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>

diff --git a/fs/coredump.c b/fs/coredump.c
index b696dc2c220d146065e174574b311c02d696b725..a8f75640ac86ec2d29cd55a253b2c3c12c7bac9b 100644 (file)
--- a/fs/coredump.c
+++ b/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))