alloc_tag: fix module allocation tags populated area calculation

vm_module_tags_populate() calculation of the populated area assumes that
area starts at a page boundary and therefore when new pages are allocation,
the end of the area is page-aligned as well. If the start of the area is
not page-aligned then allocating a page and incrementing the end of the
area by PAGE_SIZE leads to an area at the end but within the area boundary
which is not populated. Accessing this are will lead to a kernel panic.
Fix the calculation by down-aligning the start of the area and using that
as the location allocated pages are mapped to.

Link: https://lkml.kernel.org/r/20241130001423.1114965-1-surenb@google.com
Fixes: 0f9b685 ("alloc_tag: populate memory for module tags as needed")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202411132111.6a221562-lkp@intel.com
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: David Wang <00107082@163.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Sourav Panda <souravpanda@google.com>
Cc: <stable@vger.kernel.org>
Cc: Hao Ge <gehao@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/alloc_tag.c

@@ -408,19 +408,20 @@ static bool find_aligned_area(struct ma_state *mas, unsigned long section_size,
 
 static int vm_module_tags_populate(void)
 {
-	unsigned long phys_size = vm_module_tags->nr_pages << PAGE_SHIFT;
+	unsigned long phys_end = ALIGN_DOWN(module_tags.start_addr, PAGE_SIZE) +
+				 (vm_module_tags->nr_pages << PAGE_SHIFT);
+	unsigned long new_end = module_tags.start_addr + module_tags.size;
 
-	if (phys_size < module_tags.size) {
+	if (phys_end < new_end) {
 		struct page **next_page = vm_module_tags->pages + vm_module_tags->nr_pages;
-		unsigned long addr = module_tags.start_addr + phys_size;
 		unsigned long more_pages;
 		unsigned long nr;
 
-		more_pages = ALIGN(module_tags.size - phys_size, PAGE_SIZE) >> PAGE_SHIFT;
+		more_pages = ALIGN(new_end - phys_end, PAGE_SIZE) >> PAGE_SHIFT;
 		nr = alloc_pages_bulk_array_node(GFP_KERNEL | __GFP_NOWARN,
 						 NUMA_NO_NODE, more_pages, next_page);
 		if (nr < more_pages ||
-		    vmap_pages_range(addr, addr + (nr << PAGE_SHIFT), PAGE_KERNEL,
+		    vmap_pages_range(phys_end, phys_end + (nr << PAGE_SHIFT), PAGE_KERNEL,
 				     next_page, PAGE_SHIFT) < 0) {
 			/* Clean up and error out */
 			for (int i = 0; i < nr; i++)