[PATCH 3.10 046/268] USB: gadgetfs: fix use-after-free bug

From: Willy Tarreau
Date: Mon Jun 19 2017 - 14:45:04 EST

From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

commit add333a81a16abbd4f106266a2553677a165725f upstream.

Andrey Konovalov reports that fuzz testing with syzkaller causes a
KASAN use-after-free bug report in gadgetfs:

BUG: KASAN: use-after-free in gadgetfs_setup+0x208a/0x20e0 at addr ffff88003dfe5bf2
Read of size 2 by task syz-executor0/22994
CPU: 3 PID: 22994 Comm: syz-executor0 Not tainted 4.9.0-rc7+ #16
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
ffff88006df06a18 ffffffff81f96aba ffffffffe0528500 1ffff1000dbe0cd6
ffffed000dbe0cce ffff88006df068f0 0000000041b58ab3 ffffffff8598b4c8
ffffffff81f96828 1ffff1000dbe0ccd ffff88006df06708 ffff88006df06748
Call Trace:
<IRQ> [ 201.343209] [< inline >] __dump_stack lib/dump_stack.c:15
<IRQ> [ 201.343209] [<ffffffff81f96aba>] dump_stack+0x292/0x398 lib/dump_stack.c:51
[<ffffffff817e4dec>] kasan_object_err+0x1c/0x70 mm/kasan/report.c:159
[< inline >] print_address_description mm/kasan/report.c:197
[<ffffffff817e5080>] kasan_report_error+0x1f0/0x4e0 mm/kasan/report.c:286
[< inline >] kasan_report mm/kasan/report.c:306
[<ffffffff817e562a>] __asan_report_load_n_noabort+0x3a/0x40 mm/kasan/report.c:337
[< inline >] config_buf drivers/usb/gadget/legacy/inode.c:1298
[<ffffffff8322c8fa>] gadgetfs_setup+0x208a/0x20e0 drivers/usb/gadget/legacy/inode.c:1368
[<ffffffff830fdcd0>] dummy_timer+0x11f0/0x36d0 drivers/usb/gadget/udc/dummy_hcd.c:1858
[<ffffffff814807c1>] call_timer_fn+0x241/0x800 kernel/time/timer.c:1308
[< inline >] expire_timers kernel/time/timer.c:1348
[<ffffffff81482de6>] __run_timers+0xa06/0xec0 kernel/time/timer.c:1641
[<ffffffff814832c1>] run_timer_softirq+0x21/0x80 kernel/time/timer.c:1654
[<ffffffff84f4af8b>] __do_softirq+0x2fb/0xb63 kernel/softirq.c:284

The cause of the bug is subtle. The dev_config() routine gets called
twice by the fuzzer. The first time, the user data contains both a
full-speed configuration descriptor and a high-speed config
descriptor, causing dev->hs_config to be set. But it also contains an
invalid device descriptor, so the buffer containing the descriptors is
deallocated and dev_config() returns an error.

The second time dev_config() is called, the user data contains only a
full-speed config descriptor. But dev->hs_config still has the stale
pointer remaining from the first call, causing the routine to think
that there is a valid high-speed config. Later on, when the driver
dereferences the stale pointer to copy that descriptor, we get a
use-after-free access.

The fix is simple: Clear dev->hs_config if the passed-in data does not
contain a high-speed config descriptor.

Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Tested-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
drivers/usb/gadget/inode.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/drivers/usb/gadget/inode.c b/drivers/usb/gadget/inode.c
index 77b981e..3b4ef7c 100644
--- a/drivers/usb/gadget/inode.c
+++ b/drivers/usb/gadget/inode.c
@@ -1892,6 +1892,8 @@ dev_config (struct file *fd, const char __user *buf, size_t len, loff_t *ptr)
goto fail;
kbuf += total;
length -= total;
+ } else {
+ dev->hs_config = NULL;

/* could support multiple configs, using another encoding! */