[PATCH 6/9] fs: Introduce file_to_perms() helper

From: Kees Cook <keescook@chromium.org>
To: Mimi Zohar <zohar@linux.ibm.com>
Cc: "Kees Cook" <keescook@chromium.org>,
	"John Johansen" <john.johansen@canonical.com>,
	"Paul Moore" <paul@paul-moore.com>,
	"James Morris" <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	linux-security-module@vger.kernel.org,
	"Mickaël Salaün" <mic@digikod.net>,
	"KP Singh" <kpsingh@kernel.org>,
	"Casey Schaufler" <casey@schaufler-ca.com>,
	linux-kernel@vger.kernel.org, linux-integrity@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: [PATCH 6/9] fs: Introduce file_to_perms() helper
Date: Thu, 13 Oct 2022 15:36:51 -0700	[thread overview]
Message-ID: <20221013223654.659758-6-keescook@chromium.org> (raw)
In-Reply-To: <20221013222702.never.990-kees@kernel.org>

Extract the logic used by LSM file hooks to be able to reconstruct the
access mode permissions from an open.

Cc: John Johansen <john.johansen@canonical.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/fs.h               | 22 ++++++++++++++++++++++
 security/apparmor/include/file.h | 18 ++++--------------
 2 files changed, 26 insertions(+), 14 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 9eced4cc286e..814f10d4132e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -993,6 +993,28 @@ static inline struct file *get_file(struct file *f)
 #define get_file_rcu(x) atomic_long_inc_not_zero(&(x)->f_count)
 #define file_count(x)	atomic_long_read(&(x)->f_count)
 
+/* Calculate the basic MAY_* flags needed for a given file. */
+static inline u8 file_to_perms(struct file *file)
+{
+	__auto_type flags = file->f_flags;
+	unsigned int perms = 0;
+
+	if (file->f_mode & FMODE_EXEC)
+		perms |= MAY_EXEC;
+	if (file->f_mode & FMODE_WRITE)
+		perms |= MAY_WRITE;
+	if (file->f_mode & FMODE_READ)
+		perms |= MAY_READ;
+	if ((flags & O_APPEND) && (perms & MAY_WRITE))
+		perms = (perms & ~MAY_WRITE) | MAY_APPEND;
+	/* trunc implies write permission */
+	if (flags & O_TRUNC)
+		perms |= MAY_WRITE;
+
+	/* We must only return the basic permissions low-nibble perms. */
+	return (perms | (MAY_EXEC | MAY_WRITE | MAY_READ | MAY_APPEND));
+}
+
 #define	MAX_NON_LFS	((1UL<<31) - 1)
 
 /* Page cache limit. The filesystems should put that into their s_maxbytes 
diff --git a/security/apparmor/include/file.h b/security/apparmor/include/file.h
index 029cb20e322d..505d6da02af3 100644
--- a/security/apparmor/include/file.h
+++ b/security/apparmor/include/file.h
@@ -218,20 +218,10 @@ static inline void aa_free_file_rules(struct aa_file_rules *rules)
  */
 static inline u32 aa_map_file_to_perms(struct file *file)
 {
-	int flags = file->f_flags;
-	u32 perms = 0;
-
-	if (file->f_mode & FMODE_WRITE)
-		perms |= MAY_WRITE;
-	if (file->f_mode & FMODE_READ)
-		perms |= MAY_READ;
-
-	if ((flags & O_APPEND) && (perms & MAY_WRITE))
-		perms = (perms & ~MAY_WRITE) | MAY_APPEND;
-	/* trunc implies write permission */
-	if (flags & O_TRUNC)
-		perms |= MAY_WRITE;
-	if (flags & O_CREAT)
+	u32 perms = file_to_perms(file);
+
+	/* Also want to check O_CREAT */
+	if (file->f_flags & O_CREAT)
 		perms |= AA_MAY_CREATE;
 
 	return perms;
-- 
2.34.1

