Re: [PATCH v2] usb: xhci: Check endpoint is valid before dereferencing it

From: Mathias Nyman
Date: Thu Dec 22 2022 - 08:17:56 EST

On 22.12.2022 13.52, Ladislav Michl wrote:
On Thu, Dec 22, 2022 at 01:08:47PM +0200, Mathias Nyman wrote:
On 22.12.2022 11.01, Ladislav Michl wrote:
On Thu, Dec 22, 2022 at 07:29:12AM +0000, Jimmy Hu wrote:
When the host controller is not responding, all URBs queued to all
endpoints need to be killed. This can cause a kernel panic if we
dereference an invalid endpoint.

Fix this by using xhci_get_virt_ep() helper to find the endpoint and
checking if the endpoint is valid before dereferencing it.

[233311.853271] xhci-hcd xhci-hcd.1.auto: xHCI host controller not responding, assume dead
[233311.853393] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000e8

[233311.853964] pc : xhci_hc_died+0x10c/0x270
[233311.853971] lr : xhci_hc_died+0x1ac/0x270

[233311.854077] Call trace:
[233311.854085] xhci_hc_died+0x10c/0x270
[233311.854093] xhci_stop_endpoint_command_watchdog+0x100/0x1a4
[233311.854105] call_timer_fn+0x50/0x2d4
[233311.854112] expire_timers+0xac/0x2e4
[233311.854118] run_timer_softirq+0x300/0xabc
[233311.854127] __do_softirq+0x148/0x528
[233311.854135] irq_exit+0x194/0x1a8
[233311.854143] __handle_domain_irq+0x164/0x1d0
[233311.854149] gic_handle_irq.22273+0x10c/0x188
[233311.854156] el1_irq+0xfc/0x1a8
[233311.854175] lpm_cpuidle_enter+0x25c/0x418 [msm_pm]
[233311.854185] cpuidle_enter_state+0x1f0/0x764
[233311.854194] do_idle+0x594/0x6ac
[233311.854201] cpu_startup_entry+0x7c/0x80
[233311.854209] secondary_start_kernel+0x170/0x198

Fixes: 50e8725e7c42 ("xhci: Refactor command watchdog and fix split string.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jimmy Hu <hhhuuu@xxxxxxxxxx>
---
drivers/usb/host/xhci-ring.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index ddc30037f9ce..f5b0e1ce22af 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -1169,7 +1169,10 @@ static void xhci_kill_endpoint_urbs(struct xhci_hcd *xhci,
struct xhci_virt_ep *ep;
struct xhci_ring *ring;
- ep = &xhci->devs[slot_id]->eps[ep_index];
+ ep = xhci_get_virt_ep(xhci, slot_id, ep_index);
+ if (!ep)
+ return;
+

xhci_get_virt_ep also adds check for slot_id == 0. It changes behaviour,
do we really want to skip that slot? Original code went from 0 to
MAX_HC_SLOTS-1.

It seems to be off by one to me. Am I missing anything?

slot_id 0 is always invalid, so this is a good change.

I see. Now reading more carefully:
#define HCS_MAX_SLOTS(p) (((p) >> 0) & 0xff)
#define MAX_HC_SLOTS 256
So the loop should go:
for (i = 1; i <= HCS_MAX_SLOTS(xhci->hcs_params1); i++)

yes


Also, what about passing ep directly to xhci_kill_endpoint_urbs
and do the check in xhci_hc_died? Not even compile tested:

passing ep to a function named kill_endpoint_urbs() sound like the
right thing to do, but as a generic change.

I think its a good idea to first do a targeted fix for this null pointer
issue that we can send to stable fist.

Agree. But I still do not understand the root cause. There is a check
for NULL xhci->devs[i] already, so patch does not add much more, except
for test for slot_id == 0. And the eps array is just array of
struct xhci_virt_ep, not a pointers to them, so &xhci->devs[i]->eps[j]
should be always valid pointer. However struct xhci_ring in each eps
is allocated and not protected by any lock here. Is that correct?

I think root cause is that freeing xhci->devs[i] and including rings isn't
protected by the lock, this happens in xhci_free_virt_device() called by
xhci_free_dev(), which in turn may be called by usbcore at any time

So xhci->devs[i] might just suddenly disappear

Patch just checks more often if xhci->devs[i] is valid, between every endpoint.
So the race between xhci_free_virt_device() and xhci_kill_endpoint_urbs()
doesn't trigger null pointer deref as easily.



diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index ddc30037f9ce..5dac483c562a 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -1162,14 +1162,12 @@ static void xhci_kill_ring_urbs(struct xhci_hcd *xhci, struct xhci_ring *ring)
}
static void xhci_kill_endpoint_urbs(struct xhci_hcd *xhci,
- int slot_id, int ep_index)
+ struct xhci_virt_ep *ep)
{
struct xhci_td *cur_td;
struct xhci_td *tmp;
- struct xhci_virt_ep *ep;
struct xhci_ring *ring;
- ep = &xhci->devs[slot_id]->eps[ep_index];
if ((ep->ep_state & EP_HAS_STREAMS) ||
(ep->ep_state & EP_GETTING_NO_STREAMS)) {
int stream_id;
@@ -1180,18 +1178,12 @@ static void xhci_kill_endpoint_urbs(struct xhci_hcd *xhci,
if (!ring)
continue;
- xhci_dbg_trace(xhci, trace_xhci_dbg_cancel_urb,
- "Killing URBs for slot ID %u, ep index %u, stream %u",
- slot_id, ep_index, stream_id);
xhci_kill_ring_urbs(xhci, ring);
}
} else {
ring = ep->ring;
if (!ring)
return;
- xhci_dbg_trace(xhci, trace_xhci_dbg_cancel_urb,
- "Killing URBs for slot ID %u, ep index %u",
- slot_id, ep_index);
xhci_kill_ring_urbs(xhci, ring);
}
@@ -1217,6 +1209,7 @@ static void xhci_kill_endpoint_urbs(struct xhci_hcd *xhci,
void xhci_hc_died(struct xhci_hcd *xhci)
{
int i, j;
+ struct xhci_virt_ep *ep;
if (xhci->xhc_state & XHCI_STATE_DYING)
return;
@@ -1227,11 +1220,14 @@ void xhci_hc_died(struct xhci_hcd *xhci)
xhci_cleanup_command_queue(xhci);
/* return any pending urbs, remove may be waiting for them */
- for (i = 0; i <= HCS_MAX_SLOTS(xhci->hcs_params1); i++) {
+ for (i = 0; i < HCS_MAX_SLOTS(xhci->hcs_params1); i++) {
if (!xhci->devs[i])
continue;
- for (j = 0; j < 31; j++)
- xhci_kill_endpoint_urbs(xhci, i, j);
+ for (j = 0; j < EP_CTX_PER_DEV; j++) {
+ ep = &xhci->devs[i]->eps[j];
+ if (ep)
+ xhci_kill_endpoint_urbs(xhci, ep);
+ }

This does loop a bit more than the existing code.
With this change its always HCS_MAX_SLOTS * EP_CTX_PER_DEV.
Previously best case was just HCS_MAX_SLOTS.

No, that's just the same:

you're right, incorrectly read that your patch deleted the
"if (!xhci->devs[i]) continue;" lines.

Thanks
-Mathias