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From:   =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= <mic@digikod.net>
To:     James Morris <jmorris@namei.org>,
        John Johansen <john.johansen@canonical.com>,
        Kentaro Takeda <takedakn@nttdata.co.jp>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc:     =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= <mic@digikod.net>,
        Brendan Jackman <jackmanb@chromium.org>,
        Florent Revest <revest@chromium.org>,
        KP Singh <kpsingh@kernel.org>,
        Paul Moore <paul@paul-moore.com>, linux-kernel@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= <mic@linux.microsoft.com>
Subject: [RFC PATCH v1] LSM: Remove double path_rename hook calls for RENAME_EXCHANGE
Date:   Tue, 22 Feb 2022 18:53:32 +0100
Message-Id: <20220222175332.384545-1-mic@digikod.net>
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Precedence: bulk

From: Mickaël Salaün <mic@linux.microsoft.com>

In order to be able to identify a file exchange with renameat2(2) and
RENAME_EXCHANGE, which will be useful for Landlock [1], propagate the
rename flags to LSMs.  This may also improve performance because of the
switch from two set of LSM hook calls to only one, and because LSMs
using this hook may optimize the double check (e.g. only one lock,
reduce the number of path walks).

AppArmor, Landlock and Tomoyo are updated to leverage this change.  This
should not change the current behavior (same check order), except
(different level of) speed boosts.

[1] https://lore.kernel.org/r/20220221212522.320243-1-mic@digikod.net

Cc: James Morris <jmorris@namei.org>
Cc: John Johansen <john.johansen@canonical.com>
Cc: Kentaro Takeda <takedakn@nttdata.co.jp>
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20220222175332.384545-1-mic@digikod.net
---
 include/linux/lsm_hook_defs.h |  2 +-
 include/linux/lsm_hooks.h     |  1 +
 security/apparmor/lsm.c       | 30 +++++++++++++++++++++++++-----
 security/landlock/fs.c        | 12 ++++++++++--
 security/security.c           |  9 +--------
 security/tomoyo/tomoyo.c      | 11 ++++++++++-
 6 files changed, 48 insertions(+), 17 deletions(-)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 819ec92dc2a8..d8b49c9c3a8a 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -100,7 +100,7 @@ LSM_HOOK(int, 0, path_link, struct dentry *old_dentry,
 	 const struct path *new_dir, struct dentry *new_dentry)
 LSM_HOOK(int, 0, path_rename, const struct path *old_dir,
 	 struct dentry *old_dentry, const struct path *new_dir,
-	 struct dentry *new_dentry)
+	 struct dentry *new_dentry, unsigned int flags)
 LSM_HOOK(int, 0, path_chmod, const struct path *path, umode_t mode)
 LSM_HOOK(int, 0, path_chown, const struct path *path, kuid_t uid, kgid_t gid)
 LSM_HOOK(int, 0, path_chroot, const struct path *path)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 3bf5c658bc44..32cd2a7fe9fc 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -358,6 +358,7 @@
  *	@old_dentry contains the dentry structure of the old link.
  *	@new_dir contains the path structure for parent of the new link.
  *	@new_dentry contains the dentry structure of the new link.
+ *	@flags may contain rename options such as RENAME_EXCHANGE.
  *	Return 0 if permission is granted.
  * @path_chmod:
  *	Check for permission to change a mode of the file @path. The new
diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 4f0eecb67dde..900bc540656a 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -354,13 +354,16 @@ static int apparmor_path_link(struct dentry *old_dentry, const struct path *new_
 }
 
 static int apparmor_path_rename(const struct path *old_dir, struct dentry *old_dentry,
-				const struct path *new_dir, struct dentry *new_dentry)
+				const struct path *new_dir, struct dentry *new_dentry,
+				const unsigned int flags)
 {
 	struct aa_label *label;
 	int error = 0;
 
 	if (!path_mediated_fs(old_dentry))
 		return 0;
+	if ((flags & RENAME_EXCHANGE) && !path_mediated_fs(new_dentry))
+		return 0;
 
 	label = begin_current_label_crit_section();
 	if (!unconfined(label)) {
@@ -374,10 +377,27 @@ static int apparmor_path_rename(const struct path *old_dir, struct dentry *old_d
 			d_backing_inode(old_dentry)->i_mode
 		};
 
