mount_pseudo(), sget() and MS_KERNMOUNT

From: Luis Ressel
Date: Tue Dec 27 2016 - 05:57:02 EST


With Linux 4.8, the sget() function in fs/super.c got a new permission
check: It now returns -EPERM if
(!(flags & MS_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN)) .

I presume the first half is intented to detect in-kernel mounts? If so,
why doesn't mount_pseudo() (in fs/libfs.c) pass the MS_KERNMOUNT flag
to sget()?

This behaviour has caused a problem for me: During graphics driver
initalization, drm_fs_inode_new() (in drivers/gpu/drm/drm_drv.c) calls
simple_pin_fs(). The MS_KERNMOUNT flag is indeed passed down the
call chain from there, but it is lost when mount_pseudo() is called, as
that function doesn't take a 'flags' argument.

Hence, the first part of the above permission check fails. (The second
part also fails under some cicumstances due to a SELinux quirk, and
therefore the initalization of my graphics driver doesn't succeed.)

Luis Ressel

(Apologies if I'm sending this inquiry to the wrong people. I don't
know much about the kernel development workflow, so I decided to send
it to whomever spat out, plus the author of the
sget() change in 4.8.)