From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 27A02CCF2D3 for ; Mon, 19 Oct 2026 04:35:25 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 3C80F6B0119; Mon, 19 Oct 2026 00:35:24 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 378826B011A; Mon, 19 Oct 2026 00:35:24 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2909F6B011B; Mon, 19 Oct 2026 00:35:24 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id F39466B0119 for ; Mon, 19 Oct 2026 00:35:23 -0400 (EDT) Received: from smtpin12.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 85C40405D3 for ; Mon, 19 Oct 2026 04:35:23 +0000 (UTC) X-FDA: 85338111726.12.87B49F6 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf14.hostedemail.com (Postfix) with ESMTP id EA9B1100003 for ; Mon, 19 Oct 2026 04:35:21 +0000 (UTC) Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=OzQGxFq7; spf=pass (imf14.hostedemail.com: domain of sashiko-bot@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=sashiko-bot@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1792384521; h=from:from:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=4yJfhlID4fPBOMkqV1WPDOqXgcDXux6NyWocqTjnn2g=; b=mTGQDtTVNiCjYTrgaYhgdXDm5LlHDXpMqQcdlrTkGIG6rJ2WKD0GQV4MZj72gqYZSckTlx LtCEvi2RAtoCWygs6aDJrSVIfyMCSVx4G3VX+SiyOcxX8sZEgkVmwYfysuvAH59OO8lfKU JKckHLf4OP0p7jOX5S3mJdJEwSJ3/yQ= ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1792384521; b=MOpqgZfS8IbbJVt6LXk14ybJ3XSV0K84abWT9DrgSjog5fMkUWxmp4gY73S0gdXDnFEQKh gxtB2klfqRSPQNkQF6X8FD+2gGT3qpU79hO9+DC9wrE8TLb04tR/+ZBaOcELkKr0rJhykx zew0qsMnLesNJLqvtswqbV9t4k3wrbY= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=OzQGxFq7; spf=pass (imf14.hostedemail.com: domain of sashiko-bot@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=sashiko-bot@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 665EB601DB; Mon, 19 Oct 2026 04:35:21 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 404CC1F000C4; Mon, 19 Oct 2026 04:35:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792384521; bh=4yJfhlID4fPBOMkqV1WPDOqXgcDXux6NyWocqTjnn2g=; h=From:Subject:Reply-To:To:Cc:In-Reply-To:References:Date; b=OzQGxFq7LnoaH2sBKLf4N2VMO2ZNXiWKD2jjmIm7oya1wsXuWDnVXYYSdz1i7KPN9 PL2VCbQv09vKJb9fU6nZ2w0zli5D8OUDeOWUAFqIor5+PS0aXbPU81bUIfCXmnhKqv Bb2E9f+YNIAKNBmxMUAvubAT9eWsla59+tHY91iu7JKjcwWJciQhsRnIsY84PZgSsR G1CvKKxjmLaILM06FvWzPCz1E05w+Vqg3fjjKhaKJ4s3WLoGeRiIELwXLFBf9YvdPj K+4gw6q0RHz2Zsxzq3uMbB4jJbLEvLKKgJPRLH+TQPUAS31EMEvaB3FRhjj2AgaVHJ qXBNHllpoMrgA== Message-ID: From: sashiko-bot@kernel.org Subject: Re: [PATCH v2] kasan: fix runtime shadow page table synchronization Reply-To: sashiko-reviews@lists.linux.dev To: "Muchun Song" , "Andrew Morton" , "David Hildenbrand" , "Andrey Ryabinin" Cc: "Lorenzo Stoakes" , "Liam R . Howlett" , "Vlastimil Babka" , "Mike Rapoport" , "Suren Baghdasaryan" , "Michal Hocko" , "Alexander Potapenko" , "Andrey Konovalov" , "Dmitry Vyukov" , "Vincenzo Frascino" , "Dan Williams" , linux-mm@kvack.org, kasan-dev@googlegroups.com, muchun.song@linux.dev In-Reply-To: <20261018043501.3257632-1-songmuchun@bytedance.com> References: <20261018043501.3257632-1-songmuchun@bytedance.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Date: Mon, 19 Oct 2026 04:35:19 +0000 X-Stat-Signature: a8w3k86n9j6k997iapd1txwxjtjz6t48 X-Rspam-User: X-Rspamd-Queue-Id: EA9B1100003 X-Rspamd-Server: rspam10 X-HE-Tag: 1792384521-456206 X-HE-Meta: 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 7Sd0ruZ7 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Thank you for your contribution! Sashiko AI review found 2 potential issue(= s) to consider: - [High] kasan: syncing static early shadow tables breaks kasan_remove_zero= _shadow() - [High] kasan: missing kasan_p*d_table() checks