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

[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@gmail.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
---
 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

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

[Greg KH]

On Mon, Oct 10, 2022 at 09:57:20PM +0500, Mushahid Hussain wrote:
> 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@gmail.com>
> Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> ---
>  drivers/accessibility/speakup/main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

What commit id does this fix?  Should it go to stable kernels?

thanks,

greg k-h

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

[Samuel Thibault]

Hello,

Greg KH, le lun. 10 oct. 2022 19:02:46 +0200, a ecrit:
> On Mon, Oct 10, 2022 at 09:57:20PM +0500, Mushahid Hussain wrote:
> > 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@gmail.com>
> > Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> > ---
> >  drivers/accessibility/speakup/main.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> What commit id does this fix?

It is there since 2610df41489f548e235171b86895d4b49e6acb1f
("staging: speakup: Add pause command used on switching to graphical
mode")

> Should it go to stable kernels?

Yes, please.

Mushahid, you can see in Documentation/process/submitting-patches.rst
how to encode this in the patch submission.

Samuel