Ocfs2 devel - Jul 2019 - [PATCH v2 4/4] vfs: don't allow most setxattr to immutable files

Hi all,

The chattr(1) manpage has this to say about the immutable bit that
system administrators can set on 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."

Given the clause about how the file 'cannot be modified', it is
surprising that programs holding writable file descriptors can continue
to write to and truncate files after the immutable flag has been set,
but they cannot call other things such as utimes, fallocate, unlink,
link, setxattr, or reflink.

Since the immutable flag is only settable by administrators, resolve
this inconsistent behavior in favor of the documented behavior -- once
the flag is set, the file cannot be modified, period.  We presume that
administrators must be trusted to know what they're doing, and that
cutting off programs with writable fds will probably break them.

Therefore, add immutability checks to the relevant VFS functions, then
refactor the SETFLAGS and FSSETXATTR implementations to use common
argument checking functions so that we can then force pagefaults on all
the file data when setting immutability.

Note that various distro manpages points out the inconsistent behavior
of the various Linux filesystems w.r.t. immutable.  This fixes all that.

I also discovered that userspace programs can write and create writable
memory mappings to active swap files.  This is extremely bad because
this allows anyone with write privileges to corrupt system memory.  The
final patch in this series closes off that hole, at least for swap
files.

If you're going to start using this mess, you probably ought to just
pull from my git trees, which are linked below.

This has been lightly tested with fstests.  Enjoy!
Comments and questions are, as always, welcome.

--D

kernel git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=immutable-files

fstests git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfstests-dev.git/log/?h=immutable-files

From: Darrick J. Wong <darrick.wong at oracle.com>

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."

Once the flag is set, it is enforced for quite a few file operations,
such as fallocate, fpunch, fzero, rm, touch, open, etc.  However, we
don't check for immutability when doing a write(), a PROT_WRITE mmap(),
a truncate(), or a write to a previously established mmap.

If a program has an open write fd to a file that the administrator
subsequently marks immutable, the program still can change the file
contents.  Weird!

The ability to write to an immutable file does not follow the manpage
promise that immutable files cannot be modified.  Worse yet it's
inconsistent with the behavior of other syscalls which don't allow
modifications of immutable files.

Therefore, add the necessary checks to make the write, mmap, and
truncate behavior consistent with what the manpage says and consistent
with other syscalls on filesystems which support IMMUTABLE.

Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
Reviewed-by: Jan Kara <jack at suse.cz>
---
 fs/attr.c    |   13 ++++++-------
 mm/filemap.c |    3 +++
 mm/memory.c  |    4 ++++
 mm/mmap.c    |    8 ++++++--
 4 files changed, 19 insertions(+), 9 deletions(-)


diff --git a/fs/attr.c b/fs/attr.c
index d22e8187477f..1fcfdcc5b367 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -233,19 +233,18 @@ int notify_change(struct dentry * dentry, struct iattr *
attr, struct inode **de
 
 	WARN_ON_ONCE(!inode_is_locked(inode));
 
-	if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID | ATTR_TIMES_SET)) {
-		if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
-			return -EPERM;
-	}
+	if (IS_IMMUTABLE(inode))
+		return -EPERM;
+
+	if ((ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID | ATTR_TIMES_SET))
&&
+	    IS_APPEND(inode))
+		return -EPERM;
 
 	/*
 	 * If utimes(2) and friends are called with times == NULL (or both
 	 * times are UTIME_NOW), then we need to check for write permission
 	 */
 	if (ia_valid & ATTR_TOUCH) {
-		if (IS_IMMUTABLE(inode))
-			return -EPERM;
-
 		if (!inode_owner_or_capable(inode)) {
 			error = inode_permission(inode, MAY_WRITE);
 			if (error)
diff --git a/mm/filemap.c b/mm/filemap.c
index aac71aef4c61..dad85e10f5f8 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2935,6 +2935,9 @@ inline ssize_t generic_write_checks(struct kiocb *iocb,
struct iov_iter *from)
 	loff_t count;
 	int ret;
 
