[PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property

From: "Mickaël Salaün" <mic@digikod.net>
To: linux-kernel@vger.kernel.org
Cc: "Mickaël Salaün" <mic@digikod.net>,
	"Aleksa Sarai" <cyphar@cyphar.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"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>,
	"Matthew Garrett" <mjg59@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michael Kerrisk" <mtk.manpages@gmail.com>,
	"Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	"Philippe Trébuchet" <philippe.trebuchet@ssi.gouv.fr>,
	"Scott Shell" <scottsh@microsoft.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>,
	"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: [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property
Date: Tue,  5 May 2020 17:31:52 +0200	[thread overview]
Message-ID: <20200505153156.925111-3-mic@digikod.net> (raw)
In-Reply-To: <20200505153156.925111-1-mic@digikod.net>

This new MAY_EXECMOUNT flag enables to check if the underlying mount
point of an inode is marked as executable.  This is useful to implement
a security policy taking advantage of the noexec mount option.

This flag is set according to path_noexec(), which checks if a mount
point is mounted with MNT_NOEXEC or if the underlying superblock is
SB_I_NOEXEC.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
---
 fs/namei.c         | 2 ++
 include/linux/fs.h | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index a320371899cf..33b6d372e74a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2849,6 +2849,8 @@ static int may_open(const struct path *path, int acc_mode, int flag)
 		break;
 	}
 
+	/* Pass the mount point executability. */
+	acc_mode |= path_noexec(path) ? 0 : MAY_EXECMOUNT;
 	error = inode_permission(inode, MAY_OPEN | acc_mode);
 	if (error)
 		return error;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 313c934de9ee..79435fca6c3e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -103,6 +103,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 #define MAY_NOT_BLOCK		0x00000080
 /* the inode is opened with O_MAYEXEC */
 #define MAY_OPENEXEC		0x00000100
+/* the mount point is marked as executable */
+#define MAY_EXECMOUNT		0x00000200
 
 /*
  * flags in file.f_mode.  Note that FMODE_READ and FMODE_WRITE must correspond
-- 
2.26.2

