Hi folks, I'm the maintainer in Debian for strace. Trying to reproduce https://bugs.debian.org/963462 on my machine (Thinkpad T470), I've found a repeatable hard lockup running the strace testsuite. Each time it seems to have failed in a slightly different place in the testsuite (suggesting it's not one particular syscall test that's triggering the failure). I initially found this using Debian's current Buster kernel (4.19.118+2+deb10u1), then backtracking I found that 4.19.98+1+deb10u1 worked fine. I've bisected to find the failure point along the linux-4.19.y stable branch and what I've got to is the following commit: e58f543fc7c0926f31a49619c1a3648e49e8d233 is the first bad commit commit e58f543fc7c0926f31a49619c1a3648e49e8d233 Author: Jann Horn <jannh@xxxxxxxxxx> Date: Thu Sep 13 18:12:09 2018 +0200 apparmor: don't try to replace stale label in ptrace access check [ Upstream commit 1f8266ff58840d698a1e96d2274189de1bdf7969 ] As a comment above begin_current_label_crit_section() explains, begin_current_label_crit_section() must run in sleepable context because when label_is_stale() is true, aa_replace_current_label() runs, which uses prepare_creds(), which can sleep. Until now, the ptrace access check (which runs with a task lock held) violated this rule. Also add a might_sleep() assertion to begin_current_label_crit_section(), because asserts are less likely to be ignored than comments. Fixes: b2d09ae449ced ("apparmor: move ptrace checks to using labels") Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> Signed-off-by: John Johansen <john.johansen@xxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> :040000 040000 ca92f885a38c1747b812116f19de6967084a647e 865a227665e460e159502f21e8a16e6fa590bf50 M security Considering I'm running strace build tests to provoke this bug, finding the failure in a commit talking about ptrace changes does look very suspicious...! Annoyingly, I can't reproduce this on my disparate other machines here, suggesting it's maybe(?) timing related. Hope this helps - happy to give more information, test things, etc. -- Steve McIntyre, Cambridge, UK. steve@xxxxxxxxxx "Managing a volunteer open source project is a lot like herding kittens, except the kittens randomly appear and disappear because they have day jobs." -- Matt Mackall
