On Thu, Dec 08, 2016 at 05:43:09PM +1300, Eric W. Biederman wrote: > "Serge E. Hallyn" <serge@xxxxxxxxxx> writes: > > > Root in a user ns cannot be trusted to write a traditional > > security.capability xattr. If it were allowed to do so, then any > > unprivileged user on the host could map his own uid to root in a > > namespace, write the xattr, and execute the file with privilege on the > > host. > > > > This patch introduces v3 of the security.capability xattr. It builds a > > vfs_ns_cap_data struct by appending a uid_t rootid to struct > > vfs_cap_data. This is the absolute uid_t (i.e. the uid_t in > > init_user_ns) of the root id (uid 0 in a namespace) in whose namespaces > > the file capabilities may take effect. > > > > When a task in a user ns (which is privileged with CAP_SETFCAP toward > > that user_ns) asks to write v2 security.capability, the kernel will > > transparently rewrite the xattr as a v3 with the appropriate rootid. > > Subsequently, any task executing the file which has the noted kuid as > > its root uid, or which is in a descendent user_ns of such a user_ns, > > will run the file with capabilities. > > > > If a task writes a v3 security.capability, then it can provide a > > uid (valid within its own user namespace, over which it has CAP_SETFCAP) > > for the xattr. The kernel will translate that to the absolute uid, and > > write that to disk. After this, a task in the writer's namespace will > > not be able to use those capabilities, but a task in a namespace where > > the given uid is root will. > > > > Only a single security.capability xattr may be written. A task may > > overwrite the existing one so long as it was written by a user mapped > > into his own user_ns over which he has CAP_SETFCAP. > > > > This allows a simple setxattr to work, allows tar/untar to work, and > > allows us to tar in one namespace and untar in another while preserving > > the capability, without risking leaking privilege into a parent > > namespace. > > Any chance of a singed-off-by? Yes, sorry, Stéphane had pointed out that I'd apparently forgotten to do -s. Do you want me to resend the whole shebang, or does Signed-off-by: Serge Hallyn <serge@xxxxxxxxxx> suffice? (My previous iterations did have it fwiw so I don't think I could legally disavow it now :) -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
