queue-6.12/fat-avoid-parent-link-count-underflow-in-rmdir.patch

From 006cbe47745582fd16ff7e7ad4caf0127ce9a3fb Mon Sep 17 00:00:00 2001
From: Sasha Levin <sashal@kernel.org>
Date: Thu, 1 Jan 2026 19:11:48 +0800
Subject: fat: avoid parent link count underflow in rmdir

From: Zhiyu Zhang <zhiyuzhang999@gmail.com>

[ Upstream commit 8cafcb881364af5ef3a8b9fed4db254054033d8a ]

Corrupted FAT images can leave a directory inode with an incorrect
i_nlink (e.g. 2 even though subdirectories exist). rmdir then
unconditionally calls drop_nlink(dir) and can drive i_nlink to 0,
triggering the WARN_ON in drop_nlink().

Add a sanity check in vfat_rmdir() and msdos_rmdir(): only drop the
parent link count when it is at least 3, otherwise report a filesystem
error.

Link: https://lkml.kernel.org/r/20260101111148.1437-1-zhiyuzhang999@gmail.com
Fixes: 9a53c3a783c2 ("[PATCH] r/o bind mounts: unlink: monitor i_nlink")
Signed-off-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Reported-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Closes: https://lore.kernel.org/linux-fsdevel/aVN06OKsKxZe6-Kv@casper.infradead.org/T/#t
Tested-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/fat/namei_msdos.c | 7 ++++++-
 fs/fat/namei_vfat.c  | 7 ++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/fat/namei_msdos.c b/fs/fat/namei_msdos.c
index f06f6ba643cc8..c75e3791514a2 100644
--- a/fs/fat/namei_msdos.c
+++ b/fs/fat/namei_msdos.c
@@ -325,7 +325,12 @@ static int msdos_rmdir(struct inode *dir, struct dentry *dentry)
 	err = fat_remove_entries(dir, &sinfo);	/* and releases bh */
 	if (err)
 		goto out;
-	drop_nlink(dir);
+	if (dir->i_nlink >= 3)
+		drop_nlink(dir);
+	else {
+		fat_fs_error(sb, "parent dir link count too low (%u)",
+			dir->i_nlink);
+	}
 
 	clear_nlink(inode);
 	fat_truncate_time(inode, NULL, S_CTIME);
diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 15bf32c21ac0d..31d37e2b4f679 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -806,7 +806,12 @@ static int vfat_rmdir(struct inode *dir, struct dentry *dentry)
 	err = fat_remove_entries(dir, &sinfo);	/* and releases bh */
 	if (err)
 		goto out;
-	drop_nlink(dir);
+	if (dir->i_nlink >= 3)
+		drop_nlink(dir);
+	else {
+		fat_fs_error(sb, "parent dir link count too low (%u)",
+			dir->i_nlink);
+	}
 
 	clear_nlink(inode);
 	fat_truncate_time(inode, NULL, S_ATIME|S_MTIME);
-- 
2.51.0