[PATCH v2 10/11] fuse: allow splicing from filesystems mounted by real root

["Ahelenia Ziemiańska" <nabijaczleweli@nabijaczleweli.xyz>]

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FUSE tends to be installed suid 0: this allows normal users to mount
anything, including a program whose read implementation consists
of for(;;) sleep(1);, which, if splice were allowed, would sleep
forever with the pipe lock held.

Normal filesystems can only be mounted by root, and are thus deemed
safe. Extend this to when root mounts a FUSE filesystem with an
explicit check.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
---
 fs/fuse/fuse_i.h | 1 +
 fs/fuse/inode.c  | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 463c5d4ad8b4..a9ceaf10c1d2 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -532,6 +532,7 @@ struct fuse_fs_context {
 	bool no_control:1;
 	bool no_force_umount:1;
 	bool legacy_opts_show:1;
+	bool trusted:1;
 	enum fuse_dax_mode dax_mode;
 	unsigned int max_read;
 	unsigned int blksize;
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 2a6d44f91729..91108ba9acec 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1779,6 +1779,7 @@ static int fuse_get_tree(struct fs_context *fsc)
 
 	fuse_conn_init(fc, fm, fsc->user_ns, &fuse_dev_fiq_ops, NULL);
 	fc->release = fuse_free_conn;
+	fc->trusted = ctx->trusted;
 
 	fsc->s_fs_info = fm;
 
@@ -1840,6 +1841,7 @@ static int fuse_init_fs_context(struct fs_context *fsc)
 	ctx->max_read = ~0;
 	ctx->blksize = FUSE_DEFAULT_BLKSIZE;
 	ctx->legacy_opts_show = true;
+	ctx->trusted = uid_eq(current_uid(), GLOBAL_ROOT_UID);
 
 #ifdef CONFIG_BLOCK
 	if (fsc->fs_type == &fuseblk_fs_type) {
-- 
2.39.2

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