+	if (IS_IMMUTABLE(inode))
+		return -EPERM;
+
 	if (!iov_iter_count(from))
 		return 0;
 
diff --git a/mm/memory.c b/mm/memory.c
index ddf20bd0c317..abf795277f36 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2235,6 +2235,10 @@ static vm_fault_t do_page_mkwrite(struct vm_fault *vmf)
 
 	vmf->flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;
 
+	if (vmf->vma->vm_file &&
+	    IS_IMMUTABLE(vmf->vma->vm_file->f_mapping->host))
+		return VM_FAULT_SIGBUS;
+
 	ret = vmf->vma->vm_ops->page_mkwrite(vmf);
 	/* Restore original flags so that caller is not surprised */
 	vmf->flags = old_flags;
diff --git a/mm/mmap.c b/mm/mmap.c
index 7e8c3e8ae75f..b3ebca2702bf 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1483,8 +1483,12 @@ unsigned long do_mmap(struct file *file, unsigned long
addr,
 		case MAP_SHARED_VALIDATE:
 			if (flags & ~flags_mask)
 				return -EOPNOTSUPP;
-			if ((prot&PROT_WRITE) && !(file->f_mode&FMODE_WRITE))
-				return -EACCES;
+			if (prot & PROT_WRITE) {
+				if (!(file->f_mode & FMODE_WRITE))
+					return -EACCES;
+				if (IS_IMMUTABLE(file->f_mapping->host))
+					return -EPERM;
+			}
 
 			/*
 			 * Make sure we don't allow writing to an append-only

From: Darrick J. Wong <darrick.wong at oracle.com>

When we're using FS_IOC_SETFLAGS to set the immutable flag on a file, we
need to ensure that userspace can't continue to write the file after the
file becomes immutable.  To make that happen, we have to flush all the
dirty pagecache pages to disk to ensure that we can fail a page fault on
a mmap'd region, wait for pending directio to complete, and hope the
caller locked out any new writes by holding the inode lock.

Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
---
 fs/inode.c         |   21 +++++++++++++++++++--
 include/linux/fs.h |   11 +++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)


diff --git a/fs/inode.c b/fs/inode.c
index f08711b34341..65a412af3ffb 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2193,7 +2193,8 @@ EXPORT_SYMBOL(current_time);
 
 /*
  * Generic function to check FS_IOC_SETFLAGS values and reject any invalid
- * configurations.
+ * configurations.  Once we're done, prepare the inode for whatever changes
+ * are coming down the pipeline.
  *
  * Note: the caller should be holding i_mutex, or else be sure that they have
  * exclusive access to the inode structure.
@@ -2201,6 +2202,8 @@ EXPORT_SYMBOL(current_time);
 int vfs_ioc_setflags_prepare(struct inode *inode, unsigned int oldflags,
 			     unsigned int flags)
 {
+	int ret;
+
 	/*
 	 * The IMMUTABLE and APPEND_ONLY flags can only be changed by
 	 * the relevant capability.
@@ -2211,7 +2214,21 @@ int vfs_ioc_setflags_prepare(struct inode *inode,
unsigned int oldflags,
 	    !capable(CAP_LINUX_IMMUTABLE))
 		return -EPERM;
 
-	return 0;
+	/*
+	 * Now that we're done checking the new flags, flush all pending IO and
+	 * dirty mappings before setting S_IMMUTABLE on an inode via
+	 * FS_IOC_SETFLAGS.  If the flush fails we'll clear the flag before
+	 * returning error.
+	 */
+	if (!S_ISREG(inode->i_mode) || IS_IMMUTABLE(inode) ||
+	    !(flags & FS_IMMUTABLE_FL))
+		return 0;
+
+	inode_set_flags(inode, S_IMMUTABLE, S_IMMUTABLE);
+	ret = inode_drain_writes(inode);
+	if (ret)
+		inode_set_flags(inode, 0, S_IMMUTABLE);
+	return ret;
 }
 EXPORT_SYMBOL(vfs_ioc_setflags_prepare);
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 91482ab4556a..0efe749de577 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3567,4 +3567,15 @@ static inline void simple_fill_fsxattr(struct fsxattr
*fa, __u32 xflags)
 	fa->fsx_xflags = xflags;
 }
 
