| From 6cc6aafee135ba44ea748250d7d29b562ca190e3 Mon Sep 17 00:00:00 2001 |
| From: Colin Walters <walters@verbum.org> |
| Date: Fri, 4 Jan 2019 14:24:48 -0500 |
| Subject: [PATCH] backend: Compare PolkitUnixProcess uids for temporary |
| authorizations |
| |
| It turns out that the combination of `(pid, start time)` is not |
| enough to be unique. For temporary authorizations, we can avoid |
| separate users racing on pid reuse by simply comparing the uid. |
| |
| https://bugs.chromium.org/p/project-zero/issues/detail?id=1692 |
| |
| And the above original email report is included in full in a new comment. |
| |
| Reported-by: Jann Horn <jannh@google.com> |
| |
| Closes: https://gitlab.freedesktop.org/polkit/polkit/issues/75 |
| |
| CVE: CVE-2019-6133 |
| Upstream-Status: Backport [https://gitlab.freedesktop.org/polkit/polkit.git] |
| |
| Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com> |
| --- |
| src/polkit/polkitsubject.c | 2 + |
| src/polkit/polkitunixprocess.c | 71 ++++++++++++++++++- |
| .../polkitbackendinteractiveauthority.c | 39 +++++++++- |
| 3 files changed, 110 insertions(+), 2 deletions(-) |
| |
| diff --git a/src/polkit/polkitsubject.c b/src/polkit/polkitsubject.c |
| index d4c1182..ccabd0a 100644 |
| --- a/src/polkit/polkitsubject.c |
| +++ b/src/polkit/polkitsubject.c |
| @@ -99,6 +99,8 @@ polkit_subject_hash (PolkitSubject *subject) |
| * @b: A #PolkitSubject. |
| * |
| * Checks if @a and @b are equal, ie. represent the same subject. |
| + * However, avoid calling polkit_subject_equal() to compare two processes; |
| + * for more information see the `PolkitUnixProcess` documentation. |
| * |
| * This function can be used in e.g. g_hash_table_new(). |
| * |
| diff --git a/src/polkit/polkitunixprocess.c b/src/polkit/polkitunixprocess.c |
| index b02b258..78d7251 100644 |
| --- a/src/polkit/polkitunixprocess.c |
| +++ b/src/polkit/polkitunixprocess.c |
| @@ -51,7 +51,10 @@ |
| * @title: PolkitUnixProcess |
| * @short_description: Unix processs |
| * |
| - * An object for representing a UNIX process. |
| + * An object for representing a UNIX process. NOTE: This object as |
| + * designed is now known broken; a mechanism to exploit a delay in |
| + * start time in the Linux kernel was identified. Avoid |
| + * calling polkit_subject_equal() to compare two processes. |
| * |
| * To uniquely identify processes, both the process id and the start |
| * time of the process (a monotonic increasing value representing the |
| @@ -66,6 +69,72 @@ |
| * polkit_unix_process_new_for_owner() with trusted data. |
| */ |
| |
| +/* See https://gitlab.freedesktop.org/polkit/polkit/issues/75 |
| + |
| + But quoting the original email in full here to ensure it's preserved: |
| + |
| + From: Jann Horn <jannh@google.com> |
| + Subject: [SECURITY] polkit: temporary auth hijacking via PID reuse and non-atomic fork |
| + Date: Wednesday, October 10, 2018 5:34 PM |
| + |
| +When a (non-root) user attempts to e.g. control systemd units in the system |
| +instance from an active session over DBus, the access is gated by a polkit |
| +policy that requires "auth_admin_keep" auth. This results in an auth prompt |
| +being shown to the user, asking the user to confirm the action by entering the |
| +password of an administrator account. |
| + |
| +After the action has been confirmed, the auth decision for "auth_admin_keep" is |
| +cached for up to five minutes. Subject to some restrictions, similar actions can |
| +then be performed in this timespan without requiring re-auth: |
| + |
| + - The PID of the DBus client requesting the new action must match the PID of |
| + the DBus client requesting the old action (based on SO_PEERCRED information |
| + forwarded by the DBus daemon). |
| + - The "start time" of the client's PID (as seen in /proc/$pid/stat, field 22) |
| + must not have changed. The granularity of this timestamp is in the |
| + millisecond range. |
| + - polkit polls every two seconds whether a process with the expected start time |
