Re: [PATCH 1/2] fbcon: Rebuild rotated font buffer on font change

[Helge Deller]

On 10/14/26 10:51, Yao Yiqi wrote:
> fbcon_rotate_font() caches a rotated copy of the console font in
> par->rotated.buf. The buffer is sized from the font's width, height and
> charcount, and is rebuilt in fbcon_init(), fbcon_switch() and
> fbcon_modechanged(), but not in fbcon_do_set_font().
>
> When the console font is changed through KDFONTOP/KD_FONT_OP_SET, the new
> font data and geometry are installed and vc_resize() is called. The
> resulting redraw reaches the rotated bitops before the buffer is rebuilt:
> hide_cursor() drives both con_cursor() -> soft_cursor() and fbcon_putc()
> -> *_putcs(), and they index par->rotated.buf with the new cell size and
> character codes. A stale, too-small buffer is read out of bounds.
>
> KASAN reports, for example:
>
>    BUG: KASAN: slab-out-of-bounds in soft_cursor
>    Read of size 64 at addr ...
>     memcpy
>     soft_cursor
>     cw_cursor
>     fbcon_cursor
>     hide_cursor
>     redraw_screen
>     vc_do_resize
>     fbcon_do_set_font
>     fbcon_set_font
>     con_font_op
>     vt_ioctl
>
> Rebuild the buffer for the visible console right after the new font
> geometry has been installed and before any redraw, as fbcon_switch() and
> fbcon_modechanged() already do.
>
> Unlike those callers, this one does not fall back to unrotated rendering
> on failure. The "par->rotate = FB_ROTATE_UR; set_blitting_type(vc, info);"
> fallback they use does not actually switch to unrotated rendering:
> set_blitting_type() calls fbcon_set_rotation(), which immediately
> re-derives par->rotate from par->p->con_rotate. Replicating that pattern
> here would not change the outcome; a failed rebuild leaves
> par->rotated.buf NULL, so the rotated ops skip drawing, and the following
> patch retries the allocation on the next call.
>
> Fixes: 6cc50e1c5b57 ("[PATCH] fbcon: Console Rotation - Add support to rotate font bitmap")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yao Yiqi <yaoyiqi3@xxxxxxxxxx>
> ---
>   drivers/video/fbdev/core/fbcon.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
series applied for further testing to fbdev/for-next.

Thanks!
Helge