diff options
| author | Alan Stern <stern@rowland.harvard.edu> | 2026-03-15 14:30:43 -0400 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2026-04-11 14:29:56 +0200 |
| commit | 69ab97a693251d6a6093e630060a3c744fd58524 (patch) | |
| tree | 220fcb3d31bad06c25756820a4be55703205109f | |
| parent | 5cba3060ee50ba2adeb63f4bfc098551839754a7 (diff) | |
USB: dummy-hcd: Fix locking/synchronization error
commit 616a63ff495df12863692ab3f9f7b84e3fa7a66d upstream.
Syzbot testing was able to provoke an addressing exception and crash
in the usb_gadget_udc_reset() routine in
drivers/usb/gadgets/udc/core.c, resulting from the fact that the
routine was called with a second ("driver") argument of NULL. The bad
caller was set_link_state() in dummy_hcd.c, and the problem arose
because of a race between a USB reset and driver unbind.
These sorts of races were not supposed to be possible; commit
7dbd8f4cabd9 ("USB: dummy-hcd: Fix erroneous synchronization change"),
along with a few followup commits, was written specifically to prevent
them. As it turns out, there are (at least) two errors remaining in
the code. Another patch will address the second error; this one is
concerned with the first.
The error responsible for the syzbot crash occurred because the
stop_activity() routine will sometimes drop and then re-acquire the
dum->lock spinlock. A call to stop_activity() occurs in
set_link_state() when handling an emulated USB reset, after the test
of dum->ints_enabled and before the increment of dum->callback_usage.
This allowed another thread (doing a driver unbind) to sneak in and
grab the spinlock, and then clear dum->ints_enabled and dum->driver.
Normally this other thread would have to wait for dum->callback_usage
to go down to 0 before it would clear dum->driver, but in this case it
didn't have to wait since dum->callback_usage had not yet been
incremented.
The fix is to increment dum->callback_usage _before_ calling
stop_activity() instead of after. Then the thread doing the unbind
will not clear dum->driver until after the call to
usb_gadget_udc_reset() safely returns and dum->callback_usage has been
decremented again.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: syzbot+19bed92c97bee999e5db@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-usb/68fc7c9c.050a0220.346f24.023c.GAE@google.com/
Tested-by: syzbot+19bed92c97bee999e5db@syzkaller.appspotmail.com
Fixes: 7dbd8f4cabd9 ("USB: dummy-hcd: Fix erroneous synchronization change")
Cc: stable <stable@kernel.org>
Link: https://patch.msgid.link/46135f42-fdbe-46b5-aac0-6ca70492af15@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
| -rw-r--r-- | drivers/usb/gadget/udc/dummy_hcd.c | 7 |
1 files changed, 6 insertions, 1 deletions
diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c index 1cefca660773..9cebac2ebc13 100644 --- a/drivers/usb/gadget/udc/dummy_hcd.c +++ b/drivers/usb/gadget/udc/dummy_hcd.c @@ -462,8 +462,13 @@ static void set_link_state(struct dummy_hcd *dum_hcd) /* Report reset and disconnect events to the driver */ if (dum->ints_enabled && (disconnect || reset)) { - stop_activity(dum); ++dum->callback_usage; + /* + * stop_activity() can drop dum->lock, so it must + * not come between the dum->ints_enabled test + * and the ++dum->callback_usage. + */ + stop_activity(dum); spin_unlock(&dum->lock); if (reset) usb_gadget_udc_reset(&dum->gadget, dum->driver); |