This is a note to let you know that I've just added the patch titled nilfs2: use refcount_dec_and_lock() to fix potential UAF to the 5.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: nilfs2-use-refcount_dec_and_lock-to-fix-potential-uaf.patch and it can be found in the queue-5.14 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 98e2e409e76ef7781d8511f997359e9c504a95c1 Mon Sep 17 00:00:00 2001 From: Zhen Lei <thunder.leizhen@xxxxxxxxxx> Date: Tue, 7 Sep 2021 20:00:26 -0700 Subject: nilfs2: use refcount_dec_and_lock() to fix potential UAF From: Zhen Lei <thunder.leizhen@xxxxxxxxxx> commit 98e2e409e76ef7781d8511f997359e9c504a95c1 upstream. 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. Link: https://lkml.kernel.org/r/1629859428-5906-1-git-send-email-konishi.ryusuke@xxxxxxxxx Fixes: ba65ae4729bf ("nilfs2: add checkpoint tree to nilfs object") Link: https://lkml.kernel.org/r/20210723012317.4146-1-thunder.leizhen@xxxxxxxxxx Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx> Signed-off-by: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/nilfs2/the_nilfs.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-) --- a/fs/nilfs2/the_nilfs.c +++ b/fs/nilfs2/the_nilfs.c @@ -792,14 +792,13 @@ nilfs_find_or_create_root(struct the_nil 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); Patches currently in stable-queue which might be from thunder.leizhen@xxxxxxxxxx are queue-5.14/nilfs2-use-refcount_dec_and_lock-to-fix-potential-uaf.patch