powerpc/set_memory: Avoid spinlock recursion in change_page_attr()

Commit 1f9ad21c ("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 9f7853d7 ("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.
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/all/20211212112152.GA27070@sakura/
Link: https://lore.kernel.org/r/43c3c76a1175ae6dc1a3d3b5c3f7ecb48f683eea.1640344012.git.christophe.leroy@csgroup.eu

arch/powerpc/mm/pageattr.c

@@ -15,12 +15,14 @@
 #include <asm/pgtable.h>
 
 
+static pte_basic_t pte_update_delta(pte_t *ptep, unsigned long addr,
+				    unsigned long old, unsigned long new)
+{
+	return pte_update(&init_mm, addr, ptep, old & ~new, new & ~old, 0);
+}
+
 /*
- * Updates the attributes of a page in three steps:
- *
- * 1. take the page_table_lock
- * 2. install the new entry with the updated attributes
- * 3. flush the TLB
+ * Updates the attributes of a page atomically.
  *
  * This sequence is safe against concurrent updates, and also allows updating the
  * attributes of a page currently being executed or accessed.
@@ -28,41 +30,33 @@
 static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
 {
 	long action = (long)data;
-	pte_t pte;
-
-	spin_lock(&init_mm.page_table_lock);
-
-	pte = ptep_get(ptep);
 
-	/* modify the PTE bits as desired, then apply */
+	/* modify the PTE bits as desired */
 	switch (action) {
 	case SET_MEMORY_RO:
-		pte = pte_wrprotect(pte);
+		/* Don't clear DIRTY bit */
+		pte_update_delta(ptep, addr, _PAGE_KERNEL_RW & ~_PAGE_DIRTY, _PAGE_KERNEL_RO);
 		break;
 	case SET_MEMORY_RW:
-		pte = pte_mkwrite(pte_mkdirty(pte));
+		pte_update_delta(ptep, addr, _PAGE_KERNEL_RO, _PAGE_KERNEL_RW);
 		break;
 	case SET_MEMORY_NX:
-		pte = pte_exprotect(pte);
+		pte_update_delta(ptep, addr, _PAGE_KERNEL_ROX, _PAGE_KERNEL_RO);
 		break;
 	case SET_MEMORY_X:
-		pte = pte_mkexec(pte);
+		pte_update_delta(ptep, addr, _PAGE_KERNEL_RO, _PAGE_KERNEL_ROX);
 		break;
 	default:
 		WARN_ON_ONCE(1);
 		break;
 	}
 
-	pte_update(&init_mm, addr, ptep, ~0UL, pte_val(pte), 0);
-
 	/* See ptesync comment in radix__set_pte_at() */
 	if (radix_enabled())
 		asm volatile("ptesync": : :"memory");
 
 	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
 
-	spin_unlock(&init_mm.page_table_lock);
-
 	return 0;
 }