Re: [PATCH] cgroup1: fix leaked context root causing sporadic NULL deref in LTP

[Tejun Heo <tj@kernel.org>]

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.

-- 
tejun