-by: syzbot+bbe6b99feefc3a0842de@syzkaller.appspotmail.com Link: https://lore.kernel.org/all/20260307064202.353405-1-kartikey406@gmail.com/T/ [v1] Link: https://lore.kernel.org/all/20260316104536.558108-1-kartikey406@gmail.com/T/ [v2] Acked-by: Oleg Nesterov Acked-by: Michal Hocko Cc: Ben Segall Cc: Christian Brauner Cc: David Hildenbrand Cc: Dietmar Eggemann Cc: Ingo Molnar Cc: Juri Lelli Cc: Kees Cook Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Mel Gorman Cc: Mike Rapoport Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Suren Baghdasaryan Cc: Valentin Schneider Cc: Vincent Guittot Cc: Vlastimil Babka Cc: Tetsuo Handa Signed-off-by: Andrew Morton --- kernel/fork.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) (limited to 'kernel') diff --git a/kernel/fork.c b/kernel/fork.c index 5f3fdfdb14c7..8ac38beae360 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2664,8 +2664,6 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node) * * 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. */ pid_t kernel_clone(struct kernel_clone_args *args) { @@ -2700,6 +2698,9 @@ pid_t kernel_clone(struct kernel_clone_args *args) (args->pidfd == args->parent_tid)) return -EINVAL; + if (!valid_signal(args->exit_signal)) + return -EINVAL; + /* * Determine whether and which event to report to ptracer. When * called from kernel_thread or CLONE_UNTRACED is explicitly @@ -2898,11 +2899,9 @@ static noinline int copy_clone_args_from_user(struct kernel_clone_args *kargs, return -EINVAL; /* - * Verify that higher 32bits of exit_signal are unset and that - * it is a valid signal + * Verify that higher 32bits of exit_signal are unset */ - if (unlikely((args.exit_signal & ~((u64)CSIGNAL)) || - !valid_signal(args.exit_signal))) + if (unlikely(args.exit_signal & ~((u64)CSIGNAL))) return -EINVAL; if ((args.flags & CLONE_INTO_CGROUP) && -- cgit v1.2.3