Messages in this thread | | | Date | Mon, 12 Jul 2021 07:34:18 -1000 | From | Tejun Heo <> | Subject | Re: [PATCH] cgroup1: fix leaked context root causing sporadic NULL deref in LTP |
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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
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