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From: ebiederm@xmission.com (Eric W. Biederman)
To: "Mickaël Salaün" <mic@digikod.net>
Cc: linux-kernel@vger.kernel.org, "Aleksa Sarai" <cyphar@cyphar.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Eric Biggers" <ebiggers@kernel.org>,
	"Eric Chiang" <ericchiang@google.com>,
	"Florian Weimer" <fweimer@redhat.com>,
	"James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Kees Cook" <keescook@chromium.org>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
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	"Matthew Wilcox" <willy@infradead.org>,
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	"Tetsuo Handa" <penguin-kernel@I-love.SAKURA.ne.jp>,
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	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v7 2/7] exec: Move S_ISREG() check earlier
Date: Tue, 11 Aug 2020 14:27:54 -0500
Message-ID: <87o8nhm18l.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <20200723171227.446711-3-mic@digikod.net> (=?utf-8?Q?=22Micka?= =?utf-8?Q?=C3=ABl_Sala=C3=BCn=22's?= message of "Thu, 23 Jul 2020 19:12:22 +0200")

Mickaël Salaün <mic@digikod.net> writes:

> From: Kees Cook <keescook@chromium.org>
>
> The execve(2)/uselib(2) syscalls have always rejected non-regular
> files. Recently, it was noticed that a deadlock was introduced when trying
> to execute pipes, as the S_ISREG() test was happening too late. This was
> fixed in commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files
> during execve()"), but it was added after inode_permission() had already
> run, which meant LSMs could see bogus attempts to execute non-regular
> files.
>
> Move the test into the other inode type checks (which already look
> for other pathological conditions[1]). Since there is no need to use
> FMODE_EXEC while we still have access to "acc_mode", also switch the
> test to MAY_EXEC.
>
> Also include a comment with the redundant S_ISREG() checks at the end of
> execve(2)/uselib(2) to note that they are present to avoid any mistakes.

The comment is:
> +	/*
> +	 * may_open() has already checked for this, so it should be
> +	 * impossible to trip now. But we need to be extra cautious
> +	 * and check again at the very end too.
> +	 */
Those comments scare me.  Why do you need to be extra cautious?
How can the file type possibly change between may_open and anywhere?
The type of a file is immutable after it's creation.

If the comment said check just in case something went wrong with
code maintenance I could understand but that isn't what the comment
says.

Also the fallthrough change below really should be broken out into
it's own change.


> My notes on the call path, and related arguments, checks, etc:
>
> do_open_execat()
>     struct open_flags open_exec_flags = {
>         .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>         .acc_mode = MAY_EXEC,
>         ...
>     do_filp_open(dfd, filename, open_flags)
>         path_openat(nameidata, open_flags, flags)
>             file = alloc_empty_file(open_flags, current_cred());
>             do_open(nameidata, file, open_flags)
>                 may_open(path, acc_mode, open_flag)
> 		    /* new location of MAY_EXEC vs S_ISREG() test */
>                     inode_permission(inode, MAY_OPEN | acc_mode)
>                         security_inode_permission(inode, acc_mode)
>                 vfs_open(path, file)
>                     do_dentry_open(file, path->dentry->d_inode, open)
>                         /* old location of FMODE_EXEC vs S_ISREG() test */
>                         security_file_open(f)
>                         open()
>
> [1] https://lore.kernel.org/lkml/202006041910.9EF0C602@keescook/
>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/r/20200605160013.3954297-3-keescook@chromium.org
> ---
>  fs/exec.c  | 14 ++++++++++++--
>  fs/namei.c |  6 ++++--
>  fs/open.c  |  6 ------
>  3 files changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index d7c937044d10..bdc6a6eb5dce 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -141,8 +141,13 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
>  	if (IS_ERR(file))
>  		goto out;
>  
> +	/*
> +	 * may_open() has already checked for this, so it should be
> +	 * impossible to trip now. But we need to be extra cautious
> +	 * and check again at the very end too.
> +	 */
>  	error = -EACCES;
> -	if (!S_ISREG(file_inode(file)->i_mode))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
>  		goto exit;
>  
>  	if (path_noexec(&file->f_path))
> @@ -886,8 +891,13 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
>  	if (IS_ERR(file))
>  		goto out;
>  
> +	/*
> +	 * may_open() has already checked for this, so it should be
> +	 * impossible to trip now. But we need to be extra cautious
> +	 * and check again at the very end too.
> +	 */
>  	err = -EACCES;
> -	if (!S_ISREG(file_inode(file)->i_mode))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
>  		goto exit;
>
>  	if (path_noexec(&file->f_path))
> diff --git a/fs/namei.c b/fs/namei.c
> index 72d4219c93ac..a559ad943970 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2849,16 +2849,18 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>  	case S_IFLNK:
>  		return -ELOOP;
>  	case S_IFDIR:
> -		if (acc_mode & MAY_WRITE)
> +		if (acc_mode & (MAY_WRITE | MAY_EXEC))
>  			return -EISDIR;
>  		break;
>  	case S_IFBLK:
>  	case S_IFCHR:
>  		if (!may_open_dev(path))
>  			return -EACCES;
> -		/*FALLTHRU*/
> +		fallthrough;
                ^^^^^^^^^^^
That is an unrelated change and should be sent separately.

