[PATCH prototype] [2/8] Add support to override mmap exec write protection with O_FORCEWRITE

Andi Kleen
2008-03-18T01:10:21Z

pbitmaps need to write to files mapped as executable, so add
a way to bypass the normal write protection using a new O_FORCEWRITE
flag.

Right now the flag can be set from user space too. If have not
made up my mind if that is a good or a bad thing (I don't think
it has any real security implications). Probably it's more bad
than good.

Signed-off-by: Andi Kleen <andi@firstfloor.org>

---
fs/namei.c | 9 +++++++--
fs/open.c | 2 +-
include/asm-generic/fcntl.h | 4 ++++
include/linux/fs.h | 1 +
4 files changed, 13 insertions(+), 3 deletions(-)

Index: linux/fs/namei.c
===================================================================
--- linux.orig/fs/namei.c
+++ linux/fs/namei.c
@@ -334,10 +334,10 @@ int file_permission(struct file *file, i
* the inode->i_lock spinlock.
*/

-int get_write_access(struct inode * inode)
+int __get_write_access(struct inode * inode, int flags)
{
spin_lock(&inode->i_lock);
- if (atomic_read(&inode->i_writecount) < 0) {
+ if (atomic_read(&inode->i_writecount) < 0 && !(flags&O_FORCEWRITE)) {
spin_unlock(&inode->i_lock);
return -ETXTBSY;
}
@@ -347,6 +347,11 @@ int get_write_access(struct inode * inod
return 0;
}

+int get_write_access(struct inode * inode)
+{
+ return __get_write_access(inode, 0);
+}
+
int deny_write_access(struct file * file)
{
struct inode *inode = file->f_path.dentry->d_inode;
Index: linux/fs/open.c
===================================================================
--- linux.orig/fs/open.c
+++ linux/fs/open.c
@@ -742,7 +742,7 @@ static struct file *__dentry_open(struct
FMODE_PREAD | FMODE_PWRITE;
inode = dentry->d_inode;
if (f->f_mode & FMODE_WRITE) {
- error = get_write_access(inode);
+ error = __get_write_access(inode, f->f_flags & O_FORCEWRITE);
if (error)
goto cleanup_file;
}
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h
+++ linux/include/linux/fs.h
@@ -1720,6 +1720,7 @@ extern int generic_permission(struct ino
int (*check_acl)(struct inode *, int));

extern int get_write_access(struct inode *);
+extern int __get_write_access(struct inode *i, int flags);
extern int deny_write_access(struct file *);
static inline void put_write_access(struct inode * inode)
{
Index: linux/include/asm-generic/fcntl.h
===================================================================
--- linux.orig/include/asm-generic/fcntl.h
+++ linux/include/asm-generic/fcntl.h
@@ -54,6 +54,10 @@
#ifndef O_NDELAY
#define O_NDELAY O_NONBLOCK
#endif
+/* ignore ETXTBSY -- should probably hide it from user space */
+#ifndef O_FORCEWRITE
+#define O_FORCEWRITE 02000000
+#endif

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */