mm/migrate: fix do_pages_stat in compat mode

For arrays with more than 16 entries, the old code would incorrectly
advance the pages pointer by 16 words instead of 16 compat_uptr_t.  Fix by
doing the pointer arithmetic inside get_compat_pages_array where pages32
is already a correctly-typed pointer.

Discovered while working on PostgreSQL 18's new NUMA introspection code.

Link: https://lkml.kernel.org/r/aGREU0XTB48w9CwN@msg.df7cb.de
Fixes: 5b1b561 ("mm: simplify compat_sys_move_pages")
Signed-off-by: Christoph Berg <myon@debian.org>
Acked-by: David Hildenbrand <david@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Reported-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reported-by: Tomas Vondra <tomas@vondra.me>
Closes: https://www.postgresql.org/message-id/flat/6342f601-77de-4ee0-8c2a-3deb50ceac5b%40vondra.me#86402e3d80c031788f5f55b42c459471
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: df7cb

mm/migrate.c

@@ -2399,14 +2399,15 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 
 static int get_compat_pages_array(const void __user *chunk_pages[],
 				  const void __user * __user *pages,
+				  unsigned long chunk_offset,
 				  unsigned long chunk_nr)
 {
 	compat_uptr_t __user *pages32 = (compat_uptr_t __user *)pages;
 	compat_uptr_t p;
 	int i;
 
 	for (i = 0; i < chunk_nr; i++) {
-		if (get_user(p, pages32 + i))
+		if (get_user(p, pages32 + chunk_offset + i))
 			return -EFAULT;
 		chunk_pages[i] = compat_ptr(p);
 	}
@@ -2425,27 +2426,28 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
 #define DO_PAGES_STAT_CHUNK_NR 16UL
 	const void __user *chunk_pages[DO_PAGES_STAT_CHUNK_NR];
 	int chunk_status[DO_PAGES_STAT_CHUNK_NR];
+	unsigned long chunk_offset = 0;
 
 	while (nr_pages) {
 		unsigned long chunk_nr = min(nr_pages, DO_PAGES_STAT_CHUNK_NR);
 
 		if (in_compat_syscall()) {
 			if (get_compat_pages_array(chunk_pages, pages,
-						   chunk_nr))
+						   chunk_offset, chunk_nr))
 				break;
 		} else {
-			if (copy_from_user(chunk_pages, pages,
+			if (copy_from_user(chunk_pages, pages + chunk_offset,
 				      chunk_nr * sizeof(*chunk_pages)))
 				break;
 		}
 
 		do_pages_stat_array(mm, chunk_nr, chunk_pages, chunk_status);
 
-		if (copy_to_user(status, chunk_status, chunk_nr * sizeof(*status)))
+		if (copy_to_user(status + chunk_offset, chunk_status,
+				 chunk_nr * sizeof(*status)))
 			break;
 
-		pages += chunk_nr;
-		status += chunk_nr;
+		chunk_offset += chunk_nr;
 		nr_pages -= chunk_nr;
 	}
 	return nr_pages ? -EFAULT : 0;