Syzkaller found a null pointer dereference in ptp_ioctl
originating from the lack of a null check for tsevq.

general protection fault, probably for non-canonical
	address 0xdffffc000000020b: 0000 [#1] PREEMPT SMP KASAN
KASAN: probably user-memory-access in range
	[0x0000000000001058-0x000000000000105f]
CPU: 0 PID: 5053 Comm: syz-executor353 Not tainted
	6.6.0-syzkaller-10396-g4652b8e4f3ff #0
Hardware name: Google Google Compute Engine/Google Compute Engine,
	BIOS Google 10/09/2023
RIP: 0010:ptp_ioctl+0xcb7/0x1d10 drivers/ptp/ptp_chardev.c:476
...
Call Trace:
  <TASK>
  posix_clock_ioctl+0xf8/0x160 kernel/time/posix-clock.c:86
  vfs_ioctl fs/ioctl.c:51 [inline]
  __do_sys_ioctl fs/ioctl.c:871 [inline]
  __se_sys_ioctl fs/ioctl.c:857 [inline]
  __x64_sys_ioctl+0x18f/0x210 fs/ioctl.c:857
  do_syscall_x64 arch/x86/entry/common.c:51 [inline]
  do_syscall_64+0x3f/0x110 arch/x86/entry/common.c:82
  entry_SYSCALL_64_after_hwframe+0x63/0x6b

This patch fixes the issue by adding a check for tsevq and
ensuring ptp_ioctl returns with an error if tsevq is null.

Reported-by: syzbot+8a78ecea7ac1a2ea26e5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8a78ecea7ac1a2ea26e5
Fixes: c5a445b1e934 ("ptp: support event queue reader channel masks")
Signed-off-by: Yuran Pereira <redacted>
---
  drivers/ptp/ptp_chardev.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c
index 282cd7d24077..5b36c34629a0 100644
--- a/drivers/ptp/ptp_chardev.c
+++ b/drivers/ptp/ptp_chardev.c

@@ -173,6 +173,8 @@ long ptp_ioctl(struct posix_clock_context *pccontext, unsigned int cmd,
  	int enable, err = 0;
  
  	tsevq = pccontext->private_clkdata;
+	if (!tsevq)
+		return -EINVAL;
  
  	switch (cmd) {