[PATCH v2 09/11] fuse: file: limit splice_read to virtiofs

[Ahelenia Ziemiańska]

Potentially-blocking splice_reads are allowed for normal filesystems
like NFS because they're blessed by root.

FUSE is commonly used suid-root, and allows anyone to trivially create
a file that, when spliced from, will just sleep forever with the pipe
lock held.

The only way IPC to the fusing process could be avoided is if
!(ff->open_flags & FOPEN_DIRECT_IO) and the range was already cached
and we weren't past the end. Just refuse it.

virtiofs behaves like a normal filesystem and can only be mounted
by root, it's unaffected by use of a new "trusted" connection flag.
This may be extended to include real FUSE mounts by processes which
aren't suid, to match the semantics for normal filesystems.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@xxxxxxxxxxxxxxxxxx>
---
 fs/fuse/file.c      | 17 ++++++++++++++++-
 fs/fuse/fuse_i.h    |  3 +++
 fs/fuse/virtio_fs.c |  1 +
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index a660f1f21540..20bb16ddfcc9 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -3200,6 +3200,21 @@ static ssize_t fuse_copy_file_range(struct file *src_file, loff_t src_off,
 	return ret;
 }
 
+static long fuse_splice_read(struct file *in, loff_t *ppos,
+			     struct pipe_inode_info *pipe, size_t len,
+			     unsigned int flags)
+{
+	struct inode *inode = file_inode(in);
+
+	if (fuse_is_bad(inode))
+		return -EIO;
+
+	if (get_fuse_conn(inode)->trusted)
+		return filemap_splice_read(in, ppos, pipe, len, flags);
+
+	return -EINVAL;
+}
+
 static const struct file_operations fuse_file_operations = {
 	.llseek		= fuse_file_llseek,
 	.read_iter	= fuse_file_read_iter,
@@ -3212,7 +3227,7 @@ static const struct file_operations fuse_file_operations = {
 	.lock		= fuse_file_lock,
 	.get_unmapped_area = thp_get_unmapped_area,
 	.flock		= fuse_file_flock,
-	.splice_read	= filemap_splice_read,
+	.splice_read	= fuse_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.unlocked_ioctl	= fuse_file_ioctl,
 	.compat_ioctl	= fuse_file_compat_ioctl,
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 1df83eebda92..463c5d4ad8b4 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -818,6 +818,9 @@ struct fuse_conn {
 	/* Is statx not implemented by fs? */
 	unsigned int no_statx:1;
 
+	/* Do we trust this connection to always respond? */
+	bool trusted:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 5f1be1da92ce..fce0fe24899a 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1448,6 +1448,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
 	fc->delete_stale = true;
 	fc->auto_submounts = true;
 	fc->sync_fs = true;
+	fc->trusted = true;
 
 	/* Tell FUSE to split requests that exceed the virtqueue's size */
 	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
-- 
2.39.2

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