[PATCH 5.10.y] ovl: set OVL_WHITEOUTS before publishing the upper dir

[Sasha Levin <sashal@kernel.org>]

From: Chris Mason <mason@kernel.org>

[ Upstream commit cbec8610ec5aaf35df50094ff639febc23f20773 ]

Listing a directory while it is copied up causes a use-after-free of
the readdir cache.

A merge directory must never be iterated as a real upper directory:
that mode frees the cache in OVL_I(inode)->cache without looking at
its refcount, while files opened in merged mode still use it.
ovl_copy_up_workdir() publishes the upper dentry before it sets
OVL_WHITEOUTS, and ovl_dir_open() samples both without a lock, so an
open between the two stores finds the upper directory and takes it
for a real one.

With a delay between the two stores, an unprivileged process that
listed a directory during its copy-up saw only "." and "..", and its
next getdents freed the readdir cache of another descriptor.

Fix this by setting OVL_WHITEOUTS before ovl_inode_update(), so that
ovl_dir_reset() agrees with ovl_dir_open(), and by testing
OVL_TYPE_MERGE() of the path type in ovl_dir_open(), so that an open
that observes the upper dentry never iterates a merge dir as a real
dir, regardless of the order in which it observes the flag.

Fixes: b79e05aaa166 ("ovl: no direct iteration for dir with origin xattr")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[amir: test OVL_TYPE_MERGE() instead of smp_rmb() in ovl_dir_open()]
Link: https://lore.kernel.org/linux-unionfs/CAOQ4uxh-1J2OBjnDVbrrrjy1puSOW0kABvghN85ih6dtKUBTyA@mail.gmail.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
[ sashal: adjusted context for dentry-based helper calls and later inode initialization ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/overlayfs/copy_up.c | 2 +-
 fs/overlayfs/readdir.c | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index 018867303a93a..07772f54e24f2 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -630,9 +630,9 @@ static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
 	if (!c->metacopy)
 		ovl_set_upperdata(d_inode(c->dentry));
 	inode = d_inode(c->dentry);
-	ovl_inode_update(inode, temp);
 	if (S_ISDIR(inode->i_mode))
 		ovl_set_flag(OVL_WHITEOUTS, inode);
+	ovl_inode_update(inode, temp);
 unlock:
 	unlock_rename(c->workdir, c->destdir);
 
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 09a5542723787..6e13cad49ce54 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -937,7 +937,8 @@ static int ovl_dir_open(struct inode *inode, struct file *file)
 		return PTR_ERR(realfile);
 	}
 	od->realfile = realfile;
-	od->is_real = ovl_dir_is_real(file->f_path.dentry);
+	od->is_real = !OVL_TYPE_MERGE(type) &&
+		      !ovl_test_flag(OVL_WHITEOUTS, inode);
 	od->is_upper = OVL_TYPE_UPPER(type);
 	file->private_data = od;
 
-- 
2.53.0