From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from second.openwall.net (second.openwall.net [193.110.157.125]) by smtp.lore.kernel.org (Postfix) with SMTP id 785F7C3271E for ; Fri, 5 Jul 2024 00:11:34 +0000 (UTC) Received: (qmail 7547 invoked by uid 550); 5 Jul 2024 00:11:19 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 25613 invoked from network); 5 Jul 2024 00:04:13 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1720137844; bh=J7bhqQDa0BUxtW8BlJTRiOi2E8hGXU59gEoA5xrhETw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=QwxgWt6e+wmGcibU5BE9IGwyJ4Ql3BSpfzKyEgTyirg+SQT9/pX4Pbu8UeDYHXa3K bHcDIbIy8282ubQCTG9RVf8z2BGj3FJkZv1Z5ATlXNPP4S5Hj7BxwI8vbhVnkFL0Tx kZBGrQuAlC1f7G/57v67ZbKuhv6zmsOYCwAXaGhC77Ia5xlgbgXLRdK7OtUTiG7QYZ Th61o/0wqsZYUJcM88zyb4v6pCvUUSGQkwLq2+dfniroCXLoUsWoYOK8ODTz+C4iYy Wx60WNHoQrnOecRvGhfjG4Z58ujW9QM9yxO5bxrMd+7DCVBGzphEyXXx+UW8aIH827 WbHvQnbiTuXZw== Date: Thu, 4 Jul 2024 17:04:03 -0700 From: Kees Cook To: =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= Cc: Al Viro , Christian Brauner , Linus Torvalds , Paul Moore , Theodore Ts'o , Alejandro Colomar , Aleksa Sarai , Andrew Morton , Andy Lutomirski , Arnd Bergmann , Casey Schaufler , Christian Heimes , Dmitry Vyukov , 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 Subject: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2) Message-ID: <202407041656.3A05153@keescook> References: <20240704190137.696169-1-mic@digikod.net> <20240704190137.696169-2-mic@digikod.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240704190137.696169-2-mic@digikod.net> On Thu, Jul 04, 2024 at 09:01:33PM +0200, Mickaël Salaün wrote: > Add a new AT_CHECK flag to execveat(2) to check if a file would be > allowed for execution. The main use case is for script interpreters and > dynamic linkers to check execution permission according to the kernel's > security policy. Another use case is to add context to access logs e.g., > which script (instead of interpreter) accessed a file. As any > executable code, scripts could also use this check [1]. > > This is different than faccessat(2) which only checks file access > rights, but not the full context e.g. mount point's noexec, stack limit, > and all potential LSM extra checks (e.g. argv, envp, credentials). > Since the use of AT_CHECK follows the exact kernel semantic as for a > real execution, user space gets the same error codes. Nice! I much prefer this method of going through the exec machinery so we always have a single code path for these kinds of checks. > Because AT_CHECK is dedicated to user space interpreters, it doesn't > make sense for the kernel to parse the checked files, look for > interpreters known to the kernel (e.g. ELF, shebang), and return ENOEXEC > if the format is unknown. Because of that, security_bprm_check() is > never called when AT_CHECK is used. I'd like some additional comments in the code that reminds us that access control checks have finished past a certain point. [...] > diff --git a/fs/exec.c b/fs/exec.c > index 40073142288f..ea2a1867afdc 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -931,7 +931,7 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags) > .lookup_flags = LOOKUP_FOLLOW, > }; > > - if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0) > + if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH | AT_CHECK)) != 0) > return ERR_PTR(-EINVAL); > if (flags & AT_SYMLINK_NOFOLLOW) > open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW; [...] > + * To avoid race conditions leading to time-of-check to time-of-use issues, > + * AT_CHECK should be used with AT_EMPTY_PATH to check against a file > + * descriptor instead of a path. I want this enforced by the kernel. Let's not leave trivial ToCToU foot-guns around. i.e.: if ((flags & AT_CHECK) == AT_CHECK && (flags & AT_EMPTY_PATH) == 0) return ERR_PTR(-EBADF); -- Kees Cook