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From: Stas Sergeev <stsp2@yandex.ru>
To: linux-kernel@vger.kernel.org
Cc: Stas Sergeev <stsp2@yandex.ru>,
	Stefan Metzmacher <metze@samba.org>,
	Eric Biederman <ebiederm@xmission.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>,
	Christian Brauner <brauner@kernel.org>,
	Jan Kara <jack@suse.cz>,
	Jeff Layton <jlayton@kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Alexander Aring <alex.aring@gmail.com>,
	linux-fsdevel@vger.kernel.org,
	linux-api@vger.kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	=?UTF-8?q?Christian=20G=C3=B6ttsche?= <cgzones@googlemail.com>
Subject: [PATCH 2/2] openat2: add OA2_INHERIT_CRED flag
Date: Wed, 24 Apr 2024 01:46:15 +0300
Message-ID: <20240423224615.298045-3-stsp2@yandex.ru>
In-Reply-To: <20240423224615.298045-1-stsp2@yandex.ru>
References: <20240423224615.298045-1-stsp2@yandex.ru>
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This flag performs the open operation with the fs credentials
(fsuid, fsgid, group_info) that were in effect when dir_fd was opened.
This allows the process to pre-open some directories and then
change eUID (and all other UIDs/GIDs) to a less-privileged user,
retaining the ability to open/create files within these directories.

Design goal:
The idea is to provide a very light-weight sandboxing, where the
process, without the use of any heavy-weight techniques like chroot
within namespaces, can restrict the access to the set of pre-opened
directories.
This patch is just a first step to such sandboxing. If things go
well, in the future the same extension can be added to more syscalls.
These should include at least unlinkat(), renameat2() and the
not-yet-upstreamed setxattrat().

Security considerations:
- Only the bare minimal set of credentials is overridden:
  fsuid, fsgid and group_info. The rest, for example capabilities,
  are not overridden to avoid unneeded security risks.
- To avoid sandboxing escape, this patch makes sure the restricted
  lookup modes are used. Namely, RESOLVE_BENEATH or RESOLVE_IN_ROOT.
- To avoid leaking creds across exec, this patch requires O_CLOEXEC
  flag on a directory.

Use cases:
Virtual machines that deal with untrusted code, can use that
instead of a more heavy-weighted approaches.
Currently the approach is being tested on a dosemu2 VM.

Signed-off-by: Stas Sergeev <stsp2@yandex.ru>

CC: Stefan Metzmacher <metze@samba.org>
CC: Eric Biederman <ebiederm@xmission.com>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Andy Lutomirski <luto@kernel.org>
CC: Christian Brauner <brauner@kernel.org>
CC: Jan Kara <jack@suse.cz>
CC: Jeff Layton <jlayton@kernel.org>
CC: Chuck Lever <chuck.lever@oracle.com>
CC: Alexander Aring <alex.aring@gmail.com>
CC: linux-fsdevel@vger.kernel.org
CC: linux-kernel@vger.kernel.org
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Christian Göttsche <cgzones@googlemail.com>
---
 fs/internal.h                |  2 +-
 fs/namei.c                   | 61 ++++++++++++++++++++++++++++++++++--
 fs/open.c                    |  2 +-
 include/linux/fcntl.h        |  2 ++
 include/uapi/linux/openat2.h |  3 ++
 5 files changed, 66 insertions(+), 4 deletions(-)

diff --git a/fs/internal.h b/fs/internal.h
index 7ca738904e34..692b53b19aad 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -169,7 +169,7 @@ static inline void sb_end_ro_state_change(struct super_block *sb)
  * open.c
  */
 struct open_flags {
-	int open_flag;
+	u64 open_flag;
 	umode_t mode;
 	int acc_mode;
 	int intent;
diff --git a/fs/namei.c b/fs/namei.c
index 2fde2c320ae9..f34ad2b296c7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -586,6 +586,9 @@ struct nameidata {
 	int		dfd;
 	vfsuid_t	dir_vfsuid;
 	umode_t		dir_mode;
+	kuid_t		dir_open_fsuid;
+	kgid_t		dir_open_fsgid;
+	struct group_info *dir_open_groups;
 } __randomize_layout;
 
 #define ND_ROOT_PRESET 1
@@ -695,6 +698,8 @@ static void terminate_walk(struct nameidata *nd)
 	nd->depth = 0;
 	nd->path.mnt = NULL;
 	nd->path.dentry = NULL;
+	if (nd->dir_open_groups)
+		put_group_info(nd->dir_open_groups);
 }
 
