fbdev: Fix do_register_framebuffer to prevent null-ptr-deref in fb_videomode_to_var

If fb_add_videomode() in do_register_framebuffer() fails to allocate
memory for fb_videomode, it will later lead to a null-ptr dereference in
fb_videomode_to_var(), as the fb_info is registered while not having the
mode in modelist that is expected to be there, i.e. the one that is
described in fb_info->var.

================================================================
general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 1 PID: 30371 Comm: syz-executor.1 Not tainted 5.10.226-syzkaller #0
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
RIP: 0010:fb_videomode_to_var+0x24/0x610 drivers/video/fbdev/core/modedb.c:901
Call Trace:
 display_to_var+0x3a/0x7c0 drivers/video/fbdev/core/fbcon.c:929
 fbcon_resize+0x3e2/0x8f0 drivers/video/fbdev/core/fbcon.c:2071
 resize_screen drivers/tty/vt/vt.c:1176 [inline]
 vc_do_resize+0x53a/0x1170 drivers/tty/vt/vt.c:1263
 fbcon_modechanged+0x3ac/0x6e0 drivers/video/fbdev/core/fbcon.c:2720
 fbcon_update_vcs+0x43/0x60 drivers/video/fbdev/core/fbcon.c:2776
 do_fb_ioctl+0x6d2/0x740 drivers/video/fbdev/core/fbmem.c:1128
 fb_ioctl+0xe7/0x150 drivers/video/fbdev/core/fbmem.c:1203
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl fs/ioctl.c:739 [inline]
 __x64_sys_ioctl+0x19a/0x210 fs/ioctl.c:739
 do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x67/0xd1
================================================================

Even though fbcon_init() checks beforehand if fb_match_mode() in
var_to_display() fails, it can not prevent the panic because fbcon_init()
does not return error code. Considering this and the comment in the code
about fb_match_mode() returning NULL - "This should not happen" - it is
better to prevent registering the fb_info if its mode was not set
successfully. Also move fb_add_videomode() closer to the beginning of
do_register_framebuffer() to avoid having to do the cleanup on fail.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: [email protected]
Signed-off-by: Murad Masimov <[email protected]>
Signed-off-by: Helge Deller <[email protected]>

Author: Murad Masimov

drivers/video/fbdev/core/fbmem.c

@@ -388,7 +388,7 @@ static int fb_check_foreignness(struct fb_info *fi)
 
 static int do_register_framebuffer(struct fb_info *fb_info)
 {
-	int i;
+	int i, err = 0;
 	struct fb_videomode mode;
 
 	if (fb_check_foreignness(fb_info))
@@ -397,10 +397,18 @@ static int do_register_framebuffer(struct fb_info *fb_info)
 	if (num_registered_fb == FB_MAX)
 		return -ENXIO;
 
-	num_registered_fb++;
 	for (i = 0 ; i < FB_MAX; i++)
 		if (!registered_fb[i])
 			break;
+
+	if (!fb_info->modelist.prev || !fb_info->modelist.next)
+		INIT_LIST_HEAD(&fb_info->modelist);
+
+	fb_var_to_videomode(&mode, &fb_info->var);
+	err = fb_add_videomode(&mode, &fb_info->modelist);
+	if (err < 0)
+		return err;
+
 	fb_info->node = i;
 	refcount_set(&fb_info->count, 1);
 	mutex_init(&fb_info->lock);
@@ -426,16 +434,12 @@ static int do_register_framebuffer(struct fb_info *fb_info)
 	if (bitmap_empty(fb_info->pixmap.blit_y, FB_MAX_BLIT_HEIGHT))
 		bitmap_fill(fb_info->pixmap.blit_y, FB_MAX_BLIT_HEIGHT);
 
-	if (!fb_info->modelist.prev || !fb_info->modelist.next)
-		INIT_LIST_HEAD(&fb_info->modelist);
-
 	if (fb_info->skip_vt_switch)
 		pm_vt_switch_required(fb_info->device, false);
 	else
 		pm_vt_switch_required(fb_info->device, true);
 
-	fb_var_to_videomode(&mode, &fb_info->var);
-	fb_add_videomode(&mode, &fb_info->modelist);
+	num_registered_fb++;
 	registered_fb[i] = fb_info;
 
 #ifdef CONFIG_GUMSTIX_AM200EPD