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

[Herbert Xu <herbert@gondor.apana.org.au>]

On Wed, May 13, 2026 at 11:47:41AM +0900, Sergey Senozhatsky wrote:
> Adding Yosry and Herbert,
> 
> On (26/05/12 14:47), Andrew Morton 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).
> 
> I don't get this.  How can sw842_decompress() read more bytes than
> it's told to decompress.  We first compress and store the object,
> before we load and decompress, reading past the known compressed
> object size (which we pass to decompress function) should not happen.
> Yosry, Herbert, any ideas?

It sounds like a bug in 842.  I'll look into it.

Thanks for pointer.
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
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