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Date: Mon, 17 Feb 2020 22:20:22 +0100 From: Christian Brauner <christian.brauner@...ntu.com> To: James Bottomley <James.Bottomley@...senPartnership.com> Cc: Stéphane Graber <stgraber@...ntu.com>, "Eric W. Biederman" <ebiederm@...ssion.com>, Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>, Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org, containers@...ts.linux-foundation.org, smbarber@...omium.org, Seth Forshee <seth.forshee@...onical.com>, linux-security-module@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, Alexey Dobriyan <adobriyan@...il.com> Subject: Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings On Mon, Feb 17, 2020 at 01:06:08PM -0800, James Bottomley wrote: > On Fri, 2020-02-14 at 19:35 +0100, Christian Brauner wrote: > [...] > > People not as familiar with user namespaces might not be aware that > > fsid mappings already exist. Right now, fsid mappings are always > > identical to id mappings. Specifically, the kernel will lookup fsuids > > in the uid mappings and fsgids in the gid mappings of the relevant > > user namespace. > > This isn't actually entirely true: today we have the superblock user > namespace, which can be used for fsid remapping on filesystems that > support it (currently f2fs and fuse). Since this is a single shift, Note that this states "the relevant" user namespace not the caller's user namespace. And the point is true even for such filesystems. fuse does call make_kuid(fc->user_ns, attr->uid) and hence looks up the mapping in the id mappings.. This would be replaced by make_kfsuid(). > how is it going to play with s_user_ns? Do you have to understand the > superblock mapping to use this shift, or are we simply using this to > replace s_user_ns? I'm not sure what you mean by understand the superblock mapping. The case is not different from the devpts patch in this series. Fuse needs to be changed to call make_kfsuid() since it is mountable inside user namespaces at which point everthing just works.
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