Re: KASAN: slab-out-of-bounds Read in hex_string

[Alan Stern]

On Mon, 29 Apr 2019, syzbot wrote:

> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    43151d6c usb-fuzzer: main usb gadget fuzzer driver
> git tree:       https://github.com/google/kasan/tree/usb-fuzzer
> console output: https://syzkaller.appspot.com/x/log.txt?x=139ac37f200000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=4183eeef650d1234
> dashboard link: https://syzkaller.appspot.com/bug?extid=a9fefd18c7b240f19c54
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17f3b338a00000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+a9fefd18c7b240f19c54@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in hex_string+0x418/0x4b0 lib/vsprintf.c:975
> Read of size 1 at addr ffff88821a41bd38 by task kworker/0:1/12
> 
> CPU: 0 PID: 12 Comm: kworker/0:1 Not tainted 5.1.0-rc3-319004-g43151d6 #6
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
> Google 01/01/2011
> Workqueue: usb_hub_wq hub_event
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0xe8/0x16e lib/dump_stack.c:113
>   print_address_description+0x6c/0x236 mm/kasan/report.c:187
>   kasan_report.cold+0x1a/0x3c mm/kasan/report.c:317
>   hex_string+0x418/0x4b0 lib/vsprintf.c:975
>   pointer+0x460/0x910 lib/vsprintf.c:1985
>   vsnprintf+0x5a0/0x16b0 lib/vsprintf.c:2400
>   pointer+0x60b/0x910 lib/vsprintf.c:2038
>   vsnprintf+0x5a0/0x16b0 lib/vsprintf.c:2400
>   vscnprintf+0x29/0x80 lib/vsprintf.c:2499
>   vprintk_store+0x45/0x4b0 kernel/printk/printk.c:1900
>   vprintk_emit+0x210/0x5a0 kernel/printk/printk.c:1957
>   dev_vprintk_emit+0x50e/0x553 drivers/base/core.c:3185
>   dev_printk_emit+0xbf/0xf6 drivers/base/core.c:3196
>   __dev_printk+0x1ed/0x215 drivers/base/core.c:3208
>   _dev_info+0xdc/0x10e drivers/base/core.c:3254
>   dlfb_parse_vendor_descriptor drivers/video/fbdev/udlfb.c:1532 [inline]

Accessing beyond the end of the descriptor.

#syz test: https://github.com/google/kasan.git usb-fuzzer

--- a/drivers/video/fbdev/udlfb.c
+++ b/drivers/video/fbdev/udlfb.c
@@ -1511,6 +1511,7 @@ static int dlfb_parse_vendor_descriptor(
 	char *buf;
 	char *desc_end;
 	int total_len;
+	int width;
 
 	buf = kzalloc(MAX_VENDOR_DESCRIPTOR_SIZE, GFP_KERNEL);
 	if (!buf)
@@ -1529,9 +1530,10 @@ static int dlfb_parse_vendor_descriptor(
 	}
 
 	if (total_len > 5) {
+		width = min(total_len, 11);
 		dev_info(&intf->dev,
-			 "vendor descriptor length: %d data: %11ph\n",
-			 total_len, desc);
+			 "vendor descriptor length: %d data: %*ph\n",
+			 total_len, width, desc);
 
 		if ((desc[0] != total_len) || /* descriptor length */
 		    (desc[1] != 0x5f) ||   /* vendor descriptor type */