From nobody Wed Nov 27 08:45:57 2024 Received: from smtp-42a9.mail.infomaniak.ch (smtp-42a9.mail.infomaniak.ch [84.16.66.169]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C95431D015C for ; Fri, 11 Oct 2024 18:44:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=84.16.66.169 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728672291; cv=none; b=cGCrDfnp5sjerYKs55ZgPsCBu3fJHPoxwoIHbOvhZAABXKcRwiKLCnApDUN1DB2MJ+GJjod7KOlz7GpaigQ6EzhcYOxMTWgPd96qDR2R1FBZoSlTO9a7bec8hgjNHAHehdZdObwpA4YnrWOhYrEMBC1cxdv8Nx86rOxinMrf1BI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728672291; c=relaxed/simple; bh=qE4IkN8vBtssZGOjD+An+UPa82UBNAMueF7vXlqmgYA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=PB3nzb4xQYMV9oEEQ53XvdPe6NMWeKdkpWZI7/a45a+rQjBTlMmt4a2qkqNNzZXobBp/Jf3/s/PWg6E1qGvkzfflNV/IfEyTnyd+W2IMtcaEaieUpEjGA5fGVXNwMpNNOMiW8JnPr44PRt4tbwGZKrEEDwGiZ24HK61d5XTqnX0= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net; spf=pass smtp.mailfrom=digikod.net; dkim=pass (1024-bit key) header.d=digikod.net header.i=@digikod.net header.b=JhxETcDE; arc=none smtp.client-ip=84.16.66.169 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=digikod.net Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=digikod.net header.i=@digikod.net header.b="JhxETcDE" Received: from smtp-4-0000.mail.infomaniak.ch (unknown [IPv6:2001:1600:7:10:40ca:feff:fe05:0]) by smtp-4-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4XQFsg173lzKs4; Fri, 11 Oct 2024 20:44:39 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=digikod.net; s=20191114; t=1728672279; bh=oIubVU30zs/16E0Q2LZFubxJE+lcp6sykCdBAfggw8w=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=JhxETcDEKcKQm8KdwOEem6f79VAU1q6V6YaDdpiHLJeduKLLF+iOGoAmqTHH3kke6 Lwsk5uzLDQZDsOOn0K+Fyws5sSIqUc5SI+YpOXAXUo2V49SdpepkrTuOplU4uqfOE/ /FuTFg4vipnG0LrXRox2jJ0cvA+qrjVGdvAsWcrw= Received: from unknown by smtp-4-0000.mail.infomaniak.ch (Postfix) with ESMTPA id 4XQFsd69Yhz5qs; Fri, 11 Oct 2024 20:44:37 +0200 (CEST) From: =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= To: Al Viro , Christian Brauner , Kees Cook , Linus Torvalds , Paul Moore , Serge Hallyn , Theodore Ts'o Cc: =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= , Adhemerval Zanella Netto , Alejandro Colomar , Aleksa Sarai , Andrew Morton , Andy Lutomirski , Arnd Bergmann , Casey Schaufler , Christian Heimes , Dmitry Vyukov , Elliott Hughes , Eric Biggers , Eric Chiang , Fan Wu , Florian Weimer , Geert Uytterhoeven , James Morris , Jan Kara , Jann Horn , Jeff Xu , Jonathan Corbet , Jordan R Abrahams , Lakshmi Ramasubramanian , Luca Boccassi , Luis Chamberlain , "Madhavan T . Venkataraman" , Matt Bobrowski , Matthew Garrett , Matthew Wilcox , Miklos Szeredi , Mimi Zohar , Nicolas Bouchinet , Scott Shell , Shuah Khan , Stephen Rothwell , Steve Dower , Steve Grubb , Thibaut Sautereau , Vincent Strubel , Xiaoming Ni , Yin Fengwei , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, Andy Lutomirski Subject: [PATCH v20 2/6] security: Add EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits Date: Fri, 11 Oct 2024 20:44:18 +0200 Message-ID: <20241011184422.977903-3-mic@digikod.net> In-Reply-To: <20241011184422.977903-1-mic@digikod.net> References: <20241011184422.977903-1-mic@digikod.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Infomaniak-Routing: alpha The new SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and their *_LOCKED counterparts are designed to be set by processes setting up an execution environment, such as a user session, a container, or a security sandbox. Unlike other securebits, these ones can be set by unprivileged processes. Like seccomp filters or Landlock domains, the securebits are inherited across processes. When SECBIT_EXEC_RESTRICT_FILE is set, programs interpreting code should control executable resources according to execveat(2) + AT_CHECK (see previous commit). When SECBIT_EXEC_DENY_INTERACTIVE is set, a process should deny execution of user interactive commands (which excludes executable regular files). Being able to configure each of these securebits enables system administrators or owner of image containers to gradually validate the related changes and to identify potential issues (e.g. with interpreter or audit logs). It should be noted that unlike other security bits, the SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE bits are dedicated to user space willing to restrict itself. Because of that, they only make sense in the context of a trusted environment (e.g. sandbox, container, user session, full system) where the process changing its behavior (according to these bits) and all its parent processes are trusted. Otherwise, any parent process could just execute its own malicious code (interpreting a script or not), or even enforce a