ovl: consistent behavior for immutable/append-only inodes

When a lower file has immutable/append-only fileattr flags, the behavior of
overlayfs post copy up is inconsistent.

Immediattely after copy up, ovl inode still has the S_IMMUTABLE/S_APPEND
inode flags copied from lower inode, so vfs code still treats the ovl inode
as immutable/append-only.  After ovl inode evict or mount cycle, the ovl
inode does not have these inode flags anymore.

We cannot copy up the immutable and append-only fileattr flags, because
immutable/append-only inodes cannot be linked and because overlayfs will
not be able to set overlay.* xattr on the upper inodes.

Instead, if any of the fileattr flags of interest exist on the lower inode,
we store them in overlay.protattr xattr on the upper inode and we read the
flags from xattr on lookup and on fileattr_get().

This gives consistent behavior post copy up regardless of inode eviction
from cache.

When user sets new fileattr flags, we update or remove the overlay.protattr
xattr.

Storing immutable/append-only fileattr flags in an xattr instead of upper
fileattr also solves other non-standard behavior issues - overlayfs can now
copy up children of "ovl-immutable" directories and lower aliases of
"ovl-immutable" hardlinks.

Reported-by: Chengguang Xu <cgxu519@mykernel.net>
Link: https://lore.kernel.org/linux-unionfs/20201226104618.239739-1-cgxu519@mykernel.net/
Link: https://lore.kernel.org/linux-unionfs/20210210190334.1212210-5-amir73il@gmail.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 85 insertions, 0 deletions

diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 81b8f135445a..f48284a2a896 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -10,6 +10,7 @@
 #include <linux/cred.h>
 #include <linux/xattr.h>
 #include <linux/exportfs.h>
+#include <linux/fileattr.h>
 #include <linux/uuid.h>
 #include <linux/namei.h>
 #include <linux/ratelimit.h>
@@ -585,6 +586,7 @@ bool ovl_check_dir_xattr(struct super_block *sb, struct dentry *dentry,
 #define OVL_XATTR_NLINK_POSTFIX		"nlink"
 #define OVL_XATTR_UPPER_POSTFIX		"upper"
 #define OVL_XATTR_METACOPY_POSTFIX	"metacopy"
+#define OVL_XATTR_PROTATTR_POSTFIX	"protattr"
 
 #define OVL_XATTR_TAB_ENTRY(x) \
 	[x] = { [false] = OVL_XATTR_TRUSTED_PREFIX x ## _POSTFIX, \
@@ -598,6 +600,7 @@ const char *const ovl_xattr_table[][2] = {
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_NLINK),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_UPPER),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_METACOPY),
+	OVL_XATTR_TAB_ENTRY(OVL_XATTR_PROTATTR),
 };
 
 int ovl_check_setxattr(struct ovl_fs *ofs, struct dentry *upperdentry,
@@ -639,6 +642,88 @@ int ovl_set_impure(struct dentry *dentry, struct dentry *upperdentry)
 	return err;
 }
 
+
+#define OVL_PROTATTR_MAX 32 /* Reserved for future flags */
+
+void ovl_check_protattr(struct inode *inode, struct dentry *upper)
+{
+	struct ovl_fs *ofs = OVL_FS(inode->i_sb);
+	u32 iflags = inode->i_flags & OVL_PROT_I_FLAGS_MASK;
+	char buf[OVL_PROTATTR_MAX+1];
+	int res, n;
+
+	res = ovl_do_getxattr(ofs, upper, OVL_XATTR_PROTATTR, buf,
+			      OVL_PROTATTR_MAX);
+	if (res < 0)
+		return;
+
+	/*
+	 * Initialize inode flags from overlay.protattr xattr and upper inode
+	 * flags.  If upper inode has those fileattr flags set (i.e. from old
+	 * kernel), we do not clear them on ovl_get_inode(), but we will clear
+	 * them on next fileattr_set().
+	 */
+	for (n = 0; n < res; n++) {
+		if (buf[n] == 'a')
+			iflags |= S_APPEND;
+		else if (buf[n] == 'i')
+			iflags |= S_IMMUTABLE;
+		else
+			break;
+	}
+
+	if (!res || n < res) {
+		pr_warn_ratelimited("incompatible overlay.protattr format (%pd2, len=%d)\n",
+				    upper, res);
+	} else {
+		inode_set_flags(inode, iflags, OVL_PROT_I_FLAGS_MASK);
+	}
+}
+
+int ovl_set_protattr(struct inode *inode, struct dentry *upper,
+		      struct fileattr *fa)
+{
+	struct ovl_fs *ofs = OVL_FS(inode->i_sb);
+	char buf[OVL_PROTATTR_MAX];
+	int len = 0, err = 0;
+	u32 iflags = 0;
+
+	BUILD_BUG_ON(HWEIGHT32(OVL_PROT_FS_FLAGS_MASK) > OVL_PROTATTR_MAX);
+
+	if (fa->flags & FS_APPEND_FL) {
+		buf[len++] = 'a';
+		iflags |= S_APPEND;
+	}
+	if (fa->flags & FS_IMMUTABLE_FL) {
+		buf[len++] = 'i';
+		iflags |= S_IMMUTABLE;
+	}
+
+	/*
+	 * Do not allow to set protection flags when upper doesn't support
+	 * xattrs, because we do not set those fileattr flags on upper inode.
+	 * Remove xattr if it exist and all protection flags are cleared.
+	 */
+	if (len) {
+		err = ovl_check_setxattr(ofs, upper, OVL_XATTR_PROTATTR,
+					 buf, len, -EPERM);
+	} else if (inode->i_flags & OVL_PROT_I_FLAGS_MASK) {
+		err = ovl_do_removexattr(ofs, upper, OVL_XATTR_PROTATTR);
+		if (err == -EOPNOTSUPP || err == -ENODATA)
+			err = 0;
+	}
+	if (err)
+		return err;
+
+	inode_set_flags(inode, iflags, OVL_PROT_I_FLAGS_MASK);
+
+	/* Mask out the fileattr flags that should not be set in upper inode */
+	fa->flags &= ~OVL_PROT_FS_FLAGS_MASK;
+	fa->fsx_xflags &= ~OVL_PROT_FSX_FLAGS_MASK;
+
+	return 0;
+}
+
 /**
  * Caller must hold a reference to inode to prevent it from being freed while
  * it is marked inuse.