This useful behaviour is implemented for most filesystems,
and wants to be implemented for every filesystem, quoth ref:
  There is general agreement that we should standardize all file systems
  to prevent modifications even for files that were opened at the time
  the immutable flag is set.  Eventually, a change to enforce this at
  the VFS layer should be landing in mainline.

References: commit 02b016ca7f99 ("ext4: enforce the immutable flag on
 open files")
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
---
/ext4# uname -a
Linux tarta 6.18.0-10912-g416f99c3b16f-dirty #1 SMP PREEMPT_DYNAMIC Sat Dec  6 12:14:41 CET 2025 x86_64 GNU/Linux
/ext4# while sleep 1; do echo $$; done > file &
[1] 262
/ext4# chattr +i file
/ext4# sh: line 25: echo: write error: Operation not permitted
sh: line 25: echo: write error: Operation not permitted
sh: line 25: echo: write error: Operation not permitted
sh: line 25: echo: write error: Operation not permitted
fg
while sleep 1; do
    echo $$;
done > file
^C
/ext4# mount -t tmpfs tmpfs /tmp
/ext4# cd /tmp
/tmp# while sleep 1; do echo $$; done > file &
[1] 284
/tmp# chattr +i file
/tmp# sh: line 35: echo: write error: Operation not permitted
sh: line 35: echo: write error: Operation not permitted
sh: line 35: echo: write error: Operation not permitted

 mm/filemap.c | 10 ++++++++--
 mm/shmem.c   | 12 ++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index ebd75684cb0a..0b0d5cfbcd44 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3945,12 +3945,18 @@ EXPORT_SYMBOL(filemap_map_pages);
 
 vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf)
 {
-	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
+	struct file *file = vmf->vma->vm_file;
+	struct address_space *mapping = file->f_mapping;
 	struct folio *folio = page_folio(vmf->page);
 	vm_fault_t ret = VM_FAULT_LOCKED;
 
+	if (unlikely(IS_IMMUTABLE(file_inode(file)))) {
+		ret = VM_FAULT_SIGBUS;
+		goto out;
+	}
+
 	sb_start_pagefault(mapping->host->i_sb);
-	file_update_time(vmf->vma->vm_file);
+	file_update_time(file);
 	folio_lock(folio);
 	if (folio->mapping != mapping) {
 		folio_unlock(folio);
diff --git a/mm/shmem.c b/mm/shmem.c
index d578d8e765d7..5d3fbf4efb3d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1294,6 +1294,14 @@ static int shmem_setattr(struct mnt_idmap *idmap,
 	bool update_mtime = false;
 	bool update_ctime = true;
 
+	if (unlikely(IS_IMMUTABLE(inode)))
+		return -EPERM;
+
+	if (unlikely(IS_APPEND(inode) &&
+		     (attr->ia_valid & (ATTR_MODE | ATTR_UID |
+					ATTR_GID | ATTR_TIMES_SET))))
+		return -EPERM;
+
 	error = setattr_prepare(idmap, dentry, attr);
 	if (error)
 		return error;
@@ -3475,6 +3483,10 @@ static ssize_t shmem_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	ret = generic_write_checks(iocb, from);
 	if (ret <= 0)
 		goto unlock;
+	if (unlikely(IS_IMMUTABLE(inode))) {
+		ret = -EPERM;
+		goto unlock;
+	}
 	ret = file_remove_privs(file);
 	if (ret)
 		goto unlock;
-- 
2.39.5