There was an userspace regression reported after the inclusion of these backports; it's being handled in https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1813873.
The fix patch was released upstream and the SRU request was sent to kernel-team ML (thanks smb!): https://lists.ubuntu.com/archives/kernel-team/2019-January/098186.html -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/1791758 Title: ldisc crash on reopened tty Status in linux package in Ubuntu: Fix Committed Status in linux source package in Trusty: Won't Fix Status in linux source package in Xenial: Fix Committed Status in linux source package in Bionic: Fix Released Status in linux source package in Cosmic: Fix Committed Bug description: [Impact] * Line discipline code is racy when we have buffer being flush while the tty is being initialized or reinitialized. For the first problem, we have an upstream patch since January 2018: b027e2298bd5 ("tty: fix data race between tty_init_dev and flush of buf") - although it is not in Ubuntu kernel 4.4, only in kernels 4.15 and subsequent ones. * For the race between the buffer flush while tty is being reopened, we have a patch that addresses this issue recently merged for 5.0-rc1: 83d817f41070 ("tty: Hold tty_ldisc_lock() during tty_reopen()"). No Ubuntu kernel currently contains this patch, hence we're hereby submitting the SRU request. The upstream complete patch series for this is in [0]. * The approach of both patches are similar - they rely in locking/semaphore to prevent race conditions. Some additional patches are necessary to prevent correlated issues, like preventing a potential deadlock due to bad prioritization in servicing I/O over releasing tty_ldisc_lock() - refer to c96cf923a98d ("tty: Don't block on IO when ldisc change is pending"). All the necessary fixes are grouped here in this SRU request. * The symptom of the race condition between the buffer flush and the tty reopen routine is a kernel crash with the following trace: BUG: unable to handle kernel paging request at 0000000000002268 IP: [<addr>] n_tty_receive_buf_common+0x6a/0xae0 [...] Call Trace: [<addr>] ? kvm_sched_clock_read+0x1e/0x30 [<addr>] n_tty_receive_buf2+0x14/0x20 [<addr>] flush_to_ldisc+0xd5/0x120 [<addr>] process_one_work+0x156/0x400 [<addr>] worker_thread+0x11a/0x480 [...] * A kernel crash was collected from an user, analysis is present in comment #4 in this LP. [Test Case] * It is not trivial to trigger this fault, but the usual recipe is to keep accessing a machine through SSH (or keep killing getty when in IPMI serial console) and in some way run commands before the terminal is ready in that machine (like hacking some echo into ttySx or pts in an infinite loop). * We have reports of users that could reproduce this issue in their production environment, and with the patches present in this SRU request the problem was fixed. [Regression Potential] * tty subsystem is highly central and patches in that area are always delicate. For example, the upstream series [0] is a re-spin (V6) due to a hard to reproduce issue reported in the PA-RISC architecture, which was found in the V5 iteration [1] but was fixed by the patch c96cf923a98d, present in this SRU request. * The patchset [0] is present in tty-next tree since mid-November, and the patch b027e2298bd5 is available upstream since January/2018 (it's available in both Ubuntu kernels 4.15 and 4.18), so the overall likelihood of regressions is low. * These patches were sniff-tested for the 3 versions (4.4, 4.15 and 4.18) and didn't show any issues. [0] https://marc.info/?l=linux-kernel&m=154103190111795 [1] https://marc.info/?l=linux-kernel&m=153737852618183 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1791758/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
