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SubjectRe: [PATCH v4] tty: Fix out-of-bound vmalloc access in imageblit
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On 6/25/21 3:09 AM, Jiri Slaby wrote:
> 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>

I don't know how, but I didn't see this comment in the original email. I
will change it.

>
>> + * 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?

fbcon

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

Thanks,
Igor M. A. Torrente

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