fs: Don't dump core if the corefile would become world-readable.

On a filesystem like vfat, all files are created with the same owner
and mode independent of who created the file. When a vfat filesystem
is mounted with root as owner of all files and read access for everyone,
root's processes left world-readable coredumps on it (but other
users' processes only left empty corefiles when given write access
because of the uid mismatch).

Given that the old behavior was inconsistent and insecure, I don't see
a problem with changing it. Now, all processes refuse to dump core unless
the resulting corefile will only be readable by their owner.
Signed-off-by: Jann Horn <jann@thejh.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/coredump.c

@@ -685,11 +685,15 @@ void do_coredump(const siginfo_t *siginfo)
 		if (!S_ISREG(inode->i_mode))
 			goto close_fail;
 		/*
-		 * Dont allow local users get cute and trick others to coredump
-		 * into their pre-created files.
+		 * Don't dump core if the filesystem changed owner or mode
+		 * of the file during file creation. This is an issue when
+		 * a process dumps core while its cwd is e.g. on a vfat
+		 * filesystem.
 		 */
 		if (!uid_eq(inode->i_uid, current_fsuid()))
 			goto close_fail;
+		if ((inode->i_mode & 0677) != 0600)
+			goto close_fail;
 		if (!(cprm.file->f_mode & FMODE_CAN_WRITE))
 			goto close_fail;
 		if (do_truncate(cprm.file->f_path.dentry, 0, 0, cprm.file))