From: Christian Brauner Date: Mon, 15 Jun 2026 12:52:18 +0000 (+0200) Subject: fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid X-Git-Tag: v7.2-rc2~14^2~19^2~1 X-Git-Url: http://git.ipfire.org/index.cgi?a=commitdiff_plain;h=539dce1144651f7976fa418e618b0b574bf15eeb;p=thirdparty%2Flinux.git fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid vfs_tmpfile() never checked that the caller's fsuid and fsgid map into the filesystem. On an idmapped mount whose idmapping does not cover the caller's fs{u,g}id, the ->tmpfile() instance initializes the new inode through inode_init_owner(), where mapped_fsuid()/mapped_fsgid() return INVALID_UID/INVALID_GID, and the tmpfile ends up owned by (uid_t)-1. Every other creation path already refuses this: may_o_create() (O_CREAT) and may_create_dentry() (mkdir, mknod, symlink, link) bail out with -EOVERFLOW via fsuidgid_has_mapping() precisely so that an object cannot be created with an owner the filesystem cannot represent. An O_TMPFILE is no exception: it is created I_LINKABLE and linkat(2) can splice it into the namespace afterwards, so the same guarantee must hold. Add the missing fsuidgid_has_mapping() check to vfs_tmpfile(). On a non-idmapped mount the caller's fs{u,g}id always map in the superblock's user namespace, so this is a no-op there and only takes effect on an idmapped mount that does not map the caller. It applies to every filesystem that sets FS_ALLOW_IDMAP and implements ->tmpfile() (tmpfs, ext4, btrfs, xfs, f2fs, ...), and to overlayfs, whose upper-layer tmpfile creation funnels through vfs_tmpfile() via backing_tmpfile_open(). Fixes: 8e5389132ab4 ("fs: introduce fsuidgid_has_mapping() helper") Link: https://patch.msgid.link/20260615-work-idmapped-tmpfile-v1-1-754a94d81f83@kernel.org Reviewed-by: Jan Kara Signed-off-by: Christian Brauner (Amutable) --- diff --git a/fs/namei.c b/fs/namei.c index 5cc9f0f466b8..19ce43c9a6e6 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -4736,6 +4736,10 @@ int vfs_tmpfile(struct mnt_idmap *idmap, int error; int open_flag = file->f_flags; + /* A tmpfile is I_LINKABLE, so guard its owner like may_o_create(). */ + if (!fsuidgid_has_mapping(dir->i_sb, idmap)) + return -EOVERFLOW; + /* we want directory to be writable */ error = inode_permission(idmap, dir, MAY_WRITE | MAY_EXEC); if (error)