fbcon: Fix boundary checks for fbcon=vc:n1-n2 parameters

The user may use the fbcon=vc:<n1>-<n2> option to tell fbcon to take
over the given range (n1...n2) of consoles. The value for n1 and n2
needs to be a positive number and up to (MAX_NR_CONSOLES - 1).
The given values were not fully checked against those boundaries yet.

To fix the issue, convert first_fb_vc and last_fb_vc to unsigned
integers and check them against the upper boundary, and make sure that
first_fb_vc is smaller than last_fb_vc.

Cc: stable@vger.kernel.org # v4.19+
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/YpkYRMojilrtZIgM@p100

drivers/video/fbdev/core/fbcon.c

@@ -125,8 +125,8 @@ static int logo_lines;
    enums.  */
 static int logo_shown = FBCON_LOGO_CANSHOW;
 /* console mappings */
-static int first_fb_vc;
-static int last_fb_vc = MAX_NR_CONSOLES - 1;
+static unsigned int first_fb_vc;
+static unsigned int last_fb_vc = MAX_NR_CONSOLES - 1;
 static int fbcon_is_default = 1; 
 static int primary_device = -1;
 static int fbcon_has_console_bind;
@@ -440,10 +440,12 @@ static int __init fb_console_setup(char *this_opt)
 			options += 3;
 			if (*options)
 				first_fb_vc = simple_strtoul(options, &options, 10) - 1;
-			if (first_fb_vc < 0)
+			if (first_fb_vc >= MAX_NR_CONSOLES)
 				first_fb_vc = 0;
 			if (*options++ == '-')
 				last_fb_vc = simple_strtoul(options, &options, 10) - 1;
+			if (last_fb_vc < first_fb_vc || last_fb_vc >= MAX_NR_CONSOLES)
+				last_fb_vc = MAX_NR_CONSOLES - 1;
 			fbcon_is_default = 0; 
 			continue;
 		}