[PATCH 1/6] mm/fs: don't allow writes to immutable files

[Darrick J. Wong]

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."

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@xxxxxxxxxx>
---
 fs/attr.c    |   13 ++++++-------
 mm/filemap.c |    3 +++
 mm/memory.c  |    3 +++
 mm/mmap.c    |    8 ++++++--
 4 files changed, 18 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 df2006ba0cfa..65433c7ab1d8 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2940,6 +2940,9 @@ 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..4311cfdade90 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2235,6 +2235,9 @@ 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(file_inode(vmf->vma->vm_file)))
+		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..ac1e32205237 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_inode(file)))
+					return -EPERM;
+			}
 
 			/*
 			 * Make sure we don't allow writing to an append-only