Re: [PATCH RFC] core not owned by euid, mm->dumpable

[Peter Waechtler]

Am Montag, 5. April 2004 10:27 schrieb Peter Waechtler:
> With the current behavior typical server programs that switch euid
> don't dump core. This is done "for security".
> If the core file pattern exists the dump is written to but readable
> for the file owner (not necessarily root - don't argue: only chdir into
> dirs that only root can write to: think of file servers like samba)
>
> The patch below addresses this (but still not perfect).
> What I would like to see: instead of mm->dumpable=0 when calling seteuid()
> something like mm->dumpAs=root and making sure that the core is owned by
> root mode 600
>
> I could install a sighandler that calls prctl(PR_SET_DUMPABLE,1,0,0,0),
> switch euid to root, but still the formerly placed core is owned by evil
> user :(
> Then I could read the core pattern and unlink such a file - and all this
> just for the Linux platform...
>

Unlink a previously existing core.
I tried on solaris - I find the behavior (tunable with coreadm) perfect:
Switched uid? -> create as root (unlink a previously and potentially opened
core first: yes it got a new inode) then creat(O_EXCL,0600)

--- fs/namei.c.orig	2004-04-05 13:44:30.317084264 +0200
+++ fs/namei.c	2004-04-05 13:45:40.668389240 +0200
@@ -1694,24 +1694,13 @@
 	return error;
 }
 
-/*
- * Make sure that the actual truncation of the file will occur outside its
- * directory's i_sem.  Truncate can take a long time if there is a lot of
- * writeout happening, and we don't want to prevent access to the directory
- * while waiting on the I/O.
- */
-asmlinkage long sys_unlink(const char __user * pathname)
+int do_unlink(char *name)
 {
 	int error = 0;
-	char * name;
 	struct dentry *dentry;
 	struct nameidata nd;
 	struct inode *inode = NULL;
 
-	name = getname(pathname);
-	if(IS_ERR(name))
-		return PTR_ERR(name);
-
 	error = path_lookup(name, LOOKUP_PARENT, &nd);
 	if (error)
 		goto exit;
@@ -1736,8 +1725,6 @@
 exit1:
 	path_release(&nd);
 exit:
-	putname(name);
-
 	if (inode)
 		iput(inode);	/* truncate the inode here */
 	return error;
@@ -1748,6 +1735,25 @@
 	goto exit2;
 }
 
+/*
+ * Make sure that the actual truncation of the file will occur outside its
+ * directory's i_sem.  Truncate can take a long time if there is a lot of
+ * writeout happening, and we don't want to prevent access to the directory
+ * while waiting on the I/O.
+ */
+asmlinkage long sys_unlink(const char __user * pathname)
+{
+	char * name;
+
+	name = getname(pathname);
+	if(IS_ERR(name))
+		return PTR_ERR(name);
+
+	error = do_unlink(name);
+	putname(name);
+	return error;
+}
+
 int vfs_symlink(struct inode *dir, struct dentry *dentry, const char *oldname)
 {
 	int error = may_create(dir, dentry, NULL);
--- fs/exec.c.orig	2004-04-05 09:41:31.134456912 +0200
+++ fs/exec.c	2004-04-05 13:50:17.093366256 +0200
@@ -1387,9 +1387,14 @@
 		goto fail_unlock;
 
  	format_corename(corename, core_pattern, signr);
-	file = filp_open(corename, O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE, 0600);
-	if (IS_ERR(file))
-		goto fail_unlock;
+	file = filp_open(corename, O_CREAT | O_EXCL | 2 | O_NOFOLLOW | O_LARGEFILE, 0600);
+	if (IS_ERR(file)){
+		if (do_unlink(corename))
+			goto fail_unlock;
+		file = filp_open(corename, O_CREAT | O_EXCL | 2 | O_NOFOLLOW | O_LARGEFILE, 0600);
+		if (IS_ERR(file))
+			goto fail_unlock;
+	}
 	inode = file->f_dentry->d_inode;
 	if (inode->i_nlink > 1)
 		goto close_fail;	/* multiple links - don't dump */
--- include/linux/fs.h.orig	2004-04-05 09:42:52.205132296 +0200
+++ include/linux/fs.h	2004-04-05 13:50:46.119953544 +0200
@@ -1253,6 +1253,7 @@
 
 extern int open_namei(const char *, int, int, struct nameidata *);
 extern int may_open(struct nameidata *, int, int);
+extern int do_unlink(char *);
 
 extern int kernel_read(struct file *, unsigned long, char *, unsigned long);
 extern struct file * open_exec(const char *);