kernel/fork: validate exit_signal in kernel_clone()

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 <Kartikey406@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
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/
Link: https://lore.kernel.org/all/20260316104536.558108-1-kartikey406@gmail.com/T/
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Ben Segall <bsegall@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>