+/*
+ * Flush file data before changing attributes.  Caller must hold any locks
+ * required to prevent further writes to this file until we're done setting
+ * flags.
+ */
+static inline int inode_drain_writes(struct inode *inode)
+{
+	inode_dio_wait(inode);
+	return filemap_write_and_wait(inode->i_mapping);
+}
+
 #endif /* _LINUX_FS_H */

From: Darrick J. Wong <darrick.wong at oracle.com>

When we're using FS_IOC_FSSETXATTR to set the immutable flag on a file,
we need to ensure that userspace can't continue to write the file after
the file becomes immutable.  To make that happen, we have to flush all
the dirty pagecache pages to disk to ensure that we can fail a page
fault on a mmap'd region, wait for pending directio to complete, and
hope the caller locked out any new writes by holding the inode lock.

XFS has more complex locking than other FSSETXATTR implementations so we
have to keep the checking and preparation code in different functions.

Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
---
 fs/btrfs/ioctl.c   |    2 +
 fs/ext4/ioctl.c    |    2 +
 fs/f2fs/file.c     |    2 +
 fs/inode.c         |   31 +++++++++++++++++++++++
 fs/xfs/xfs_ioctl.c |   71 +++++++++++++++++++++++++++++++++++++++-------------
 include/linux/fs.h |    3 ++
 6 files changed, 90 insertions(+), 21 deletions(-)


diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 3cd66efdb99d..aeffe3fd99c4 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -420,7 +420,7 @@ static int btrfs_ioctl_fssetxattr(struct file *file, void
__user *arg)
 
 	simple_fill_fsxattr(&old_fa,
 			    btrfs_inode_flags_to_xflags(binode->flags));
-	ret = vfs_ioc_fssetxattr_check(inode, &old_fa, &fa);
+	ret = vfs_ioc_fssetxattr_prepare(inode, &old_fa, &fa);
 	if (ret)
 		goto out_unlock;
 
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 566dfac28b3f..69810e59f89a 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1109,7 +1109,7 @@ long ext4_ioctl(struct file *filp, unsigned int cmd,
unsigned long arg)
 
 		inode_lock(inode);
 		ext4_fill_fsxattr(inode, &old_fa);
-		err = vfs_ioc_fssetxattr_check(inode, &old_fa, &fa);
+		err = vfs_ioc_fssetxattr_prepare(inode, &old_fa, &fa);
 		if (err)
 			goto out;
 		flags = (ei->i_flags & ~EXT4_FL_XFLAG_VISIBLE) |
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 8799468724f9..b47f22eb483e 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2825,7 +2825,7 @@ static int f2fs_ioc_fssetxattr(struct file *filp, unsigned
long arg)
 	inode_lock(inode);
 
