intel_th: msu: Fix window switching without windows

Commit 6cac7866 ("intel_th: msu: Add a sysfs attribute to trigger
window switch") adds a NULL pointer dereference in the case when there are
no windows allocated:

> BUG: kernel NULL pointer dereference, address: 0000000000000000
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: 0000 1 SMP
> CPU: 5 PID: 1110 Comm: bash Not tainted 5.5.0-rc1+ #1
> RIP: 0010:msc_win_switch+0xa/0x80 [intel_th_msu]
> Call Trace:
> ? win_switch_store+0x9b/0xc0 [intel_th_msu]
> dev_attr_store+0x17/0x30
> sysfs_kf_write+0x3e/0x50
> kernfs_fop_write+0xda/0x1b0
> __vfs_write+0x1b/0x40
> vfs_write+0xb9/0x1a0
> ksys_write+0x67/0xe0
> __x64_sys_write+0x1a/0x20
> do_syscall_64+0x57/0x1d0
> entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fix that by disallowing window switching with multiwindow buffers without
windows.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: 6cac7866 ("intel_th: msu: Add a sysfs attribute to trigger window switch")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reported-by: Ammy Yi <ammy.yi@intel.com>
Tested-by: Ammy Yi <ammy.yi@intel.com>
Cc: stable@vger.kernel.org # v5.2+
Link: https://lore.kernel.org/r/20191217115527.74383-5-alexander.shishkin@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/intel_th/msu.c

@@ -1676,10 +1676,13 @@ static int intel_th_msc_init(struct msc *msc)
 	return 0;
 }
 
-static void msc_win_switch(struct msc *msc)
+static int msc_win_switch(struct msc *msc)
 {
 	struct msc_window *first;
 
+	if (list_empty(&msc->win_list))
+		return -EINVAL;
+
 	first = list_first_entry(&msc->win_list, struct msc_window, entry);
 
 	if (msc_is_last_win(msc->cur_win))
@@ -1691,6 +1694,8 @@ static void msc_win_switch(struct msc *msc)
 	msc->base_addr = msc_win_base_dma(msc->cur_win);
 
 	intel_th_trace_switch(msc->thdev);
+
+	return 0;
 }
 
 /**
@@ -2025,16 +2030,15 @@ win_switch_store(struct device *dev, struct device_attribute *attr,
 	if (val != 1)
 		return -EINVAL;
 
+	ret = -EINVAL;
 	mutex_lock(&msc->buf_mutex);
 	/*
 	 * Window switch can only happen in the "multi" mode.
 	 * If a external buffer is engaged, they have the full
 	 * control over window switching.
 	 */
-	if (msc->mode != MSC_MODE_MULTI || msc->mbuf)
-		ret = -ENOTSUPP;
-	else
-		msc_win_switch(msc);
+	if (msc->mode == MSC_MODE_MULTI && !msc->mbuf)
+		ret = msc_win_switch(msc);
 	mutex_unlock(&msc->buf_mutex);
 
 	return ret ? ret : size;