Re: KASAN: use-after-free Read in bit_putcs

Tetsuo Handa
2020-09-27T00:25:34Z

On 2020/09/27 4:39, Peilin Ye wrote:
> On Sun, Sep 27, 2020 at 01:25:17AM +0900, Tetsuo Handa wrote:
>> Since I don't know the meaning of "struct vt_consize"->v_clin (which is commented
>> with "/* number of pixel rows per character */" but does it mean font size ?),
>> I don't know why we can assign that value to vcp->vc_font.height via
>>
>> if (v.v_clin)
>> vcp->vc_font.height = v.v_clin;
>>
>> in vt_resizex(). While ioctl(PIO_FONT) needs to pass vc->vc_sw->con_font_set()
>> check in con_font_set(), ioctl(VT_RESIZEX) does not pass it in vt_resizex()...
>>
>> Since this problem does not happen if I remove
>>
>> if (v.v_clin)
>> vcp->vc_font.height = v.v_clin;
>
> Hi Tetsuo!
>
>> from vt_resizex(), I guess that some variables are getting confused by change
>> of vc->vc_font.height ...
>
> Yes, see bit_putcs():
>
> (drivers/video/fbdev/core/bitblit.c)
> static void bit_putcs(struct vc_data *vc, struct fb_info *info,
> const unsigned short *s, int count, int yy, int xx,
> int fg, int bg)
> {
> struct fb_image image;
> u32 width = DIV_ROUND_UP(vc->vc_font.width, 8);
> u32 cellsize = width * vc->vc_font.height;
> ^^^^^^^^ ^^^^^^^^^^^^^^
>
> `cellsize` is now too large. Later, in bit_putcs_aligned():
>
> while (cnt--) {
> src = vc->vc_font.data + (scr_readw(s++)&
> charmask)*cellsize;
> ^^^^^^^^
>
> `src` goes out of bounds of the data buffer. At first glance I guess
> this is an out-of-bound read reported as a use-after-free read? The
> crashlog says:

How this OOB access is reported varies.

>
> To resolve this out-of-bound issue for now, I think the easiest way
> is to add a range check in bit_putcs(), or bit_putcs_aligned().
>
> ...but yeah, that `VT_RESIZEX` ioctl looks really buggy, and is already
> causing more issues:

At least, since not all fonts have height == 32 (e.g. font_vga_8x8 is height == 8),
allow changing vc->vc_font.height with only

if (v.v_clin > 32)
return -EINVAL;

validation in VT_RESIZEX looks wrong. This needs more validations.

By the way, can we find a user of VT_RESIZEX? As far as I googled with "VT_RESIZEX",
I couldn't find a userspace program; only explanation of VT_RESIZEX and kernel patches
related to VT_RESIZEX are found. Also, while console_ioctl(4) man page says

Any parameter may be set to zero, indicating "no change"

, the assignment

if (!v.v_vlin)
v.v_vlin = vc->vc_scan_lines;

changes the meaning to

If v_vlin parameter is set to zero, the value for associated console is copied
to each console (instead of preserving current value for that console)

. Maybe for now we can try this (effectively making VT_RESIZEX == VT_RESIZE) ?

vt_ioctl.c | 57 ++++++++++-----------------------------------------------
1 file changed, 10 insertions(+), 47 deletions(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index a4e520bdd521..bc33938e2f20 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -773,58 +773,21 @@ static int vt_resizex(struct vc_data *vc, struct vt_consize __user *cs)
if (copy_from_user(&v, cs, sizeof(struct vt_consize)))
return -EFAULT;

- /* FIXME: Should check the copies properly */
- if (!v.v_vlin)
- v.v_vlin = vc->vc_scan_lines;
-
- if (v.v_clin) {
- int rows = v.v_vlin / v.v_clin;
- if (v.v_rows != rows) {
- if (v.v_rows) /* Parameters don't add up */
- return -EINVAL;
- v.v_rows = rows;
- }
- }
-
- if (v.v_vcol && v.v_ccol) {
- int cols = v.v_vcol / v.v_ccol;
- if (v.v_cols != cols) {
- if (v.v_cols)
- return -EINVAL;
- v.v_cols = cols;
- }
- }
-
- if (v.v_clin > 32)
- return -EINVAL;
+ if (v.v_vlin)
+ pr_info_once("\"struct vt_consize\"->v_vlin is ignored. Please report if you need this.\n");
+ if (v.v_clin)
+ pr_info_once("\"struct vt_consize\"->v_clin is ignored. Please report if you need this.\n");

+ console_lock();
for (i = 0; i < MAX_NR_CONSOLES; i++) {
- struct vc_data *vcp;
+ vc = vc_cons[i].d;

- if (!vc_cons[i].d)
- continue;
- console_lock();
- vcp = vc_cons[i].d;
- if (vcp) {
- int ret;
- int save_scan_lines = vcp->vc_scan_lines;
- int save_font_height = vcp->vc_font.height;
-
- if (v.v_vlin)
- vcp->vc_scan_lines = v.v_vlin;
- if (v.v_clin)
- vcp->vc_font.height = v.v_clin;
- vcp->vc_resize_user = 1;
- ret = vc_resize(vcp, v.v_cols, v.v_rows);
- if (ret) {
- vcp->vc_scan_lines = save_scan_lines;
- vcp->vc_font.height = save_font_height;
- console_unlock();
- return ret;
- }
+ if (vc) {
+ vc->vc_resize_user = 1;
+ vc_resize(vc, v.v_cols, v.v_rows);
}
- console_unlock();
}
+ console_unlock();

return 0;
}