On Sun, Sep 27, 2020 at 01:25:17AM +0900, Tetsuo Handa wrote:
A simplified reproducer and debug printk() patch shown below reported that vc_font.height is increased to 9 via ioctl(VT_RESIZEX) after it was once decreased from 16 to 2 via ioctl(PIO_FONT).
Since vc_resize() with v.v_rows == 0 preserves current vc->vc_rows value, this reproducer is bypassing
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; } }
check by setting v.v_vlin == 1 and v.v_clin == 9.
If v.v_vcol > 0 and v.v_vcol != vc->vc_cols (though this reproducer is passing v.v_vcol == 0), tty_do_resize() from vc_do_resize() from vc_resize() can make "struct tty_struct"->winsize.ws_ypixel = 1 despite "struct tty_struct"->winsize.vc->vc_rows = vc->vc_rows (which is usually larger than 1). Does such winsize (a row has 1 / vc->vc_rows pixel) make sense?
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:
[ 149.732103][ T6693] Allocated by task 6667: [ 149.732115][ T6693] kasan_save_stack (mm/kasan/common.c:48) [ 149.732121][ T6693] __kasan_kmalloc.constprop.0 (mm/kasan/common.c:56 mm/kasan/common.c:461) [ 149.732126][ T6693] __kmalloc (mm/slab.c:3656 mm/slab.c:3664) [ 149.732133][ T6693] alloc_pipe_info (fs/pipe.c:810) [ 149.732139][ T6693] create_pipe_files (fs/pipe.c:883 fs/pipe.c:914) [ 149.732145][ T6693] do_pipe2 (fs/pipe.c:965 fs/pipe.c:1012)
I'm not sure, but I don't think a buffer allocated in fs/pipe.c is related here. Maybe they just live near each other on the heap?
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:
KASAN: global-out-of-bounds Read in fbcon_get_font Link: https://syzkaller.appspot.com/bug?id=08b8be45afea11888776f897895aef9ad1c3ecf...
This was also caused by `VT_RESIZEX`...
Thank you, Peilin Ye