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SubjectRe: [PATCH v4] tty: Fix out-of-bound vmalloc access in imageblit
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On 24. 06. 21, 22:27, Igor Matheus Andrade Torrente wrote:
> This issue happens when a userspace program does an ioctl
> FBIOPUT_VSCREENINFO passing the fb_var_screeninfo struct
> containing only the fields xres, yres, and bits_per_pixel
> with values.
>
> If this struct is the same as the previous ioctl, the
> vc_resize() detects it and doesn't call the resize_screen(),
> leaving the fb_var_screeninfo incomplete. And this leads to
> the updatescrollmode() calculates a wrong value to
> fbcon_display->vrows, which makes the real_y() return a
> wrong value of y, and that value, eventually, causes
> the imageblit to access an out-of-bound address value.
>
> To solve this issue I made the resize_screen() be called
> even if the screen does not need any resizing, so it will
> "fix and fill" the fb_var_screeninfo independently.
>
> Reported-and-tested-by: syzbot+858dc7a2f7ef07c2c219@syzkaller.appspotmail.com
> Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
> ---
> drivers/tty/vt/vt.c | 17 ++++++++++++++++-
> 1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index fa1548d4f94b..e342f5c905bc 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -1220,7 +1220,22 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
> new_screen_size = new_row_size * new_rows;
>
> if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
> - return 0;
> + /* This function is being called here to cover the case

This was already commented on v3:
<quote>
Please do not use networking style comments, use normal ones:
/*
* This function...
</quote>

> + * where the userspace calls the FBIOPUT_VSCREENINFO twice,
> + * passing the same fb_var_screeninfo containing the fields:
> + * yres/xres equal to a number non-multiple of vc_font.height
> + * and yres_virtual/xres_virtual equal to number lesser than the
> + * vc_font.height and yres/xres.
> + * In the second call, the struct fb_var_screeninfo isn't
> + * being modified by the underlying driver because of the
> + * if above, and this causes the fbcon_display->vrows to become
> + * negative and it eventually leads to out-of-bound
> + * access by the imageblit function.
> + * To give the correct values to the struct and to not have
> + * to deal with possible errors from the code below, we call
> + * the resize_screen here as well.
> + */
> + return resize_screen(vc, new_cols, new_rows, user);

I suppose you need to add { } after the if given how its body grew.

With what all console drivers have you tested this with?

>
> if (new_screen_size > KMALLOC_MAX_SIZE || !new_screen_size)
> return -EINVAL;
>


--
js
suse labs

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