execve strangeness

PATCH Andrew Morton 1 participant 1 message Jul 1, 2001

Subsystems (2)

VFS exec and binfmt

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Andrew Morton

author

2001-07-01 16:12 UTC #0 lore raw

Try this, as root:

[root@mnm akpm]# /var/log/messages
bash: /var/log/messages: Text file busy

Strange return value, that.

It happens because vfs_permission() sees CAP_DAC_OVERRIDE and returns "yes" on a file which has no `x' bits set. Then open_exec() falls through to deny_write_access() which sees that the file is open for writing.

If the file is _not_ open for writing then the "WTF" test in prepare_binprm() is what stops us from executing the file. So the test there is definitely needed.

Moving the "WTF" test into open_exec() definitely fixes things up, but I think the real bug is in vfs_permission().

fs/exec.c +6 -1

@@ -349,6 +349,8 @@
 		file = ERR_PTR(-EACCES);
 		if (!IS_NOEXEC(inode) && S_ISREG(inode->i_mode)) {
 			int err = permission(inode, MAY_EXEC);
+			if (!err && !(inode->i_mode & 0111))
+				err = -EACCES;
 			file = ERR_PTR(err);
 			if (!err) {
 				file = dentry_open(nd.dentry, nd.mnt, O_RDONLY);

@@ -606,7 +608,10 @@
 	struct inode * inode = bprm->file->f_dentry->d_inode;
 
 	mode = inode->i_mode;
-	/* Huh? We had already checked for MAY_EXEC, WTF do we check this? */
+	/*
+	 * Check execute perms again - if the caller has CAP_DAC_OVERRIDE,
+	 * vfs_permission lets a non-executable through
+	 */
 	if (!(mode & 0111))	/* with at least _one_ execute bit set */
 		return -EACCES;
 	if (bprm->file->f_op == NULL)