CVE-2021-47632: powerpc/set_memory: Avoid spinlock recursion in change_page_attr()

CVE-2021-47632: powerpc/set_memory: Avoid spinlock recursion in change_page_attr()

[Greg Kroah-Hartman]

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

powerpc/set_memory: Avoid spinlock recursion in change_page_attr()

Commit 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines")
included a spin_lock() to change_page_attr() in order to
safely perform the three step operations. But then
commit 9f7853d7609d ("powerpc/mm: Fix set_memory_*() against
concurrent accesses") modify it to use pte_update() and do
the operation safely against concurrent access.

In the meantime, Maxime reported some spinlock recursion.

[   15.351649] BUG: spinlock recursion on CPU#0, kworker/0:2/217
[   15.357540]  lock: init_mm+0x3c/0x420, .magic: dead4ead, .owner: kworker/0:2/217, .owner_cpu: 0
[   15.366563] CPU: 0 PID: 217 Comm: kworker/0:2 Not tainted 5.15.0+ #523
[   15.373350] Workqueue: events do_free_init
[   15.377615] Call Trace:
[   15.380232] [e4105ac0] [800946a4] do_raw_spin_lock+0xf8/0x120 (unreliable)
[   15.387340] [e4105ae0] [8001f4ec] change_page_attr+0x40/0x1d4
[   15.393413] [e4105b10] [801424e0] __apply_to_page_range+0x164/0x310
[   15.400009] [e4105b60] [80169620] free_pcp_prepare+0x1e4/0x4a0
[   15.406045] [e4105ba0] [8016c5a0] free_unref_page+0x40/0x2b8
[   15.411979] [e4105be0] [8018724c] kasan_depopulate_vmalloc_pte+0x6c/0x94
[   15.418989] [e4105c00] [801424e0] __apply_to_page_range+0x164/0x310
[   15.425451] [e4105c50] [80187834] kasan_release_vmalloc+0xbc/0x134
[   15.431898] [e4105c70] [8015f7a8] __purge_vmap_area_lazy+0x4e4/0xdd8
[   15.438560] [e4105d30] [80160d10] _vm_unmap_aliases.part.0+0x17c/0x24c
[   15.445283] [e4105d60] [801642d0] __vunmap+0x2f0/0x5c8
[   15.450684] [e4105db0] [800e32d0] do_free_init+0x68/0x94
[   15.456181] [e4105dd0] [8005d094] process_one_work+0x4bc/0x7b8
[   15.462283] [e4105e90] [8005d614] worker_thread+0x284/0x6e8
[   15.468227] [e4105f00] [8006aaec] kthread+0x1f0/0x210
[   15.473489] [e4105f40] [80017148] ret_from_kernel_thread+0x14/0x1c

Remove the read / modify / write sequence to make the operation atomic
and remove the spin_lock() in change_page_attr().

To do the operation atomically, we can't use pte modification helpers
anymore. Because all platforms have different combination of bits, it
is not easy to use those bits directly. But all have the
_PAGE_KERNEL_{RO/ROX/RW/RWX} set of flags. All we need it to compare
two sets to know which bits are set or cleared.

For instance, by comparing _PAGE_KERNEL_ROX and _PAGE_KERNEL_RO you
know which bit gets cleared and which bit get set when changing exec
permission.

The Linux kernel CVE team has assigned CVE-2021-47632 to this issue.


Affected and fixed versions
===========================

	Fixed in 5.15.34 with commit 6def4eaf0391f24be541633a954c0e4876858b1e
	Fixed in 5.16.20 with commit 96917107e67846f1d959ed03be281048efad14c5
	Fixed in 5.17.3 with commit 6ebe5ca2cbe438a688f2ae238ef5a0b0b5f3468a
	Fixed in 5.18 with commit a4c182ecf33584b9b2d1aa9dad073014a504c01f

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47632
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/powerpc/mm/pageattr.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/6def4eaf0391f24be541633a954c0e4876858b1e
	https://git.kernel.org/stable/c/96917107e67846f1d959ed03be281048efad14c5
	https://git.kernel.org/stable/c/6ebe5ca2cbe438a688f2ae238ef5a0b0b5f3468a
	https://git.kernel.org/stable/c/a4c182ecf33584b9b2d1aa9dad073014a504c01f