[PATCH] kernel/fork: validate exit_signal in clone() syscall

[Deepanshu Kartikey <kartikey406@gmail.com>]

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), causing
a WARN_ON in do_notify_parent() when the process exits:

  WARNING: kernel/signal.c:2174 do_notify_parent+0xc7e/0xd70

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 comment above kernel_clone() states that callers are expected to
validate exit_signal. clone3() correctly does this:

  if (unlikely((args.exit_signal & ~((u64)CSIGNAL)) ||
               !valid_signal(args.exit_signal)))
          return -EINVAL;

The clone() syscall has no such check. Add the missing valid_signal()
check to clone(), consistent with the existing validation in clone3().

Fixes: 3f2c788a1314 ("fork: prevent accidental access to clone3 features")
Reported-by: syzbot+bbe6b99feefc3a0842de@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bbe6b99feefc3a0842de
Tested-by: syzbot+bbe6b99feefc3a0842de@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
---
 kernel/fork.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 947a8dbce06a..dbe26ac6ca10 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2845,7 +2845,8 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
 		.stack		= newsp,
 		.tls		= tls,
 	};
-
+	if (!valid_signal(args.exit_signal))
+		return -EINVAL;
 	return kernel_clone(&args);
 }
 #endif
-- 
2.43.0