From 1e7107c5ef44431bc1ebbd4c353f1d7c22e5f2ec Mon Sep 17 00:00:00 2001 From: Paul Gortmaker Date: Wed, 16 Jun 2021 08:51:57 -0400 Subject: cgroup1: fix leaked context root causing sporadic NULL deref in LTP MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Cc: Tejun Heo Cc: Zefan Li Cc: Johannes Weiner Cc: stable@vger.kernel.org # v5.1+ Reported-by: Richard Purdie Fixes: 71d883c37e8d ("cgroup_do_mount(): massage calling conventions") Signed-off-by: Paul Gortmaker Signed-off-by: Tejun Heo --- include/linux/fs_context.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include') diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h index e2bc16300c82..6b54982fc5f3 100644 --- a/include/linux/fs_context.h +++ b/include/linux/fs_context.h @@ -141,6 +141,7 @@ extern int vfs_get_tree(struct fs_context *fc); extern void put_fs_context(struct fs_context *fc); extern int vfs_parse_fs_param_source(struct fs_context *fc, struct fs_parameter *param); +extern void fc_drop_locked(struct fs_context *fc); /* * sget() wrappers to be called from the ->get_tree() op. -- cgit v1.2.3