[PATCH 4/6] vfs: don't allow most setxattr to immutable files

["Darrick J. Wong" <darrick.wong@xxxxxxxxxx>]

From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

The chattr manpage has this to say about immutable files:

"A file with the 'i' attribute cannot be modified: it cannot be deleted
or renamed, no link can be created to this file, most of the file's
metadata can not be modified, and the file can not be opened in write
mode."

However, we don't actually check the immutable flag in the setattr code,
which means that we can update inode flags and project ids and extent
size hints on supposedly immutable files.  Therefore, reject setflags
and fssetxattr calls on an immutable file if the file is immutable and
will remain that way.

Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
---
 fs/inode.c |   31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)


diff --git a/fs/inode.c b/fs/inode.c
index a3757051fd55..adfb458bf533 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2184,6 +2184,17 @@ int vfs_ioc_setflags_check(struct inode *inode, int oldflags, int flags)
 	    !capable(CAP_LINUX_IMMUTABLE))
 		return -EPERM;
 
+	/*
+	 * We aren't allowed to change any other flags if the immutable flag is
+	 * already set and is not being unset.
+	 */
+	if ((oldflags & FS_IMMUTABLE_FL) &&
+	    (flags & FS_IMMUTABLE_FL)) {
+		if ((oldflags & ~FS_IMMUTABLE_FL) !=
+		    (flags & ~FS_IMMUTABLE_FL))
+			return -EPERM;
+	}
+
 	return 0;
 }
 EXPORT_SYMBOL(vfs_ioc_setflags_check);
@@ -2226,6 +2237,26 @@ int vfs_ioc_fssetxattr_check(struct inode *inode, const struct fsxattr *old_fa,
 	    !S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
 		return -EINVAL;
 
+	/*
+	 * We aren't allowed to change any fields if the immutable flag is
+	 * already set and is not being unset.
+	 */
+	if ((old_fa->fsx_xflags & FS_XFLAG_IMMUTABLE) &&
+	    (fa->fsx_xflags & FS_XFLAG_IMMUTABLE)) {
+		if ((old_fa->fsx_xflags & ~FS_XFLAG_IMMUTABLE) !=
+		    (fa->fsx_xflags & ~FS_XFLAG_IMMUTABLE))
+			return -EPERM;
+		if (old_fa->fsx_projid != fa->fsx_projid)
+			return -EPERM;
+		if ((fa->fsx_xflags & (FS_XFLAG_EXTSIZE |
+				       FS_XFLAG_EXTSZINHERIT)) &&
+		    old_fa->fsx_extsize != fa->fsx_extsize)
+			return -EPERM;
+		if ((old_fa->fsx_xflags & FS_XFLAG_COWEXTSIZE) &&
+		    old_fa->fsx_cowextsize != fa->fsx_cowextsize)
+			return -EPERM;
+	}
+
 	/* Extent size hints of zero turn off the flags. */
 	if (fa->fsx_extsize == 0)
 		fa->fsx_xflags &= ~(FS_XFLAG_EXTSIZE | FS_XFLAG_EXTSZINHERIT);