vgacon, x86/boot: Prevent faulty bootparams.screeninfo from causing harm

If a zero for the number of colums or rows manages to slip through,
gotoxy() will underflow vc->vc_pos, causing the next action on the
referenced memory to end with a page fault.

Make the check in vgacon more pessimistic to prevent that. Also change
a similar check in the x86 boot code with a similar problem.

Signed-off-by: Jan H. Schönherr <redacted>
---
Let me know, if I should split this into two patches.

 arch/x86/boot/compressed/misc.c | 3 +--
 drivers/video/console/vgacon.c  | 5 ++---
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index a0838ab..c14217c 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c

@@ -116,8 +116,7 @@ void __putstr(const char *s)
 		}
 	}
 
-	if (boot_params->screen_info.orig_video_mode = 0 &&
-	    lines = 0 && cols = 0)
+	if (lines = 0 || cols = 0)
 		return;
 
 	x = boot_params->screen_info.orig_x;

diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index dc06cb6..445b1dc 100644
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c

@@ -398,9 +398,8 @@ static const char *vgacon_startup(void)
 #endif
 	}
 
-	/* boot_params.screen_info initialized? */
-	if ((screen_info.orig_video_mode  = 0) &&
-	    (screen_info.orig_video_lines = 0) &&
+	/* boot_params.screen_info reasonably initialized? */
+	if ((screen_info.orig_video_lines = 0) ||
 	    (screen_info.orig_video_cols  = 0))
 		goto no_vga;
 

-- 
2.3.1.dirty

On Sun, Aug 27, 2017 at 03:56:46PM +0200, Jan H. Schönherr wrote:

If a zero for the number of colums or rows manages to slip through,
gotoxy() will underflow vc->vc_pos, causing the next action on the
referenced memory to end with a page fault.

Make the check in vgacon more pessimistic to prevent that. Also change
a similar check in the x86 boot code with a similar problem.

Signed-off-by: Jan H. Schönherr <redacted>
---
Let me know, if I should split this into two patches.

 arch/x86/boot/compressed/misc.c | 3 +--
 drivers/video/console/vgacon.c  | 5 ++---
 2 files changed, 3 insertions(+), 5 deletions(-)

Why send this patch to me?

And yes, of course you have to split this up, these two files have
nothing to do with each other...

greg k-h

On 08/27/2017 05:05 PM, Greg Kroah-Hartman wrote:

On Sun, Aug 27, 2017 at 03:56:46PM +0200, Jan H. Schönherr wrote:

quoted

If a zero for the number of colums or rows manages to slip through,
gotoxy() will underflow vc->vc_pos, causing the next action on the
referenced memory to end with a page fault.

Make the check in vgacon more pessimistic to prevent that. Also change
a similar check in the x86 boot code with a similar problem.

Signed-off-by: Jan H. Schönherr <redacted>
---
Let me know, if I should split this into two patches.

 arch/x86/boot/compressed/misc.c | 3 +--
 drivers/video/console/vgacon.c  | 5 ++---
 2 files changed, 3 insertions(+), 5 deletions(-)

Why send this patch to me?

Because there isn't a clear maintainer: The drivers/video catch-all in MAINTAINERS
is captioned as "Framebuffer layer",  but all recent patches to vgacon.c went through
your hands.

And yes, of course you have to split this up, these two files have
nothing to do with each other...

Okay, then ignore this patch, and I'll split this down. I'll send the vgacon.c part
through Bartlomiej Zolnierkiewicz, then.

Regards
Jan