[PATCH] Speakup: fix a segfault caused by switching consoles

[Mushahid Hussain]

This patch fixes a segfault by adding a null check on synth in
speakup_con_update(). The segfault can be reproduced as follows:

	- Login into a text console

	- Load speakup and speakup_soft modules

	- Remove speakup_soft

	- Switch to a graphics console

This is caused by lack of a null check on `synth` in
speakup_con_update().

Here's the sequence that causes the segfault:

	- When we remove the speakup_soft, synth_release() sets the synth
	  to null.

	- After that, when we change the virtual console to graphics
	  console, vt_notifier_call() is fired, which then calls
	  speakup_con_update().

	- Inside speakup_con_update() there's no null check on synth,
	  so it calls synth_printf().

	- Inside synth_printf(), synth_buffer_add() and synth_start(),
	  both access synth, when it is null and causing a segfault.

Therefore adding a null check on synth solves the issue.

Signed-off-by: Mushahid Hussain <mushi.shar@xxxxxxxxx>
Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>
---
 drivers/accessibility/speakup/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/accessibility/speakup/main.c b/drivers/accessibility/speakup/main.c
index f52265293482..73db0cb44fc7 100644
--- a/drivers/accessibility/speakup/main.c
+++ b/drivers/accessibility/speakup/main.c
@@ -1778,7 +1778,7 @@ static void speakup_con_update(struct vc_data *vc)
 {
 	unsigned long flags;
 
-	if (!speakup_console[vc->vc_num] || spk_parked)
+	if (!speakup_console[vc->vc_num] || spk_parked || !synth)
 		return;
 	if (!spin_trylock_irqsave(&speakup_info.spinlock, flags))
 		/* Speakup output, discard */
-- 
2.38.0