seccomp filter to mask these bits. Such a secure environment can be achieved with an appropriate access control (e.g. mount's noexec option, file access rights, LSM policy) and an enlighten ld.so checking that libraries are allowed for execution e.g., to protect against illegitimate use of LD_PRELOAD. Ptrace restrictions according to these securebits would not make sense because of the processes' trust assumption. Scripts may need some changes to deal with untrusted data (e.g. stdin, environment variables), but that is outside the scope of the kernel. See chromeOS's documentation about script execution control and the related threat model: https://www.chromium.org/chromium-os/developer-library/guides/security/noex= ec-shell-scripts/ Cc: Al Viro Cc: Andy Lutomirski Cc: Christian Brauner Cc: Kees Cook Cc: Paul Moore Cc: Serge Hallyn Signed-off-by: Micka=C3=ABl Sala=C3=BCn Link: https://lore.kernel.org/r/20241011184422.977903-3-mic@digikod.net Reviewed-by: Serge Hallyn --- Changes since v19: * Replace SECBIT_SHOULD_EXEC_CHECK and SECBIT_SHOULD_EXEC_RESTRICT with SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE: https://lore.kernel.org/all/20240710.eiKohpa4Phai@digikod.net/ * Remove the ptrace restrictions, suggested by Andy. * Improve documentation according to the discussion with Jeff. New design since v18: https://lore.kernel.org/r/20220104155024.48023-3-mic@digikod.net --- include/uapi/linux/securebits.h | 113 +++++++++++++++++++++++++++++++- security/commoncap.c | 29 ++++++-- 2 files changed, 135 insertions(+), 7 deletions(-) diff --git a/include/uapi/linux/securebits.h b/include/uapi/linux/securebit= s.h index d6d98877ff1a..351b6ecefc76 100644 --- a/include/uapi/linux/securebits.h +++ b/include/uapi/linux/securebits.h @@ -52,10 +52,121 @@ #define SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED \ (issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE_LOCKED)) =20 +/* + * The SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE securebi= ts + * are intended for script interpreters and dynamic linkers to enforce a + * consistent execution security policy handled by the kernel. + * + * Whether an interpreter should check these securebits or not depends on = the + * security risk of running malicious scripts with respect to the execution + * environment, and whether the kernel can check if a script is trustworth= y or + * not. For instance, Python scripts running on a server can use arbitrary + * syscalls and access arbitrary files. Such interpreters should then be + * enlighten to use these securebits and let users define their security + * policy. However, a JavaScript engine running in a web browser should + * already be sandboxed and then should not be able to harm the user's + * environment. + * + * When SECBIT_EXEC_RESTRICT_FILE is set, a process should only interpret = or + * execute a file if a call to execveat(2) with the related file descripto= r and + * the AT_CHECK flag succeed. + * + * This secure bit may be set by user session managers, service managers, + * container runtimes, sandboxer tools... Except for test environments, t= he + * related SECBIT_EXEC_RESTRICT_FILE_LOCKED bit should also be set. + * + * Programs should only enforce consistent restrictions according to the + * securebits but without relying on any other user-controlled configurati= on. + * Indeed, the use case for these securebits is to only trust executable c= ode + * vetted by the system configuration (through the kernel), so we should be + * careful to not let untrusted users control this configuration. + * + * However, script interpreters may still use user configuration such as + * environment variables as long as it is not a way to disable the secureb= its + * checks. For instance, the PATH and LD_PRELOAD variables can be set by a + * script's caller. Changing these variables may lead to unintended code + * executions, but only from vetted executable programs, which is OK. For= this + * to make sense, the system should provide a consistent security policy to + * avoid arbitrary code execution e.g., by enforcing a write xor execute + * policy. + * + * SECBIT_EXEC_RESTRICT_FILE is complementary and should also be checked. + */ +#define SECURE_EXEC_RESTRICT_FILE 8 +#define SECURE_EXEC_RESTRICT_FILE_LOCKED 9 /* make bit-8 immutable */ + +#define SECBIT_EXEC_RESTRICT_FILE (issecure_mask(SECURE_EXEC_RESTRICT_FILE= )) +#define SECBIT_EXEC_RESTRICT_FILE_LOCKED \ + (issecure_mask(SECURE_EXEC_RESTRICT_FILE_LOCKED)) + +/* + * When SECBIT_EXEC_DENY_INTERACTIVE is set, a process should never interp= ret + * interactive user commands (e.g. scripts). However, if such commands are + * passed through a file descriptor (e.g. stdin), its content should be + * interpreted if a call to execveat(2) with the related