Re: [syzbot] BUG: unable to handle kernel paging request in vga16fb_imageblit (2)

On 2021/05/03 22:41, Tetsuo Handa wrote:

Therefore, I guess that the problem is that fbcon_putcs() from do_update_region() from
redraw_screen() from vt_kdsetmode(KD_TEXT) from ioctl(fd, KDSETMODE, KD_TEXT) tries to
redraw 2 x 16640 despite memory amount allocated for actual screen is only 80 x 30.

I don't know how to fix this problem...

Daniel Vetter suggested me that parameter validation is missing/wrong somewhere, for
resize requests that don't fit should be rejected. Thus, I'm thinking how to add
parameter validation.

Like a diff shown bottom, adding a hook for validating whether rows / columns are
small enough (VGA_FB_PHYS_LEN bytes starting from VGA_FB_PHYS ?) survives the

----------
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/kd.h>
#include <linux/vt.h>

int main(int argc, char *argv[])
{
        const int fd = open("/dev/char/4:1", O_RDWR);
        struct vt_sizes vt = { 0x4100, 2 };

        ioctl(fd, KDSETMODE, KD_GRAPHICS);
        ioctl(fd, VT_RESIZE, &vt);
        ioctl(fd, KDSETMODE, KD_TEXT);
        return 0;
}
----------

reproducer. But I don't know how to calculate upper boundary values for vga16fb_tty_resize(),
for I even don't know where these values are derived from...

Also, currently resize_screen() calls vc->vc_sw->con_resize() only if vc->vc_mode != KD_GRAPHICS.
But we need to unconditionally call vga16fb_tty_resize() in order to survive the reproducer; we
need to prevent vc->vc_{rows,cols} from exceeding the upper boundary values regardless of current
vc->vc_mode setting. The "vc->vc_mode != KD_GRAPHICS" check predates the git history, and I don't
know the side effect of removing this check...

----------
e400b6ec4ede4 drivers/char/vt.c   (Antonino A. Daplas   2007-10-16 01:29:35 -0700 1168) static inline int resize_screen(struct vc_data *vc, int width, int height,
e400b6ec4ede4 drivers/char/vt.c   (Antonino A. Daplas   2007-10-16 01:29:35 -0700 1169)                                 int user)
^1da177e4c3f4 drivers/char/vt.c   (Linus Torvalds       2005-04-16 15:20:36 -0700 1170) {
^1da177e4c3f4 drivers/char/vt.c   (Linus Torvalds       2005-04-16 15:20:36 -0700 1171)         /* Resizes the resolution of the display adapater */
^1da177e4c3f4 drivers/char/vt.c   (Linus Torvalds       2005-04-16 15:20:36 -0700 1172)         int err = 0;
^1da177e4c3f4 drivers/char/vt.c   (Linus Torvalds       2005-04-16 15:20:36 -0700 1173)
^1da177e4c3f4 drivers/char/vt.c   (Linus Torvalds       2005-04-16 15:20:36 -0700 1174)         if (vc->vc_mode != KD_GRAPHICS && vc->vc_sw->con_resize)
e400b6ec4ede4 drivers/char/vt.c   (Antonino A. Daplas   2007-10-16 01:29:35 -0700 1175)                 err = vc->vc_sw->con_resize(vc, width, height, user);
e400b6ec4ede4 drivers/char/vt.c   (Antonino A. Daplas   2007-10-16 01:29:35 -0700 1176)
^1da177e4c3f4 drivers/char/vt.c   (Linus Torvalds       2005-04-16 15:20:36 -0700 1177)         return err;
^1da177e4c3f4 drivers/char/vt.c   (Linus Torvalds       2005-04-16 15:20:36 -0700 1178) }
----------

I need more help from those who know this area.

 drivers/tty/vt/vt.c              |    2 +-
 drivers/video/fbdev/core/fbcon.c |    7 +++++++
 drivers/video/fbdev/vga16fb.c    |    8 ++++++++
 include/linux/fb.h               |    3 +++
 4 files changed, 19 insertions(+), 1 deletion(-)

----------

diff --git a/include/linux/fb.h b/include/linux/fb.h
index a8dccd23c249..870384afb5e9 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h

