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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 12 May 2026 03:06:58 +0530 Kartik Nair <contact.kartikn@gmail.com> wrote:

> 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>

Thanks.

> --- 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]);

Decompressing uninitialized memory sounds rather bad, so I'll add a
cc:stable to this.

I think the Fixes: target is 56e5a103a721 ("zsmalloc: prefer the the
original page's node for compressed data").  Can people please check
this when reviewing?