 	f2fs_fill_fsxattr(inode, &old_fa);
-	err = vfs_ioc_fssetxattr_check(inode, &old_fa, &fa);
+	err = vfs_ioc_fssetxattr_prepare(inode, &old_fa, &fa);
 	if (err)
 		goto out;
 	flags = (fi->i_flags & ~F2FS_FL_XFLAG_VISIBLE) |
diff --git a/fs/inode.c b/fs/inode.c
index 65a412af3ffb..cf07378e5731 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2293,3 +2293,34 @@ int vfs_ioc_fssetxattr_check(struct inode *inode, const
struct fsxattr *old_fa,
 	return 0;
 }
 EXPORT_SYMBOL(vfs_ioc_fssetxattr_check);
+
+/*
+ * Generic function to check FS_IOC_FSSETXATTR values and reject any invalid
+ * configurations.  If none are found, flush all pending IO and dirty mappings
+ * before setting S_IMMUTABLE on an inode.  If the flush fails we'll clear
the
+ * flag before returning error.
+ *
+ * Note: the caller must hold whatever locks are necessary to block any other
+ * threads from starting a write to the file.
+ */
+int vfs_ioc_fssetxattr_prepare(struct inode *inode,
+			       const struct fsxattr *old_fa,
+			       struct fsxattr *fa)
+{
+	int ret;
+
+	ret = vfs_ioc_fssetxattr_check(inode, old_fa, fa);
+	if (ret)
+		return ret;
+
+	if (!S_ISREG(inode->i_mode) || IS_IMMUTABLE(inode) ||
+	    !(fa->fsx_xflags & FS_XFLAG_IMMUTABLE))
+		return 0;
+
+	inode_set_flags(inode, S_IMMUTABLE, S_IMMUTABLE);
+	ret = inode_drain_writes(inode);
+	if (ret)
+		inode_set_flags(inode, 0, S_IMMUTABLE);
+	return ret;
+}
+EXPORT_SYMBOL(vfs_ioc_fssetxattr_prepare);
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index fe29aa61293c..552f18554c48 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1057,6 +1057,30 @@ xfs_ioctl_setattr_xflags(
 	return 0;
 }
 
+/*
+ * If we're setting immutable on a regular file, we need to prevent new
writes.
+ * Once we've done that, we must wait for all the other writes to complete.
+ *
+ * The caller must use @join_flags to release the locks which are held on @ip
+ * regardless of return value.
+ */
+static int
+xfs_ioctl_setattr_drain_writes(
+	struct xfs_inode	*ip,
+	const struct fsxattr	*fa,
+	int			*join_flags)
+{
+	struct inode		*inode = VFS_I(ip);
+
+	if (!S_ISREG(inode->i_mode) || !(fa->fsx_xflags &
FS_XFLAG_IMMUTABLE))
+		return 0;
+
+	*join_flags = XFS_IOLOCK_EXCL | XFS_MMAPLOCK_EXCL;
+	xfs_ilock(ip, *join_flags);
+
+	return inode_drain_writes(inode);
+}
+
 /*
  * If we are changing DAX flags, we have to ensure the file is clean and any
  * cached objects in the address space are invalidated and removed. This
@@ -1064,6 +1088,9 @@ xfs_ioctl_setattr_xflags(
  * operation. The locks need to be held until the transaction has been
committed
  * so that the cache invalidation is atomic with respect to the DAX flag
  * manipulation.
+ *
+ * The caller must use @join_flags to release the locks which are held on @ip
+ * regardless of return value.
  */
 static int
 xfs_ioctl_setattr_dax_invalidate(
@@ -1075,8 +1102,6 @@ xfs_ioctl_setattr_dax_invalidate(
 	struct super_block	*sb = inode->i_sb;
 	int			error;
 
-	*join_flags = 0;
-
 	/*
 	 * It is only valid to set the DAX flag on regular files and
 	 * directories on filesystems where the block size is equal to the page
@@ -1102,21 +1127,15 @@ xfs_ioctl_setattr_dax_invalidate(
 		return 0;
 
 	/* lock, flush and invalidate mapping in preparation for flag change */
-	xfs_ilock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL);
-	error = filemap_write_and_wait(inode->i_mapping);
-	if (error)
-		goto out_unlock;
-	error = invalidate_inode_pages2(inode->i_mapping);
-	if (error)
-		goto out_unlock;
-
-	*join_flags = XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL;
-	return 0;
-
-out_unlock:
-	xfs_iunlock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL);
-	return error;
+	if (*join_flags == 0) {
+		*join_flags = XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL;
+		xfs_ilock(ip, *join_flags);
+		error = filemap_write_and_wait(inode->i_mapping);
+		if (error)
+			return error;
+	}
 