-		error = aa_path_perm(OP_RENAME_SRC, label, &old_path, 0,
-				     MAY_READ | AA_MAY_GETATTR | MAY_WRITE |
-				     AA_MAY_SETATTR | AA_MAY_DELETE,
-				     &cond);
+		if (flags & RENAME_EXCHANGE) {
+			struct path_cond cond_exchange = {
+				i_uid_into_mnt(mnt_userns, d_backing_inode(new_dentry)),
+				d_backing_inode(new_dentry)->i_mode
+			};
+
+			error = aa_path_perm(OP_RENAME_SRC, label, &new_path, 0,
+					     MAY_READ | AA_MAY_GETATTR | MAY_WRITE |
+					     AA_MAY_SETATTR | AA_MAY_DELETE,
+					     &cond_exchange);
+			if (!error)
+				error = aa_path_perm(OP_RENAME_DEST, label, &old_path,
+						     0, MAY_WRITE | AA_MAY_SETATTR |
+						     AA_MAY_CREATE, &cond_exchange);
+		}
+
+		if (!error)
+			error = aa_path_perm(OP_RENAME_SRC, label, &old_path, 0,
+					     MAY_READ | AA_MAY_GETATTR | MAY_WRITE |
+					     AA_MAY_SETATTR | AA_MAY_DELETE,
+					     &cond);
 		if (!error)
 			error = aa_path_perm(OP_RENAME_DEST, label, &new_path,
 					     0, MAY_WRITE | AA_MAY_SETATTR |
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 97b8e421f617..7e57fca6e814 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -574,10 +574,12 @@ static inline u32 maybe_remove(const struct dentry *const dentry)
 static int hook_path_rename(const struct path *const old_dir,
 		struct dentry *const old_dentry,
 		const struct path *const new_dir,
-		struct dentry *const new_dentry)
+		struct dentry *const new_dentry,
+		const unsigned int flags)
 {
 	const struct landlock_ruleset *const dom =
 		landlock_get_current_domain();
+	u32 exchange_access = 0;
 
 	if (!dom)
 		return 0;
@@ -585,11 +587,17 @@ static int hook_path_rename(const struct path *const old_dir,
 	if (old_dir->dentry != new_dir->dentry)
 		/* Gracefully forbids reparenting. */
 		return -EXDEV;
+	if (flags & RENAME_EXCHANGE) {
+		if (unlikely(d_is_negative(new_dentry)))
+			return -ENOENT;
+		exchange_access =
+			get_mode_access(d_backing_inode(new_dentry)->i_mode);
+	}
 	if (unlikely(d_is_negative(old_dentry)))
 		return -ENOENT;
 	/* RENAME_EXCHANGE is handled because directories are the same. */
 	return check_access_path(dom, old_dir, maybe_remove(old_dentry) |
-			maybe_remove(new_dentry) |
+			maybe_remove(new_dentry) | exchange_access |
 			get_mode_access(d_backing_inode(old_dentry)->i_mode));
 }
 
diff --git a/security/security.c b/security/security.c
index 22261d79f333..8634da4cfd46 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1184,15 +1184,8 @@ int security_path_rename(const struct path *old_dir, struct dentry *old_dentry,
 		     (d_is_positive(new_dentry) && IS_PRIVATE(d_backing_inode(new_dentry)))))
 		return 0;
 
-	if (flags & RENAME_EXCHANGE) {
-		int err = call_int_hook(path_rename, 0, new_dir, new_dentry,
-					old_dir, old_dentry);
-		if (err)
-			return err;
-	}
-
 	return call_int_hook(path_rename, 0, old_dir, old_dentry, new_dir,
-				new_dentry);
+				new_dentry, flags);
 }
 EXPORT_SYMBOL(security_path_rename);
 
diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index b6a31901f289..71e82d855ebf 100644
--- a/security/tomoyo/tomoyo.c
+++ b/security/tomoyo/tomoyo.c
@@ -264,17 +264,26 @@ static int tomoyo_path_link(struct dentry *old_dentry, const struct path *new_di
  * @old_dentry: Pointer to "struct dentry".
  * @new_parent: Pointer to "struct path".
  * @new_dentry: Pointer to "struct dentry".
+ * @flags: Rename options.
  *
  * Returns 0 on success, negative value otherwise.
  */
 static int tomoyo_path_rename(const struct path *old_parent,
 			      struct dentry *old_dentry,
 			      const struct path *new_parent,
-			      struct dentry *new_dentry)
+			      struct dentry *new_dentry,
+			      const unsigned int flags)
 {
 	struct path path1 = { .mnt = old_parent->mnt, .dentry = old_dentry };
 	struct path path2 = { .mnt = new_parent->mnt, .dentry = new_dentry };
 
+	if (flags & RENAME_EXCHANGE) {
+		const int err = tomoyo_path2_perm(TOMOYO_TYPE_RENAME, &path2,
+				&path1);
+
+		if (err)
+			return err;
+	}
 	return tomoyo_path2_perm(TOMOYO_TYPE_RENAME, &path1, &path2);
 }
 

base-commit: cfb92440ee71adcc2105b0890bb01ac3cddb8507
-- 
2.35.1