On Mon, 21 Feb 2022 07:27:32 +0100 Juergen Gross wrote:
On 20.02.22 14:42, Marek Marczykowski-Górecki wrote:
xennet_destroy_queues() relies on info->netdev->real_num_tx_queues to delete queues. Since d7dac083414eb5bb99a6d2ed53dc2c1b405224e5 ("net-sysfs: update the queue counts in the unregistration path"), unregister_netdev() indirectly sets real_num_tx_queues to 0. Those two facts together means, that xennet_destroy_queues() called from xennet_remove() cannot do its job, because it's called after unregister_netdev(). This results in kfree-ing queues that are still linked in napi, which ultimately crashes:
BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] PREEMPT SMP PTI CPU: 1 PID: 52 Comm: xenwatch Tainted: G W 5.16.10-1.32.fc32.qubes.x86_64+ #226 RIP: 0010:free_netdev+0xa3/0x1a0 Code: ff 48 89 df e8 2e e9 00 00 48 8b 43 50 48 8b 08 48 8d b8 a0 fe ff ff 48 8d a9 a0 fe ff ff 49 39 c4 75 26 eb 47 e8 ed c1 66 ff <48> 8b 85 60 01 00 00 48 8d 95 60 01 00 00 48 89 ef 48 2d 60 01 00 RSP: 0000:ffffc90000bcfd00 EFLAGS: 00010286 RAX: 0000000000000000 RBX: ffff88800edad000 RCX: 0000000000000000 RDX: 0000000000000001 RSI: ffffc90000bcfc30 RDI: 00000000ffffffff RBP: fffffffffffffea0 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000001 R12: ffff88800edad050 R13: ffff8880065f8f88 R14: 0000000000000000 R15: ffff8880066c6680 FS: 0000000000000000(0000) GS:ffff8880f3300000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000000 CR3: 00000000e998c006 CR4: 00000000003706e0 Call Trace: <TASK> xennet_remove+0x13d/0x300 [xen_netfront] xenbus_dev_remove+0x6d/0xf0 __device_release_driver+0x17a/0x240 device_release_driver+0x24/0x30 bus_remove_device+0xd8/0x140 device_del+0x18b/0x410 ? _raw_spin_unlock+0x16/0x30 ? klist_iter_exit+0x14/0x20 ? xenbus_dev_request_and_reply+0x80/0x80 device_unregister+0x13/0x60 xenbus_dev_changed+0x18e/0x1f0 xenwatch_thread+0xc0/0x1a0 ? do_wait_intr_irq+0xa0/0xa0 kthread+0x16b/0x190 ? set_kthread_struct+0x40/0x40 ret_from_fork+0x22/0x30 </TASK>Fix this by calling xennet_destroy_queues() from xennet_close() too, when real_num_tx_queues is still available. This ensures that queues are destroyed when real_num_tx_queues is set to 0, regardless of how unregister_netdev() was called.
Originally reported at https://github.com/QubesOS/qubes-issues/issues/7257
Fixes: d7dac083414eb5bb9 ("net-sysfs: update the queue counts in the unregistration path") Cc: stable@vger.kernel.org # 5.16+ Signed-off-by: Marek Marczykowski-Górecki marmarek@invisiblethingslab.com
While this fixes the issue, I'm not sure if that is the correct thing to do. xennet_remove() calls xennet_destroy_queues() under rtnl_lock, which may be important here? Just moving xennet_destroy_queues() before
I checked some of the call paths leading to xennet_close(), and all of those contained an ASSERT_RTNL(), so it seems the rtnl_lock is already taken here. Could you test with adding an ASSERT_RTNL() in xennet_destroy_queues()?
unregister_netdev() in xennet_remove() did not helped - it crashed in another way (use-after-free in xennet_close()).
Yes, this would need to basically do the xennet_close() handling in xennet_destroy() instead, which I believe is not really an option.
I think the patch makes open/close asymmetric, tho. After ifup ; ifdown; the next ifup will fail because queues are already destroyed, no? IOW xennet_open() expects the queues were created at an earlier stage.
Maybe we can move the destroy to ndo_uninit? (and create to ndo_init?)