+	return invalidate_inode_pages2(inode->i_mapping);
 }
 
 /*
@@ -1325,6 +1344,12 @@ xfs_ioctl_setattr(
 			return code;
 	}
 
+	code = xfs_ioctl_setattr_drain_writes(ip, fa, &join_flags);
+	if (code) {
+		xfs_iunlock(ip, join_flags);
+		goto error_free_dquots;
+	}
+
 	/*
 	 * Changing DAX config may require inode locking for mapping
 	 * invalidation. These need to be held all the way to transaction commit
@@ -1333,8 +1358,10 @@ xfs_ioctl_setattr(
 	 * appropriately.
 	 */
 	code = xfs_ioctl_setattr_dax_invalidate(ip, fa, &join_flags);
-	if (code)
+	if (code) {
+		xfs_iunlock(ip, join_flags);
 		goto error_free_dquots;
+	}
 
 	tp = xfs_ioctl_setattr_get_trans(ip, join_flags);
 	if (IS_ERR(tp)) {
@@ -1484,6 +1511,12 @@ xfs_ioc_setxflags(
 	if (error)
 		return error;
 
+	error = xfs_ioctl_setattr_drain_writes(ip, &fa, &join_flags);
+	if (error) {
+		xfs_iunlock(ip, join_flags);
+		goto out_drop_write;
+	}
+
 	/*
 	 * Changing DAX config may require inode locking for mapping
 	 * invalidation. These need to be held all the way to transaction commit
@@ -1492,8 +1525,10 @@ xfs_ioc_setxflags(
 	 * appropriately.
 	 */
 	error = xfs_ioctl_setattr_dax_invalidate(ip, &fa, &join_flags);
-	if (error)
+	if (error) {
+		xfs_iunlock(ip, join_flags);
 		goto out_drop_write;
+	}
 
 	tp = xfs_ioctl_setattr_get_trans(ip, join_flags);
 	if (IS_ERR(tp)) {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0efe749de577..73a8bd789e36 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3560,6 +3560,9 @@ int vfs_ioc_setflags_prepare(struct inode *inode, unsigned
int oldflags,
 
 int vfs_ioc_fssetxattr_check(struct inode *inode, const struct fsxattr *old_fa,
 			     struct fsxattr *fa);
+int vfs_ioc_fssetxattr_prepare(struct inode *inode,
+			       const struct fsxattr *old_fa,
+			       struct fsxattr *fa);
 
 static inline void simple_fill_fsxattr(struct fsxattr *fa, __u32 xflags)
 {

From: Darrick J. Wong <darrick.wong at oracle.com>

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 at oracle.com>
---
 fs/inode.c |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)


diff --git a/fs/inode.c b/fs/inode.c
index cf07378e5731..4261c709e50e 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2214,6 +2214,14 @@ int vfs_ioc_setflags_prepare(struct inode *inode,
unsigned int oldflags,
 	    !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)
&&
+	    oldflags != flags)
+		return -EPERM;
+
 	/*
 	 * Now that we're done checking the new flags, flush all pending IO and
 	 * dirty mappings before setting S_IMMUTABLE on an inode via
@@ -2284,6 +2292,25 @@ 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 != fa->fsx_xflags)
+			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);

From: Darrick J. Wong <darrick.wong at oracle.com>

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 at oracle.com>
---
v2: use memcmp instead of open coding a bunch of checks
---
 fs/inode.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index cf07378e5731..31f694e405fe 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2214,6 +2214,14 @@ int vfs_ioc_setflags_prepare(struct inode *inode,
unsigned int oldflags,
 	    !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)
&&
+	    oldflags != flags)
+		return -EPERM;
+
 	/*
 	 * Now that we're done checking the new flags, flush all pending IO and
 	 * dirty mappings before setting S_IMMUTABLE on an inode via
@@ -2284,6 +2292,15 @@ 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) &&
+	    memcmp(fa, old_fa, offsetof(struct fsxattr, fsx_pad)))
+		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);