+ zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch added to mm-hotfixes-unstable branch

+ zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: zsmalloc: zero-initialize zspage memory to prevent KMSAN uninit reads
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days

------------------------------------------------------
From: Kartik Nair <contact.kartikn@gmail.com>
Subject: zsmalloc: zero-initialize zspage memory to prevent KMSAN uninit reads
Date: Tue, 12 May 2026 03:06:58 +0530

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.

Link: https://lore.kernel.org/20260511213658.25273-1-contact.kartikn@gmail.com
Fixes: 56e5a103a721 ("zsmalloc: prefer the the original page's node for compressed data")
Reported-by: syzbot+8f77ff6144a73f0cf71b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8f77ff6144a73f0cf71b
Signed-off-by: Kartik Nair <contact.kartikn@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/zsmalloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/zsmalloc.c~zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads
+++ a/mm/zsmalloc.c
@@ -951,7 +951,7 @@ static struct zspage *alloc_zspage(struc
 	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]);
_

Patches currently in -mm which might be from contact.kartikn@gmail.com are

zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch

Re: + zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch added to mm-hotfixes-unstable branch

[Sergey Senozhatsky]

On (26/05/12 14:48), Andrew Morton wrote:
> The patch titled
>      Subject: zsmalloc: zero-initialize zspage memory to prevent KMSAN uninit reads
> has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
>      zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch
> 
> This patch will shortly appear at
>      https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zsmalloc-zero-initialize-zspage-memory-to-prevent-kmsan-uninit-reads.patch
> 
> This patch will later appear in the mm-hotfixes-unstable branch at
>     git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
> 
> Before you just go and hit "reply", please:
>    a) Consider who else should be cc'ed
>    b) Prefer to cc a suitable mailing list as well
>    c) Ideally: find the original patch on the mailing list and do a
>       reply-to-all to that, adding suitable additional cc's
> 
> *** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
> 
> The -mm tree is included into linux-next via various
> branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
> and is updated there most days
> 
> ------------------------------------------------------
> From: Kartik Nair <contact.kartikn@gmail.com>
> Subject: zsmalloc: zero-initialize zspage memory to prevent KMSAN uninit reads
> Date: Tue, 12 May 2026 03:06:58 +0530
> 
> 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.

This is very unlikely to fix anything, we should not have out-of-bounds
reads in the first place.