 /* path_put is needed afterwards regardless of success or failure */
@@ -2414,6 +2419,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 			get_fs_pwd(current->fs, &nd->path);
 			nd->inode = nd->path.dentry->d_inode;
 		}
+		nd->dir_open_fsuid = current_cred()->fsuid;
+		nd->dir_open_fsgid = current_cred()->fsgid;
+		nd->dir_open_groups = get_current_groups();
 	} else {
 		/* Caller must check execute permissions on the starting path component */
 		struct fd f = fdget_raw(nd->dfd);
@@ -2437,6 +2445,10 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 			path_get(&nd->path);
 			nd->inode = nd->path.dentry->d_inode;
 		}
+		nd->dir_open_fsuid = f.file->f_cred->fsuid;
+		nd->dir_open_fsgid = f.file->f_cred->fsgid;
+		nd->dir_open_groups = get_group_info(
+				f.file->f_cred->group_info);
 		fdput(f);
 	}
 
@@ -3776,6 +3788,29 @@ static int do_o_path(struct nameidata *nd, unsigned flags, struct file *file)
 	return error;
 }
 
+static const struct cred *openat2_override_creds(struct nameidata *nd)
+{
+	const struct cred *old_cred;
+	struct cred *override_cred;
+
+	override_cred = prepare_creds();
+	if (!override_cred)
+		return NULL;
+
+	override_cred->fsuid = nd->dir_open_fsuid;
+	override_cred->fsgid = nd->dir_open_fsgid;
+	override_cred->group_info = nd->dir_open_groups;
+
+	override_cred->non_rcu = 1;
+
+	old_cred = override_creds(override_cred);
+
+	/* override_cred() gets its own ref */
+	put_cred(override_cred);
+
+	return old_cred;
+}
+
 static struct file *path_openat(struct nameidata *nd,
 			const struct open_flags *op, unsigned flags)
 {
@@ -3794,8 +3829,28 @@ static struct file *path_openat(struct nameidata *nd,
 		error = do_o_path(nd, flags, file);
 	} else {
 		const char *s = path_init(nd, flags);
-		file = alloc_empty_file(op->open_flag, current_cred());
-		error = PTR_ERR_OR_ZERO(file);
+		const struct cred *old_cred = NULL;
+
+		error = 0;
+		if (op->open_flag & OA2_INHERIT_CRED) {
+			/* Make sure to work only with restricted
+			 * look-up modes.
+			 */
+			if (!(nd->flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT)))
+				error = -EPERM;
+			/* Only work with O_CLOEXEC dirs. */
+			if (!get_close_on_exec(nd->dfd))
+				error = -EPERM;
+
+			if (!error)
+				old_cred = openat2_override_creds(nd);
+		}
+		if (!error) {
+			file = alloc_empty_file(op->open_flag, current_cred());
+			error = PTR_ERR_OR_ZERO(file);
+		} else {
+			file = ERR_PTR(error);
+		}
 		if (!error) {
 			while (!(error = link_path_walk(s, nd)) &&
 			       (s = open_last_lookups(nd, file, op)) != NULL)
@@ -3803,6 +3858,8 @@ static struct file *path_openat(struct nameidata *nd,
 		}
 		if (!error)
 			error = do_open(nd, file, op);
+		if (old_cred)
+			revert_creds(old_cred);
 		terminate_walk(nd);
 		if (IS_ERR(file))
 			return file;
diff --git a/fs/open.c b/fs/open.c
index ee8460c83c77..6be013182a35 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1225,7 +1225,7 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 	 * values before calling build_open_flags(), but openat2(2) checks all
 	 * of its arguments.
 	 */
-	if (flags & ~VALID_OPEN_FLAGS)
+	if (flags & ~VALID_OPENAT2_FLAGS)
 		return -EINVAL;
 	if (how->resolve & ~VALID_RESOLVE_FLAGS)
 		return -EINVAL;
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index a332e79b3207..b71f8b162102 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -12,6 +12,8 @@
 	 FASYNC	| O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
 	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
 
+#define VALID_OPENAT2_FLAGS (VALID_OPEN_FLAGS | OA2_INHERIT_CRED)
+
 /* List of all valid flags for the how->resolve argument: */
 #define VALID_RESOLVE_FLAGS \
 	(RESOLVE_NO_XDEV | RESOLVE_NO_MAGICLINKS | RESOLVE_NO_SYMLINKS | \
diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h
index a5feb7604948..cdd676a10b62 100644
--- a/include/uapi/linux/openat2.h
+++ b/include/uapi/linux/openat2.h
@@ -40,4 +40,7 @@ struct open_how {
 					return -EAGAIN if that's not
 					possible. */
 
+/* openat2-specific flags go to upper 4 bytes. */
+#define OA2_INHERIT_CRED		(1ULL << 32)
+
 #endif /* _UAPI_LINUX_OPENAT2_H */
-- 
2.44.0