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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	stable@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
	Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Richard Purdie <richard.purdie@linuxfoundation.org>,
	Paul Gortmaker <paul.gortmaker@windriver.com>
Subject: [PATCH 5.10 06/24] cgroup1: fix leaked context root causing sporadic NULL deref in LTP
Date: Thu, 29 Jul 2021 15:54:26 +0200	[thread overview]
Message-ID: <20210729135137.470133128@linuxfoundation.org> (raw)
In-Reply-To: <20210729135137.267680390@linuxfoundation.org>

From: Paul Gortmaker <paul.gortmaker@windriver.com>

commit 1e7107c5ef44431bc1ebbd4c353f1d7c22e5f2ec upstream.

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>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/internal.h              |    1 -
 include/linux/fs_context.h |    1 +
 kernel/cgroup/cgroup-v1.c  |    4 +---
 3 files changed, 2 insertions(+), 4 deletions(-)

--- a/fs/internal.h
+++ b/fs/internal.h
@@ -64,7 +64,6 @@ extern void __init chrdev_init(void);
  */
 extern const struct fs_context_operations legacy_fs_context_ops;
 extern int parse_monolithic_mount_data(struct fs_context *, void *);
-extern void fc_drop_locked(struct fs_context *);
 extern void vfs_clean_context(struct fs_context *fc);
 extern int finish_clean_context(struct fs_context *fc);
 
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -139,6 +139,7 @@ extern int vfs_parse_fs_string(struct fs
 extern int generic_parse_monolithic(struct fs_context *fc, void *data);
 extern int vfs_get_tree(struct fs_context *fc);
 extern void put_fs_context(struct fs_context *fc);
+extern void fc_drop_locked(struct fs_context *fc);
 
 /*
  * sget() wrappers to be called from the ->get_tree() op.
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -1225,9 +1225,7 @@ int cgroup1_get_tree(struct fs_context *
 		ret = cgroup_do_get_tree(fc);
 
 	if (!ret && percpu_ref_is_dying(&ctx->root->cgrp.self.refcnt)) {
-		struct super_block *sb = fc->root->d_sb;
-		dput(fc->root);
-		deactivate_locked_super(sb);
+		fc_drop_locked(fc);
 		ret = 1;
 	}
 



  parent reply	other threads:[~2021-07-29 14:01 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 13:54 [PATCH 5.10 00/24] 5.10.55-rc1 review Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 01/24] tools: Allow proper CC/CXX/... override with LLVM=1 in Makefile.include Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 02/24] [PATCH] io_uring: fix link timeout refs Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 03/24] KVM: x86: determine if an exception has an error code only when injecting it Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 04/24] af_unix: fix garbage collect vs MSG_PEEK Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 05/24] workqueue: fix UAF in pwq_unbound_release_workfn() Greg Kroah-Hartman
2021-07-29 13:54 ` Greg Kroah-Hartman [this message]
2021-07-29 13:54 ` [PATCH 5.10 07/24] net/802/mrp: fix memleak in mrp_request_join() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 08/24] net/802/garp: fix memleak in garp_request_join() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 09/24] net: annotate data race around sk_ll_usec Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 10/24] sctp: move 198 addresses from unusable to private scope Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 11/24] rcu-tasks: Dont delete holdouts within trc_inspect_reader() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 12/24] rcu-tasks: Dont delete holdouts within trc_wait_for_one_reader() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 13/24] ipv6: allocate enough headroom in ip6_finish_output2() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 14/24] drm/ttm: add a check against null pointer dereference Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 15/24] hfs: add missing clean-up in hfs_fill_super Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 16/24] hfs: fix high memory mapping in hfs_bnode_read Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 17/24] hfs: add lock nesting notation to hfs_find_init Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 18/24] firmware: arm_scmi: Fix possible scmi_linux_errmap buffer overflow Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 19/24] firmware: arm_scmi: Fix range check for the maximum number of pending messages Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 20/24] cifs: fix the out of range assignment to bit fields in parse_server_interfaces Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 21/24] iomap: remove the length variable in iomap_seek_data Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 22/24] iomap: remove the length variable in iomap_seek_hole Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 23/24] ARM: dts: versatile: Fix up interrupt controller node names Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 5.10 24/24] ipv6: ip6_finish_output2: set sk into newly allocated nskb Greg Kroah-Hartman
2021-07-29 19:47 ` [PATCH 5.10 00/24] 5.10.55-rc1 review Pavel Machek
2021-07-29 22:53 ` Shuah Khan
2021-07-30  4:58   ` Greg Kroah-Hartman
2021-07-29 23:36 ` Florian Fainelli
2021-07-30  2:40 ` Shuah Khan
2021-07-30  6:23 ` Naresh Kamboju
2021-07-30 10:12 ` Sudip Mukherjee
2021-07-31  4:43 ` Guenter Roeck
2021-07-31  7:51 ` Fox Chen

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