| + still exists. If not, the temporary auth entry is purged. |
| + |
| +Without the start time check, this would obviously be buggy because an attacker |
| +could simply wait for the legitimate client to disappear, then create a new |
| +client with the same PID. |
| + |
| +Unfortunately, the start time check is bypassable because fork() is not atomic. |
| +Looking at the source code of copy_process() in the kernel: |
| + |
| + p->start_time = ktime_get_ns(); |
| + p->real_start_time = ktime_get_boot_ns(); |
| + [...] |
| + retval = copy_thread_tls(clone_flags, stack_start, stack_size, p, tls); |
| + if (retval) |
| + goto bad_fork_cleanup_io; |
| + |
| + if (pid != &init_struct_pid) { |
| + pid = alloc_pid(p->nsproxy->pid_ns_for_children); |
| + if (IS_ERR(pid)) { |
| + retval = PTR_ERR(pid); |
| + goto bad_fork_cleanup_thread; |
| + } |
| + } |
| + |
| +The ktime_get_boot_ns() call is where the "start time" of the process is |
| +recorded. The alloc_pid() call is where a free PID is allocated. In between |
| +these, some time passes; and because the copy_thread_tls() call between them can |
| +access userspace memory when sys_clone() is invoked through the 32-bit syscall |
| +entry point, an attacker can even stall the kernel arbitrarily long at this |
| +point (by supplying a pointer into userspace memory that is associated with a |
| +userfaultfd or is backed by a custom FUSE filesystem). |
| + |
| +This means that an attacker can immediately call sys_clone() when the victim |
| +process is created, often resulting in a process that has the exact same start |
| +time reported in procfs; and then the attacker can delay the alloc_pid() call |
| +until after the victim process has died and the PID assignment has cycled |
| +around. This results in an attacker process that polkit can't distinguish from |
| +the victim process. |
| +*/ |
| + |
| + |
| /** |
| * PolkitUnixProcess: |
| * |
| diff --git a/src/polkitbackend/polkitbackendinteractiveauthority.c b/src/polkitbackend/polkitbackendinteractiveauthority.c |
| index a1630b9..80e8141 100644 |
| --- a/src/polkitbackend/polkitbackendinteractiveauthority.c |
| +++ b/src/polkitbackend/polkitbackendinteractiveauthority.c |
| @@ -3031,6 +3031,43 @@ temporary_authorization_store_free (TemporaryAuthorizationStore *store) |
| g_free (store); |
| } |
| |
| +/* See the comment at the top of polkitunixprocess.c */ |
| +static gboolean |
| +subject_equal_for_authz (PolkitSubject *a, |
| + PolkitSubject *b) |
| +{ |
| + if (!polkit_subject_equal (a, b)) |
| + return FALSE; |
| + |
| + /* Now special case unix processes, as we want to protect against |
| + * pid reuse by including the UID. |
| + */ |
| + if (POLKIT_IS_UNIX_PROCESS (a) && POLKIT_IS_UNIX_PROCESS (b)) { |
| + PolkitUnixProcess *ap = (PolkitUnixProcess*)a; |
| + int uid_a = polkit_unix_process_get_uid ((PolkitUnixProcess*)a); |
| + PolkitUnixProcess *bp = (PolkitUnixProcess*)b; |
| + int uid_b = polkit_unix_process_get_uid ((PolkitUnixProcess*)b); |
| + |
| + if (uid_a != -1 && uid_b != -1) |
| + { |
| + if (uid_a == uid_b) |
| + { |
| + return TRUE; |
| + } |
| + else |
| + { |
| + g_printerr ("denying slowfork; pid %d uid %d != %d!\n", |
| + polkit_unix_process_get_pid (ap), |
| + uid_a, uid_b); |
| + return FALSE; |
| + } |
| + } |
| + /* Fall through; one of the uids is unset so we can't reliably compare */ |
| + } |
| + |
| + return TRUE; |
| +} |
| + |
| static gboolean |
| temporary_authorization_store_has_authorization (TemporaryAuthorizationStore *store, |
| PolkitSubject *subject, |
| @@ -3073,7 +3110,7 @@ temporary_authorization_store_has_authorization (TemporaryAuthorizationStore *st |
| TemporaryAuthorization *authorization = l->data; |
| |
| if (strcmp (action_id, authorization->action_id) == 0 && |
| - polkit_subject_equal (subject_to_use, authorization->subject)) |
| + subject_equal_for_authz (subject_to_use, authorization->subject)) |
| { |
| ret = TRUE; |
| if (out_tmp_authz_id != NULL) |
| -- |
| 2.20.1 |
| |