On Mon, Jun 13, 2022 at 01:15:17PM +0200, Christian Brauner wrote:
When calling setattr_prepare() to determine the validity of the attributes the ia_{g,u}id fields contain the value that will be written to inode->i_{g,u}id. This is exactly the same for idmapped and non-idmapped mounts and allows callers to pass in the values they want to see written to inode->i_{g,u}id.
When group ownership is changed a caller whose fsuid owns the inode can change the group of the inode to any group they are a member of. When searching through the caller's groups we need to use the gid mapped according to the idmapped mount otherwise we will fail to change ownership for unprivileged users.
Consider a caller running with fsuid and fsgid 1000 using an idmapped mount that maps id 65534 to 1000 and 65535 to 1001. Consequently, a file owned by 65534:65535 in the filesystem will be owned by 1000:1001 in the idmapped mount.
The caller now requests the gid of the file to be changed to 1000 going through the idmapped mount. In the vfs we will immediately map the requested gid to the value that will need to be written to inode->i_gid and place it in attr->ia_gid. Since this idmapped mount maps 65534 to 1000 we place 65534 in attr->ia_gid.
When we check whether the caller is allowed to change group ownership we first validate that their fsuid matches the inode's uid. The inode->i_uid is 65534 which is mapped to uid 1000 in the idmapped mount. Since the caller's fsuid is 1000 we pass the check.
We now check whether the caller is allowed to change inode->i_gid to the requested gid by calling in_group_p(). This will compare the passed in gid to the caller's fsgid and search the caller's additional groups.
Since we're dealing with an idmapped mount we need to pass in the gid mapped according to the idmapped mount. This is akin to checking whether a caller is privileged over the future group the inode is owned by. And that needs to take the idmapped mount into account. Note, all helpers are nops without idmapped mounts.
New regression test sent to xfstests.
Link: https://github.com/lxc/lxd/issues/10537 Fixes: 2f221d6f7b88 ("attr: handle idmapped mounts") Cc: Seth Forshee seth.forshee@digitalocean.com Cc: Christoph Hellwig hch@lst.de Cc: Aleksa Sarai cyphar@cyphar.com Cc: Al Viro viro@zeniv.linux.org.uk Cc: stable@vger.kernel.org # 5.15+ CC: linux-fsdevel@vger.kernel.org Signed-off-by: Christian Brauner (Microsoft) brauner@kernel.org
Looks correct to me.
Reviewed-by: Seth Forshee sforshee@digitalocean.com