@@ -289,14 +289,17 @@ struct fb_ops {
 
 	/* teardown any resources to do with this framebuffer */
 	void (*fb_destroy)(struct fb_info *info);
 
 	/* called at KDB enter and leave time to prepare the console */
 	int (*fb_debug_enter)(struct fb_info *info);
 	int (*fb_debug_leave)(struct fb_info *info);
+
+	/* Check if resizing TTY to these sizes is safe. */
+	int (*fb_tty_resize)(struct fb_info *info, unsigned int cols, unsigned int rows);
 };
 
 #ifdef CONFIG_FB_TILEBLITTING
 #define FB_TILE_CURSOR_NONE        0
 #define FB_TILE_CURSOR_UNDERLINE   1
 #define FB_TILE_CURSOR_LOWER_THIRD 2
 #define FB_TILE_CURSOR_LOWER_HALF  3

diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
index 1e8a38a7967d..54aa6a99b09c 100644
--- a/drivers/video/fbdev/vga16fb.c
+++ b/drivers/video/fbdev/vga16fb.c

@@ -1266,27 +1266,35 @@ static void vga16fb_destroy(struct fb_info *info)
 {
 	iounmap(info->screen_base);
 	fb_dealloc_cmap(&info->cmap);
 	/* XXX unshare VGA regions */
 	framebuffer_release(info);
 }
 
+static int vga16fb_tty_resize(struct fb_info *info, unsigned int cols, unsigned int rows)
+{
+	if (cols <= 80 && rows <= 30)
+		return 0;
+	return -EINVAL;
+}
+
 static const struct fb_ops vga16fb_ops = {
 	.owner		= THIS_MODULE,
 	.fb_open        = vga16fb_open,
 	.fb_release     = vga16fb_release,
 	.fb_destroy	= vga16fb_destroy,
 	.fb_check_var	= vga16fb_check_var,
 	.fb_set_par	= vga16fb_set_par,
 	.fb_setcolreg 	= vga16fb_setcolreg,
 	.fb_pan_display = vga16fb_pan_display,
 	.fb_blank 	= vga16fb_blank,
 	.fb_fillrect	= vga16fb_fillrect,
 	.fb_copyarea	= vga16fb_copyarea,
 	.fb_imageblit	= vga16fb_imageblit,
+	.fb_tty_resize  = vga16fb_tty_resize,
 };
 
 #ifndef MODULE
 static int __init vga16fb_setup(char *options)
 {
 	char *this_opt;
 	

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 3406067985b1..c0eac87a2a56 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c

@@ -1990,14 +1990,21 @@ static int fbcon_resize(struct vc_data *vc, unsigned int width,
 		if (pitch <= 0)
 			return -EINVAL;
 		size = CALC_FONTSZ(vc->vc_font.height, pitch, vc->vc_font.charcount);
 		if (size > FNTSIZE(vc->vc_font.data))
 			return -EINVAL;
 	}
 
+	if (info->fbops && info->fbops->fb_tty_resize) {
+		int err = info->fbops->fb_tty_resize(info, width, height);
+
+		if (err)
+			return err;
+	}
+
 	virt_w = FBCON_SWAP(ops->rotate, width, height);
 	virt_h = FBCON_SWAP(ops->rotate, height, width);
 	virt_fw = FBCON_SWAP(ops->rotate, vc->vc_font.width,
 				 vc->vc_font.height);
 	virt_fh = FBCON_SWAP(ops->rotate, vc->vc_font.height,
 				 vc->vc_font.width);
 	var.xres = virt_w * virt_fw;

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 01645e87b3d5..fa1548d4f94b 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c

@@ -1167,15 +1167,15 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
 
 static inline int resize_screen(struct vc_data *vc, int width, int height,
 				int user)
 {
 	/* Resizes the resolution of the display adapater */
 	int err = 0;
 
-	if (vc->vc_mode != KD_GRAPHICS && vc->vc_sw->con_resize)
+	if (vc->vc_sw->con_resize)
 		err = vc->vc_sw->con_resize(vc, width, height, user);
 
 	return err;
 }
 
 /**
  *	vc_do_resize	-	resizing method for the tty
----------