PM: hibernate: Fix copying the zero bitmap to safe pages

The following crash is observed 100% of the time during resume from
the hibernation on a x86 QEMU system.

[   12.931887]  ? __die_body+0x1a/0x60
[   12.932324]  ? page_fault_oops+0x156/0x420
[   12.932824]  ? search_exception_tables+0x37/0x50
[   12.933389]  ? fixup_exception+0x21/0x300
[   12.933889]  ? exc_page_fault+0x69/0x150
[   12.934371]  ? asm_exc_page_fault+0x26/0x30
[   12.934869]  ? get_buffer.constprop.0+0xac/0x100
[   12.935428]  snapshot_write_next+0x7c/0x9f0
[   12.935929]  ? submit_bio_noacct_nocheck+0x2c2/0x370
[   12.936530]  ? submit_bio_noacct+0x44/0x2c0
[   12.937035]  ? hib_submit_io+0xa5/0x110
[   12.937501]  load_image+0x83/0x1a0
[   12.937919]  swsusp_read+0x17f/0x1d0
[   12.938355]  ? create_basic_memory_bitmaps+0x1b7/0x240
[   12.938967]  load_image_and_restore+0x45/0xc0
[   12.939494]  software_resume+0x13c/0x180
[   12.939994]  resume_store+0xa3/0x1d0

The commit being fixed introduced a bug in copying the zero bitmap
to safe pages. A temporary bitmap is allocated with PG_ANY flag in
prepare_image() to make a copy of zero bitmap after the unsafe pages
are marked. Freeing this temporary bitmap with PG_UNSAFE_KEEP later
results in an inconsistent state of unsafe pages. Since free bit is
left as is for this temporary bitmap after free, these pages are
treated as unsafe pages when they are allocated again. This results
in incorrect calculation of the number of pages pre-allocated for the
image.

nr_pages = (nr_zero_pages + nr_copy_pages) - nr_highmem - allocated_unsafe_pages;

The allocate_unsafe_pages is estimated to be higher than the actual
which results in running short of pages in safe_pages_list. Hence the
crash is observed in get_buffer() due to NULL pointer access of
safe_pages_list.

Fix this issue by creating the temporary zero bitmap from safe pages
(free bit not set) so that the corresponding free bits can be cleared
while freeing this bitmap.

Fixes: 005e8ddd ("PM: hibernate: don't store zero pages in the image file")
Suggested-by: : Brian Geffon <bgeffon@google.com>
Signed-off-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Reviewed-by: Brian Geffon <bgeffon@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/snapshot.c

@@ -2647,7 +2647,7 @@ static int prepare_image(struct memory_bitmap *new_bm, struct memory_bitmap *bm,
 	memory_bm_free(bm, PG_UNSAFE_KEEP);
 
 	/* Make a copy of zero_bm so it can be created in safe pages */
-	error = memory_bm_create(&tmp, GFP_ATOMIC, PG_ANY);
+	error = memory_bm_create(&tmp, GFP_ATOMIC, PG_SAFE);
 	if (error)
 		goto Free;
 
@@ -2660,7 +2660,7 @@ static int prepare_image(struct memory_bitmap *new_bm, struct memory_bitmap *bm,
 		goto Free;
 
 	duplicate_memory_bitmap(zero_bm, &tmp);
-	memory_bm_free(&tmp, PG_UNSAFE_KEEP);
+	memory_bm_free(&tmp, PG_UNSAFE_CLEAR);
 	/* At this point zero_bm is in safe pages and it can be used for restoring. */
 
 	if (nr_highmem > 0) {