file descriptor a= nd + * the AT_CHECK flag succeed. + * + * For instance, script interpreters called with a script snippet as argum= ent + * should always deny such execution if SECBIT_EXEC_DENY_INTERACTIVE is se= t. + * + * This secure bit may be set by user session managers, service managers, + * container runtimes, sandboxer tools... Except for test environments, t= he + * related SECBIT_EXEC_DENY_INTERACTIVE_LOCKED bit should also be set. + * + * See the SECBIT_EXEC_RESTRICT_FILE documentation. + * + * Here is the expected behavior for a script interpreter according to + * combination of any exec securebits: + * + * 1. SECURE_EXEC_RESTRICT_FILE=3D0 SECURE_EXEC_DENY_INTERACTIVE=3D0 (defa= ult) + * Always interpret scripts, and allow arbitrary user commands. + * =3D> No threat, everyone and everything is trusted, but we can get a= head of + * potential issues thanks to the call to execveat with AT_CHECK whi= ch + * should always be performed but ignored by the script interpreter. + * Indeed, this check is still important to enable systems administr= ators + * to verify requests (e.g. with audit) and prepare for migration to= a + * secure mode. + * + * 2. SECURE_EXEC_RESTRICT_FILE=3D1 SECURE_EXEC_DENY_INTERACTIVE=3D0 + * Deny script interpretation if they are not executable, but allow + * arbitrary user commands. + * =3D> The threat is (potential) malicious scripts run by trusted (and= not + * fooled) users. That can protect against unintended script execut= ions + * (e.g. sh /tmp/*.sh). This makes sense for (semi-restricted) user + * sessions. + * + * 3. SECURE_EXEC_RESTRICT_FILE=3D0 SECURE_EXEC_DENY_INTERACTIVE=3D1 + * Always interpret scripts, but deny arbitrary user commands. + * =3D> This use case may be useful for secure services (i.e. without + * interactive user session) where scripts' integrity is verified (e= .g. + * with IMA/EVM or dm-verity/IPE) but where access rights might not = be + * ready yet. Indeed, arbitrary interactive commands would be much = more + * difficult to check. + * + * 4. SECURE_EXEC_RESTRICT_FILE=3D1 SECURE_EXEC_DENY_INTERACTIVE=3D1 + * Deny script interpretation if they are not executable, and also deny + * any arbitrary user commands. + * =3D> The threat is malicious scripts run by untrusted users (but tru= sted + * code). This makes sense for system services that may only execute + * trusted scripts. + */ +#define SECURE_EXEC_DENY_INTERACTIVE 10 +#define SECURE_EXEC_DENY_INTERACTIVE_LOCKED 11 /* make bit-10 immutable */ + +#define SECBIT_EXEC_DENY_INTERACTIVE \ + (issecure_mask(SECURE_EXEC_DENY_INTERACTIVE)) +#define SECBIT_EXEC_DENY_INTERACTIVE_LOCKED \ + (issecure_mask(SECURE_EXEC_DENY_INTERACTIVE_LOCKED)) + #define SECURE_ALL_BITS (issecure_mask(SECURE_NOROOT) | \ issecure_mask(SECURE_NO_SETUID_FIXUP) | \ issecure_mask(SECURE_KEEP_CAPS) | \ - issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE)) + issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE) | \ + issecure_mask(SECURE_EXEC_RESTRICT_FILE) | \ + issecure_mask(SECURE_EXEC_DENY_INTERACTIVE)) #define SECURE_ALL_LOCKS (SECURE_ALL_BITS << 1) =20 +#define SECURE_ALL_UNPRIVILEGED (issecure_mask(SECURE_EXEC_RESTRICT_FILE) = | \ + issecure_mask(SECURE_EXEC_DENY_INTERACTIVE)) + #endif /* _UAPI_LINUX_SECUREBITS_H */ diff --git a/security/commoncap.c b/security/commoncap.c index cefad323a0b1..52ea01acb453 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -1302,21 +1302,38 @@ int cap_task_prctl(int option, unsigned long arg2, = unsigned long arg3, & (old->securebits ^ arg2)) /*[1]*/ || ((old->securebits & SECURE_ALL_LOCKS & ~arg2)) /*[2]*/ || (arg2 & ~(SECURE_ALL_LOCKS | SECURE_ALL_BITS)) /*[3]*/ - || (cap_capable(current_cred(), - current_cred()->user_ns, - CAP_SETPCAP, - CAP_OPT_NONE) !=3D 0) /*[4]*/ /* * [1] no changing of bits that are locked * [2] no unlocking of locks * [3] no setting of unsupported bits - * [4] doing anything requires privilege (go read about - * the "sendmail capabilities bug") */ ) /* cannot change a locked bit */ return -EPERM; =20 + /* + * Doing anything requires privilege (go read about the + * "sendmail capabilities bug"), except for unprivileged bits. + * Indeed, the SECURE_ALL_UNPRIVILEGED bits are not + * restrictions enforced by the kernel but by user space on + * itself. + */ + if (cap_capable(current_cred(), current_cred()->user_ns, + CAP_SETPCAP, CAP_OPT_NONE) !=3D 0) { + const unsigned long unpriv_and_locks =3D + SECURE_ALL_UNPRIVILEGED | + SECURE_ALL_UNPRIVILEGED << 1; + const unsigned long changed =3D old->securebits ^ arg2; + + /* For legacy reason, denies non-change. */ + if (!changed) + return -EPERM; + + /* Denies privileged changes. */ + if (changed & ~unpriv_and_locks) + return -EPERM; + } + new =3D prepare_creds(); if (!new) return -ENOMEM; --=20 2.46.1