Re: [syzbot] KASAN: vmalloc-out-of-bounds Write in imageblit (2)

[Helge Deller]

On 7/31/22 15:55, Khalid Masum wrote:
> On 7/30/22 23:25, Helge Deller wrote:
>> On 7/29/22 08:51, Khalid Masum wrote:
>>> Here is a simplified reproducer for the issue:
>>>
>>> https://gist.githubusercontent.com/Labnann/923d6b9b3a19848fc129637b839b8a55/raw/a68271fcc724569735fe27f80817e561b3ff629a/reproducer.c
>>
>> The reproducer does this:
>> ioctl(3, TIOCLINUX, TIOCL_SETSEL, selection: xs:3  ys:0  xe:0 ye:0 mode:0)  = 0
>> -> sets the text selection area
>> ioctl(4, KDFONTOP)  with op=0 (con_font_set), charcount=512  width=8  height=32, 0x20000000) = 0
>> -> changes the font size.
>>
>> It does not crash with current Linus' head (v5.19-rc8).
>> Kernel v5.16, which was used by this KASAN report, hasn't received backports
>> since months, so I tried stable kernel v5.15.58 instead, and this
>> kernel crashed with the reproducer.
>>
>> The reproducer brings up two issues with current code:
>> 1. The reproducer uses ioctl(TIOCLINUX, TIOCL_SETSEL) and hands over (invalid)
>> zero-values for ys and ye for the starting lines.
>> This is wrong, since the API seems to expect a "1" as the very first line for the selection.
>
> Why do you think that API expects a 1?

because the code in drivers/tty/vt/selection.c indicates this:
See:
static int vc_selection(struct vc_data *vc, struct tiocl_selection *v, struct tty_struct *tty)
{
        int ps, pe;
...
        v->xs = min_t(u16, v->xs - 1, vc->vc_cols - 1);
        v->ys = min_t(u16, v->ys - 1, vc->vc_rows - 1);
        v->xe = min_t(u16, v->xe - 1, vc->vc_cols - 1);
        v->ye = min_t(u16, v->ye - 1, vc->vc_rows - 1);
...
        ps = v->ys * vc->vc_size_row + (v->xs << 1);
        pe = v->ye * vc->vc_size_row + (v->xe << 1);

The userspace-provided xs,ys,xe and ye values are decremented by one
before ps and pe (the pointers into the text screen array) are calculated.
So, for the xs...ye values in the range from 1..max-screen-lines/columns
are expected.

Helge