fs: make dumpable=2 require fully qualified path
When the suid_dumpable sysctl is set to "2", and there is no core dump
pipe defined in the core_pattern sysctl, a local user can cause core files
to be written to root-writable directories, potentially with
user-controlled content.
This means an admin can unknowningly reintroduce a variation of
CVE-2006-2451, allowing local users to gain root privileges.
$ cat /proc/sys/fs/suid_dumpable
2
$ cat /proc/sys/kernel/core_pattern
core
$ ulimit -c unlimited
$ cd /
$ ls -l core
ls: cannot access core: No such file or directory
$ touch core
touch: cannot touch `core': Permission denied
$ OHAI="evil-string-here" ping localhost >/dev/null 2>&1 &
$ pid=$!
$ sleep 1
$ kill -SEGV $pid
$ ls -l core
-rw------- 1 root kees 458752 Jun 21 11:35 core
$ sudo strings core | grep evil
OHAI=evil-string-here
While cron has been fixed to abort reading a file when there is any
parse error, there are still other sensitive directories that will read
any file present and skip unparsable lines.
Instead of introducing a suid_dumpable=3 mode and breaking all users of
mode 2, this only disables the unsafe portion of mode 2 (writing to disk
via relative path). Most users of mode 2 (e.g. Chrome OS) already use
a core dump pipe handler, so this change will not break them. For the
situations where a pipe handler is not defined but mode 2 is still
active, crash dumps will only be written to fully qualified paths. If a
relative path is defined (e.g. the default "core" pattern), dump
attempts will trigger a printk yelling about the lack of a fully
qualified path.
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alan Cox <alan@linux.intel.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/exec.c b/fs/exec.c
index e95aeed..95aae3f 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -2111,6 +2111,7 @@
int retval = 0;
int flag = 0;
int ispipe;
+ bool need_nonrelative = false;
static atomic_t core_dump_count = ATOMIC_INIT(0);
struct coredump_params cprm = {
.signr = signr,
@@ -2136,14 +2137,16 @@
if (!cred)
goto fail;
/*
- * We cannot trust fsuid as being the "true" uid of the
- * process nor do we know its entire history. We only know it
- * was tainted so we dump it as root in mode 2.
+ * We cannot trust fsuid as being the "true" uid of the process
+ * nor do we know its entire history. We only know it was tainted
+ * so we dump it as root in mode 2, and only into a controlled
+ * environment (pipe handler or fully qualified path).
*/
if (__get_dumpable(cprm.mm_flags) == 2) {
/* Setuid core dump mode */
flag = O_EXCL; /* Stop rewrite attacks */
cred->fsuid = GLOBAL_ROOT_UID; /* Dump root private */
+ need_nonrelative = true;
}
retval = coredump_wait(exit_code, &core_state);
@@ -2223,6 +2226,14 @@
if (cprm.limit < binfmt->min_coredump)
goto fail_unlock;
+ if (need_nonrelative && cn.corename[0] != '/') {
+ printk(KERN_WARNING "Pid %d(%s) can only dump core "\
+ "to fully qualified path!\n",
+ task_tgid_vnr(current), current->comm);
+ printk(KERN_WARNING "Skipping core dump\n");
+ goto fail_unlock;
+ }
+
cprm.file = filp_open(cn.corename,
O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE | flag,
0600);