[PATCH] zsmalloc: zero-initialize zspage memory to prevent KMSAN uninit reads

[Kartik Nair <contact.kartikn@gmail.com>]

Pages allocated via alloc_zpdesc() use alloc_pages_node() without
__GFP_ZERO, leaving physical memory uninitialized. When a compressed
object spans two physical pages in a zspage, zs_obj_read_sg_begin()
sets up a scatterlist pointing directly at the raw second page. If the
second page was freshly allocated and never written beyond the object
boundary, KMSAN detects reads of uninitialized memory downstream in
the decompressor (e.g. sw842_decompress reading the CRC trailer).

Fix this by passing __GFP_ZERO to alloc_zpdesc() in alloc_zspage() so
all pages backing a zspage are zero-initialized at allocation time.

Reported-by: syzbot+8f77ff6144a73f0cf71b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8f77ff6144a73f0cf71b
Signed-off-by: Kartik Nair <contact.kartikn@gmail.com>
---
 mm/zsmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 63128ddb7..5bbd417d3 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -951,7 +951,7 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
 	for (i = 0; i < class->pages_per_zspage; i++) {
 		struct zpdesc *zpdesc;
 
-		zpdesc = alloc_zpdesc(gfp, nid);
+		zpdesc = alloc_zpdesc(gfp | __GFP_ZERO, nid);
 		if (!zpdesc) {
 			while (--i >= 0) {
 				zpdesc_dec_zone_page_state(zpdescs[i]);
-- 
2.39.5 (Apple Git-154)