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Subject[PATCH] vt_ioctl: make VT_RESIZEX behave like VT_RESIZE
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Date
syzbot is reporting UAF/OOB read at bit_putcs()/soft_cursor() [1][2], for
vt_resizex() from ioctl(VT_RESIZEX) allows setting font height larger than
actual font height calculated by con_font_set() from ioctl(PIO_FONT).
Since fbcon_set_font() from con_font_set() allocates minimal amount of
memory based on actual font height calculated by con_font_set(),
use of vt_resizex() can cause UAF/OOB read for font data.

VT_RESIZEX was introduced in Linux 1.3.3, but it is unclear that what
comes to the "+ more" part, and I couldn't find a user of VT_RESIZEX.

#define VT_RESIZE 0x5609 /* set kernel's idea of screensize */
#define VT_RESIZEX 0x560A /* set kernel's idea of screensize + more */

So far we are not aware of syzbot reports caused by setting non-zero value
to v_vlin parameter. But given that it is possible that nobody is using
VT_RESIZEX, we can try removing support for v_clin and v_vlin parameters.

Therefore, this patch effectively makes VT_RESIZEX behave like VT_RESIZE,
with emitting a message if somebody is still using v_clin and/or v_vlin
parameters.

[1] https://syzkaller.appspot.com/bug?id=32577e96d88447ded2d3b76d71254fb855245837
[2] https://syzkaller.appspot.com/bug?id=6b8355d27b2b94fb5cedf4655e3a59162d9e48e3

Reported-by: syzbot <syzbot+b308f5fd049fbbc6e74f@syzkaller.appspotmail.com>
Reported-by: syzbot <syzbot+16469b5e8e5a72e9131e@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
drivers/tty/vt/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;
}
--
2.25.1
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