On Wed, Jun 16, 2021 at 08:51:57AM -0400, Paul Gortmaker wrote:
Richard reported sporadic (roughly one in 10 or so) null dereferences and other strange behaviour for a set of automated LTP tests. Things like:
BUG: kernel NULL pointer dereference, address: 0000000000000008 #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: 0 PID: 1516 Comm: umount Not tainted 5.10.0-yocto-standard #1 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-48-gd9c812dda519-prebuilt.qemu.org 04/01/2014 RIP: 0010:kernfs_sop_show_path+0x1b/0x60
...or these others:
RIP: 0010:do_mkdirat+0x6a/0xf0 RIP: 0010:d_alloc_parallel+0x98/0x510 RIP: 0010:do_readlinkat+0x86/0x120
There were other less common instances of some kind of a general scribble but the common theme was mount and cgroup and a dubious dentry triggering the NULL dereference. I was only able to reproduce it under qemu by replicating Richard's setup as closely as possible - I never did get it to happen on bare metal, even while keeping everything else the same.
In commit 71d883c37e8d ("cgroup_do_mount(): massage calling conventions") we see this as a part of the overall change:
struct cgroup_subsys *ss;
struct dentry *dentry;[...]
dentry = cgroup_do_mount(&cgroup_fs_type, fc->sb_flags, root,CGROUP_SUPER_MAGIC, ns);[...]
if (percpu_ref_is_dying(&root->cgrp.self.refcnt)) {struct super_block *sb = dentry->d_sb;dput(dentry);
ret = cgroup_do_mount(fc, CGROUP_SUPER_MAGIC, ns);if (!ret && percpu_ref_is_dying(&root->cgrp.self.refcnt)) {struct super_block *sb = fc->root->d_sb;dput(fc->root); deactivate_locked_super(sb); msleep(10); return restart_syscall(); }
In changing from the local "*dentry" variable to using fc->root, we now export/leave that dentry pointer in the file context after doing the dput() in the unlikely "is_dying" case. With LTP doing a crazy amount of back to back mount/unmount [testcases/bin/cgroup_regression_5_1.sh] the unlikely becomes slightly likely and then bad things happen.
A fix would be to not leave the stale reference in fc->root as follows:
dput(fc->root);
fc->root = NULL;deactivate_locked_super(sb);
...but then we are just open-coding a duplicate of fc_drop_locked() so we simply use that instead.
Cc: Al Viro viro@zeniv.linux.org.uk Cc: Tejun Heo tj@kernel.org Cc: Zefan Li lizefan.x@bytedance.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: stable@vger.kernel.org # v5.1+ Reported-by: Richard Purdie richard.purdie@linuxfoundation.org Fixes: 71d883c37e8d ("cgroup_do_mount(): massage calling conventions") Signed-off-by: Paul Gortmaker paul.gortmaker@windriver.com
I dropped the ball on this and this didn't get pushed. Re-applied to for-5.14-fixes. Will send out in a few days.
Thanks.