>  	case S_IFIFO:
>  	case S_IFSOCK:
> +		if (acc_mode & MAY_EXEC)
> +			return -EACCES;
>  		flag &= ~O_TRUNC;
>  		break;
>  	}
> diff --git a/fs/open.c b/fs/open.c
> index 6cd48a61cda3..623b7506a6db 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -784,12 +784,6 @@ static int do_dentry_open(struct file *f,
>  		return 0;
>  	}
>  
> -	/* Any file opened for execve()/uselib() has to be a regular file. */
> -	if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) {
> -		error = -EACCES;
> -		goto cleanup_file;
> -	}
> -
>  	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
>  		error = get_write_access(inode);
>  		if (unlikely(error))

  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-23 17:12 [PATCH v7 0/7] Add support for O_MAYEXEC Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 1/7] exec: Change uselib(2) IS_SREG() failure to EACCES Mickaël Salaün
2020-08-11 18:59   ` Eric W. Biederman
2020-08-11 19:14     ` Eric W. Biederman
2020-07-23 17:12 ` [PATCH v7 2/7] exec: Move S_ISREG() check earlier Mickaël Salaün
2020-08-11 19:27   ` Eric W. Biederman [this message]
2020-07-23 17:12 ` [PATCH v7 3/7] exec: Move path_noexec() " Mickaël Salaün
2020-08-11 19:36   ` Eric W. Biederman
2020-08-13 15:31     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 4/7] fs: Introduce O_MAYEXEC flag for openat2(2) Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-07-27  4:21   ` Al Viro
2020-07-27  5:27     ` Florian Weimer
2020-07-27 19:46       ` Mickaël Salaün
2020-08-11 19:51   ` Eric W. Biederman
2020-08-13 14:36     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-08-11 19:58   ` Eric W. Biederman
2020-08-13 14:49     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 6/7] selftest/openat2: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-07-23 17:12 ` [PATCH v7 7/7] ima: add policy support for the new file open MAY_OPENEXEC flag Mickaël Salaün
2020-07-24 19:04   ` Kees Cook
2020-07-24 11:20 ` [PATCH v7 0/7] Add support for O_MAYEXEC Thibaut Sautereau
2020-07-24 19:06 ` Kees Cook
2020-07-25 11:15   ` Christian Brauner
2020-08-10 20:11   ` Mickaël Salaün
2020-08-10 20:21     ` Al Viro
2020-08-10 22:09       ` David Laight
2020-08-10 22:28         ` Al Viro
2020-08-10 22:47           ` Mickaël Salaün
2020-08-11  8:09             ` David Laight
2020-08-11  8:50               ` Mickaël Salaün
2020-08-10 22:43       ` Mickaël Salaün
2020-08-10 23:03         ` Jann Horn
2020-08-11  8:48           ` Mickaël Salaün
2020-08-11 13:56             ` Mimi Zohar
2020-08-11 14:02               ` Matthew Wilcox
2020-08-11 14:30                 ` Mimi Zohar
2020-08-11 17:18             ` Deven Bowers
2020-08-10 23:05         ` Al Viro
2020-08-11  8:49           ` Mickaël Salaün

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