Description: incorrect unprivileged overlayfs permission checking
References:
 https://www.openwall.com/lists/oss-security/2020/10/13/6
Notes:
 carnil> Additionally to the three fixing commits
 carnil> 130fdbc3d1f9966dd4230709c30f3768bccd3065 ("ovl: pass correct
 carnil> flags for opening real directory") and
 carnil> 292f902a40c11f043a5ca1305a114da0e523eaa3 ("ovl: call secutiry
 carnil> hook in ovl_real_ioctl()") might be wanted (see oss-security
 carnil> post).
 carnil> Only exploitable when unprivileged user namespaces are enabled.
 bwh> I think it's only exploitable when unprivileged user namespace
 bwh> are enabled, *and* mounting of overlayfs is permitted in all
 bwh> user namespaces.  This is not possible in the upstream or stable
 bwh> kernels, or in a default Debian configuration, but we do provide
 bwh> run-time configuration knobs to enable these.
Bugs:
upstream: released (5.8-rc1) [48bd024b8a40d73ad6b086de2615738da0c7004f, 56230d956739b9cb1cbde439d76227d77979a04d, 05acefb4872dae89e772729efb194af754c877e8]
6.1-upstream-stable: N/A "Fixed before branch point"
5.10-upstream-stable: N/A "Fixed before branch point"
4.19-upstream-stable: N/A "Vulnerable configuration not possible"
4.9-upstream-stable: N/A "Vulnerable configuration not possible"
sid: released (5.8.7-1)
6.1-bookworm-security: N/A "Fixed before branch point"
5.10-bullseye-security: N/A "Fixed before branching point"
4.19-buster-security: needed
4.9-stretch-security: N/A "Vulnerable configuration not possible"