Hi, On Fri, Jul 23, 2021 at 10:28 AM Zhen Lei <thunder.leizhen@xxxxxxxxxx> wrote: > > When the refcount is decreased to 0, the resource reclamation branch is > entered. Before CPU0 reaches the race point (1), CPU1 may obtain the > spinlock and traverse the rbtree to find 'root', see nilfs_lookup_root(). > Although CPU1 will call refcount_inc() to increase the refcount, it is > obviously too late. CPU0 will release 'root' directly, CPU1 then accesses > 'root' and triggers UAF. > > Use refcount_dec_and_lock() to ensure that both the operations of decrease > refcount to 0 and link deletion are lock protected eliminates this risk. > > CPU0 CPU1 > nilfs_put_root(): > <-------- (1) > spin_lock(&nilfs->ns_cptree_lock); > rb_erase(&root->rb_node, &nilfs->ns_cptree); > spin_unlock(&nilfs->ns_cptree_lock); > > kfree(root); > <-------- use-after-free > > ======================================================================== > refcount_t: underflow; use-after-free. > WARNING: CPU: 2 PID: 9476 at lib/refcount.c:28 \ > refcount_warn_saturate+0x1cf/0x210 lib/refcount.c:28 > Modules linked in: > CPU: 2 PID: 9476 Comm: syz-executor.0 Not tainted 5.10.45-rc1+ #3 > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), ... > RIP: 0010:refcount_warn_saturate+0x1cf/0x210 lib/refcount.c:28 > ... ... > Call Trace: > __refcount_sub_and_test include/linux/refcount.h:283 [inline] > __refcount_dec_and_test include/linux/refcount.h:315 [inline] > refcount_dec_and_test include/linux/refcount.h:333 [inline] > nilfs_put_root+0xc1/0xd0 fs/nilfs2/the_nilfs.c:795 > nilfs_segctor_destroy fs/nilfs2/segment.c:2749 [inline] > nilfs_detach_log_writer+0x3fa/0x570 fs/nilfs2/segment.c:2812 > nilfs_put_super+0x2f/0xf0 fs/nilfs2/super.c:467 > generic_shutdown_super+0xcd/0x1f0 fs/super.c:464 > kill_block_super+0x4a/0x90 fs/super.c:1446 > deactivate_locked_super+0x6a/0xb0 fs/super.c:335 > deactivate_super+0x85/0x90 fs/super.c:366 > cleanup_mnt+0x277/0x2e0 fs/namespace.c:1118 > __cleanup_mnt+0x15/0x20 fs/namespace.c:1125 > task_work_run+0x8e/0x110 kernel/task_work.c:151 > tracehook_notify_resume include/linux/tracehook.h:188 [inline] > exit_to_user_mode_loop kernel/entry/common.c:164 [inline] > exit_to_user_mode_prepare+0x13c/0x170 kernel/entry/common.c:191 > syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:266 > do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > There is no reproduction program, and the above is only theoretical > analysis. > > Fixes: ba65ae4729bf ("nilfs2: add checkpoint tree to nilfs object") > Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx> Thank you for sending this patch. I understand the purpose of the correction. I feel we need more review for the life cycle management of nilfs_root structure related to kobject. Anyway, I would like to apply this patch itself. Thanks, Ryusuke Konishi > --- > fs/nilfs2/the_nilfs.c | 9 ++++----- > 1 file changed, 4 insertions(+), 5 deletions(-) > > diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c > index 8b7b01a380ce..c8bfc01da5d7 100644 > --- a/fs/nilfs2/the_nilfs.c > +++ b/fs/nilfs2/the_nilfs.c > @@ -792,14 +792,13 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 cno) > > void nilfs_put_root(struct nilfs_root *root) > { > - if (refcount_dec_and_test(&root->count)) { > - struct the_nilfs *nilfs = root->nilfs; > + struct the_nilfs *nilfs = root->nilfs; > > - nilfs_sysfs_delete_snapshot_group(root); > - > - spin_lock(&nilfs->ns_cptree_lock); > + if (refcount_dec_and_lock(&root->count, &nilfs->ns_cptree_lock)) { > rb_erase(&root->rb_node, &nilfs->ns_cptree); > spin_unlock(&nilfs->ns_cptree_lock); > + > + nilfs_sysfs_delete_snapshot_group(root); > iput(root->ifile); > > kfree(root); > -- > 2.25.1 >