memblock: don't touch memblock arrays when memblock_free() is called late

When memblock_free() is called after memblock_discard() on architectures
that don't select ARCH_KEEP_MEMBLOCK, it tries to update memblock.reserved
that was already discarded and it causes use-after-free, for example

[    8.514775] BUG: KASAN: use-after-free in memblock_isolate_range+0x4ac/0x650
[    8.514775] Read of size 8 at addr ffff88a07fe6a000 by task swapper/0/1
[    8.514775] Call Trace:
[    8.514775]  <TASK>
[    8.514775]  kasan_report+0xb2/0x1b0
[    8.514775]  memblock_isolate_range+0x4ac/0x650
[    8.514775]  memblock_phys_free+0xc4/0x190
[    8.514775]  housekeeping_late_init+0x257/0x280
[    8.514775]  do_one_initcall+0xaa/0x470
[    8.514775]  do_initcalls+0x1b4/0x1f0
[    8.514775]  kernel_init_freeable+0x4b5/0x550
[    8.514775]  kernel_init+0x1c/0x150
[    8.514775]  ret_from_fork+0x5dc/0x8e0
[    8.514775]  ret_from_fork_asm+0x1a/0x30
[    8.514775]  </TASK>

Make sure memblock_free() updates memblock.reserved only when called early
enough or when ARCH_KEEP_MEMBLOCK is enabled.

Reported-by: Waiman Long <longman@redhat.com>
Reported-by: Breno Leitao <leitao@debian.org>
Closes: https://lore.kernel.org/all/20260505051821.1107133-1-longman@redhat.com
Tested-by: Waiman Long <longman@redhat.com>
Tested-by: Breno Leitao <leitao@debian.org>
Fixes: 87ce9e8 ("memblock, treewide: make memblock_free() handle late freeing")
Link: https://patch.msgid.link/20260513105122.502506-1-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: rppt

mm/memblock.c

@@ -989,13 +989,15 @@ void __init_memblock memblock_free(void *ptr, size_t size)
 int __init_memblock memblock_phys_free(phys_addr_t base, phys_addr_t size)
 {
 	phys_addr_t end = base + size - 1;
-	int ret;
+	int ret = 0;
 
 	memblock_dbg("%s: [%pa-%pa] %pS\n", __func__,
 		     &base, &end, (void *)_RET_IP_);
 
 	kmemleak_free_part_phys(base, size);
-	ret = memblock_remove_range(&memblock.reserved, base, size);
+
+	if (!slab_is_available() || IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK))
+		ret = memblock_remove_range(&memblock.reserved, base, size);
 
 	if (slab_is_available())
 		__free_reserved_area(base, base + size, -1);