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

On Sun, Sep 27, 2020 at 05:28:12PM +0900, Tetsuo Handa wrote:

Well, vt_io_ioctl(PIO_FONT) initializes "struct console_font_op op;" with

  op.width = 8;
  op.height = 0;
  op.charcount = 256;

and calls con_font_set() from con_font_op(). But the "/* Need to guess font height [compat] */"
chunk in con_font_set() guesses font's height due to being initialized with op.height = 0.
Then, con_font_set() calls fbcon_set_font() via vc->vc_sw->con_font_set(), and fbcon_set_font()
allocates minimal amount of memory for font data based on font's height calcllated by con_font_set().

Therefore, any attempt to change font's height (like vt_resizex()) larger than font's height
calculated by con_font_set() can cause OOB read of memory block for font data. If we allocate
maximal amount of memory for any font, OOB read of memory block for font data should not happen.

----------------------------------------

static char fontdata[8192] = { 2 };

[  227.065369] bit_putcs: width=1 cellsize=1 count€ maxcnt�92 scan_align=0 buf_align=0 image.height=1
[  227.066254] bit_putcs: width=1 cellsize=1 count€ maxcnt�92 scan_align=0 buf_align=0 image.height=1
[  227.067642] vcÿff8880d69b4000 v.v_rows=0 v.v_cols=0 v.v_vlin=1 v.v_clin=9 v.v_vcol=0 v.v_ccol=0 ret=0
[  227.067699] vcpÿff8880d69b4000 before: ->vc_rowsH0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines@0 ->vc_font.height=9 save_font_height=1
[  227.067774] vcpÿff8880d69b4000 after: ->vc_rowsH0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines@0 ->vc_font.height=9 save_font_height=1 ret=0
[  227.067831] vcpÿff8880cac4b000 before: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height
[  227.067891] vcpÿff8880cac4b000 after: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height ret=0
[  227.067947] vcpÿff8880c6180000 before: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height
[  227.068007] vcpÿff8880c6180000 after: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height ret=0
[  227.068063] vcpÿff8880d6b84000 before: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height
[  227.068123] vcpÿff8880d6b84000 after: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height ret=0
[  227.068179] vcpÿff8880ca8c0000 before: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height
[  227.068255] vcpÿff8880ca8c0000 after: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height ret=0
[  227.068455] vcpÿff8880cbd5d000 before: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height
[  227.068515] vcpÿff8880cbd5d000 after: ->vc_rows0 ->vc_cols€ ->vc_scan_lines=1 save_scan_lines=0 ->vc_font.height=9 save_font_height ret=0
[  227.084709] =================================
[  227.084729] BUG: KASAN: slab-out-of-bounds in soft_cursor+0x34e/0x4a0
[  227.084748] Read of size 9 at addr ffff8880c98d5930 by task a.out/1662

Very interesting, I remember seeing this on the syzbot dashboard...

Yes, I guess it is this one:
	KASAN: slab-out-of-bounds Read in soft_cursor
	https://syzkaller.appspot.com/bug?idk8355d27b2b94fb5cedf4655e3a59162d9e48e3

There is a `0x560aul` ioctl() in the reproducer, which is `VT_RESIZEX`.

Thank you,
Peilin Ye