linux-kernel - [PATCH V9 2/4] fuse: Trace daemon creds

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Date:   Thu, 24 Sep 2020 14:13:16 +0100
From:   Alessio Balsini <balsini@...roid.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Akilesh Kailash <akailash@...gle.com>,
        Amir Goldstein <amir73il@...il.com>,
        Antonio SJ Musumeci <trapexit@...wn.link>,
        David Anderson <dvander@...gle.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Martijn Coenen <maco@...roid.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Lawrence <paullawrence@...gle.com>,
        Stefano Duo <stefanoduo@...gle.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>,
        fuse-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH V9 2/4] fuse: Trace daemon creds

Add a reference to the FUSE daemon credentials, so that they can be used to
temporarily raise the user credentials when accessing lower file system
files in passthrough.

When using FUSE passthrough, read/write operations are directly forwarded
to the lower file system file, but there is no guarantee that the process
that is triggering the request has the right permissions to access the
lower file system.
By default, in the non-passthrough use case, it is the daemon that handles
the read/write operations, that can be performed to the lower file system
with the daemon privileges.
When passthrough is active, instead, the read/write operation is directly
applied to the lower file system, so to keep the same behavior as before,
the calling process temporarily receives the same credentials as the
daemon, that should be removed as soon as the operation completes.

Signed-off-by: Alessio Balsini <balsini@...roid.com>
---
 fs/fuse/fuse_i.h | 3 +++
 fs/fuse/inode.c  | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 6c5166447905..67bf5919f8d6 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -524,6 +524,9 @@ struct fuse_conn {
 	/** The group id for this mount */
 	kgid_t group_id;
 
+	/** Creds of process which created this mount point */
+	const struct cred *creator_cred;
+
 	/** The pid namespace for this mount */
 	struct pid_namespace *pid_ns;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index eb223130a917..d22407bfa959 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -654,6 +654,8 @@ void fuse_conn_put(struct fuse_conn *fc)
 			fiq->ops->release(fiq);
 		put_pid_ns(fc->pid_ns);
 		put_user_ns(fc->user_ns);
+		if (fc->creator_cred)
+			put_cred(fc->creator_cred);
 		fc->release(fc);
 	}
 }
@@ -1203,6 +1205,12 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 	fc->allow_other = ctx->allow_other;
 	fc->user_id = ctx->user_id;
 	fc->group_id = ctx->group_id;
+	fc->creator_cred = prepare_creds();
+	if (!fc->creator_cred) {
+		err = -ENOMEM;
+		goto err_dev_free;
+	}
+
 	fc->max_read = max_t(unsigned, 4096, ctx->max_read);
 	fc->destroy = ctx->destroy;
 	fc->no_control = ctx->no_control;
-- 
2.28.0.681.g6f77f65b4e-goog