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@xxxxxxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v5.1+
> Reported-by: Richard Purdie <richard.purdie@xxxxxxxxxxxxxxxxxxx>
> Fixes: 71d883c37e8d ("cgroup_do_mount(): massage calling conventions")
> Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
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.
--
tejun
