[v3] vgacon: Fix a UAF in vgacon_invert_region

[zhangxiaoxu (A)]

Please ignore this patch.
Thanks.

在 2020/3/4 10:10, Zhang Xiaoxu 写道:
> When syzkaller tests, there is a UAF:
>    BUG: KASan: use after free in vgacon_invert_region+0x9d/0x110 at addr
>      ffff880000100000
>    Read of size 2 by task syz-executor.1/16489
>    page:ffffea0000004000 count:0 mapcount:-127 mapping:          (null)
>    index:0x0
>    page flags: 0xfffff00000000()
>    page dumped because: kasan: bad access detected
>    CPU: 1 PID: 16489 Comm: syz-executor.1 Not tainted
>    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>    rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014
>    Call Trace:
>      [<ffffffffb119f309>] dump_stack+0x1e/0x20
>      [<ffffffffb04af957>] kasan_report+0x577/0x950
>      [<ffffffffb04ae652>] __asan_load2+0x62/0x80
>      [<ffffffffb090f26d>] vgacon_invert_region+0x9d/0x110
>      [<ffffffffb0a39d95>] invert_screen+0xe5/0x470
>      [<ffffffffb0a21dcb>] set_selection+0x44b/0x12f0
>      [<ffffffffb0a3bfae>] tioclinux+0xee/0x490
>      [<ffffffffb0a1d114>] vt_ioctl+0xff4/0x2670
>      [<ffffffffb0a0089a>] tty_ioctl+0x46a/0x1a10
>      [<ffffffffb052db3d>] do_vfs_ioctl+0x5bd/0xc40
>      [<ffffffffb052e2f2>] SyS_ioctl+0x132/0x170
>      [<ffffffffb11c9b1b>] system_call_fastpath+0x22/0x27
>      Memory state around the buggy address:
>       ffff8800000fff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>       00 00
>       ffff8800000fff80: 00 00 00 00 00 00 00 00 00 00 00 00 00
>       00 00 00
>      >ffff880000100000: ff ff ff ff ff ff ff ff ff ff ff ff ff
>       ff ff ff
> 
> It can be reproduce in the linux mainline by the program:
>    #include <stdio.h>
>    #include <stdlib.h>
>    #include <unistd.h>
>    #include <fcntl.h>
>    #include <sys/types.h>
>    #include <sys/stat.h>
>    #include <sys/ioctl.h>
>    #include <linux/vt.h>
> 
>    struct tiocl_selection {
>      unsigned short xs;      /* X start */
>      unsigned short ys;      /* Y start */
>      unsigned short xe;      /* X end */
>      unsigned short ye;      /* Y end */
>      unsigned short sel_mode; /* selection mode */
>    };
> 
>    #define TIOCL_SETSEL    2
>    struct tiocl {
>      unsigned char type;
>      unsigned char pad;
>      struct tiocl_selection sel;
>    };
> 
>    int main()
>    {
>      int fd = 0;
>      const char *dev = "/dev/char/4:1";
> 
>      struct vt_consize v = {0};
>      struct tiocl tioc = {0};
> 
>      fd = open(dev, O_RDWR, 0);
> 
>      v.v_rows = 3346;
>      ioctl(fd, VT_RESIZEX, &v);
> 
>      tioc.type = TIOCL_SETSEL;
>      ioctl(fd, TIOCLINUX, &tioc);
> 
>      return 0;
>    }
> 
> When resize the screen, update the 'vc->vc_size_row' to the new_row_size,
> but when 'set_origin' in 'vgacon_set_origin', vgacon use 'vga_vram_base'
> for 'vc_origin' and 'vc_visible_origin', not 'vc_screenbuf'. It maybe
> smaller than 'vc_screenbuf'. When TIOCLINUX, use the new_row_size to calc
> the offset, it maybe larger than the vga_vram_size in vgacon driver, then
> bad access.
> Also, if set an larger screenbuf firstly, then set an more larger
> screenbuf, when copy old_origin to new_origin, a bad access may happen.
> 
> So, If the screen size larger than vga_vram, resize screen should be
> failed. This alse fix CVE-2020-8649 and CVE-2020-8647.
> 
> Fixes: 0aec4867dca14 ("[PATCH] SVGATextMode fix")
> Reference: CVE-2020-8647 and CVE-2020-8649
> Reported-by: Hulk Robot <hulkci at huawei.com>
> Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5 at huawei.com>
> ---
>   drivers/video/console/vgacon.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
> index de7b8382aba9..95e2fece7e91 100644
> --- a/drivers/video/console/vgacon.c
> +++ b/drivers/video/console/vgacon.c
> @@ -1316,6 +1316,9 @@ static int vgacon_font_get(struct vc_data *c, struct console_font *font)
>   static int vgacon_resize(struct vc_data *c, unsigned int width,
>   			 unsigned int height, unsigned int user)
>   {
> +	if ((width >> 1) * height > vga_vram_size)
> +		return -EINVAL;
> +
>   	if (width % 2 || width > screen_info.orig_video_cols ||
>   	    height > (screen_info.orig_video_lines * vga_default_font_height)/
>   	    c->vc_font.height)
>