in kasan_populate_early_sh= adow() -- commit 84e267a96cb1eab3b9fd041e188f62b5f7899dd0 Author: Muchun Song kasan: fix runtime shadow page table synchronization This commit tracks page table modifications during KASAN zero-shadow population and calls arch_sync_kernel_mappings() once after the walk. It al= so avoids replacing already-populated page table entries during aligned shadow population and retains page tables referenced by synchronized entries during teardown. > diff --git a/mm/kasan/init.c b/mm/kasan/init.c > index 66a8838879876..f94f461d864ca 100644 > --- a/mm/kasan/init.c > +++ b/mm/kasan/init.c [ ... ] > @@ -115,25 +116,26 @@ static int __ref zero_pmd_populate(pud_t *pud, unsi= gned long addr, > do { > next =3D pmd_addr_end(addr, end); > =20 > - if (IS_ALIGNED(addr, PMD_SIZE) && end - addr >=3D PMD_SIZE) { > + if (IS_ALIGNED(addr, PMD_SIZE) && end - addr >=3D PMD_SIZE && > + pmd_none(*pmd)) { > pmd_populate_kernel(&init_mm, pmd, > lm_alias(kasan_early_shadow_pte)); > + *mask |=3D PGTBL_PMD_MODIFIED; > continue; > } [Severity: High] What happens in zero_pmd_populate(), zero_pud_populate(), zero_p4d_populate= (), and kasan_populate_early_shadow() when the page table entry is not none because it already points to the shared static early shadow table (kasan_pte_table(*pmd), kasan_pmd_table(*pud), kasan_pud_table(*p4d), or kasan_p4d_table(*pgd))? For example, in arch/loongarch/mm/kasan_init.c, kasan_init() pre-populates swapper_pg_dir across the entire shadow range with kasan_early_shadow_* bef= ore calling kasan_populate_early_shadow(): arch/loongarch/mm/kasan_init.c:kasan_init() { /* Maps everything to a single page of zeroes */ kasan_pgd_populate(KASAN_SHADOW_START, KASAN_SHADOW_END, NUMA_NO_NODE, tru= e); kasan_populate_early_shadow(mem_to_shadow((void *)VMALLOC_START), mem_to_shadow((void *)KFENCE_AREA_END)); } Because *p4d, *pud, and *pmd point to kasan_early_shadow_pud, kasan_early_shadow_pmd, and kasan_early_shadow_pte rather than invalid_pud_table, invalid_pmd_table, and invalid_pte_table, p4d_none(*p4d), pud_none(*pud), and pmd_none(*pmd) all evaluate to false. Instead of skipping aligned PGDIR_SIZE, P4D_SIZE, PUD_SIZE, and PMD_SIZE blocks, kasan_populate_early_shadow() descends into zero_pte_populate() for every single PMD across the multi-terabyte VMALLOC_START..KFENCE_AREA_END shadow range, executing billions of redundant set_pte_at() calls on the same shared kasan_early_shadow_pte array (and on 16KB-page LoongArch configs whe= re mem_to_shadow(VMALLOC_START) is not PMD-aligned, prematurely write-protecti= ng kasan_early_shadow_pte entries while kasan_pg_dir is still active). The same descent into the shared static kasan_early_shadow_* tables happens= at runtime if kasan_add_zero_shadow() -> kasan_populate_early_shadow() is call= ed again on a range where kasan_remove_zero_shadow() previously retained a synchronized kasan_early_shadow_* entry. Should these aligned fast paths also check whether the entry already points= to kasan_early_shadow_{p4d,pud,pmd,pte} (and skip descending without needing to re-populate or set the modified mask)? [ ... ] > @@ -377,6 +420,9 @@ static void kasan_remove_pmd_table(pmd_t *pmd, unsign= ed long addr, > continue; > =20 > if (kasan_pte_table(*pmd)) { > + if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PMD_MODIFIED) > + continue; > + > if (IS_ALIGNED(addr, PMD_SIZE) && > IS_ALIGNED(next, PMD_SIZE)) { > pmd_clear(pmd); > @@ -403,6 +449,9 @@ static void kasan_remove_pud_table(pud_t *pud, unsign= ed long addr, > continue; > =20 > if (kasan_pmd_table(*pud)) { > + if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PUD_MODIFIED) > + continue; > + > if (IS_ALIGNED(addr, PUD_SIZE) && > IS_ALIGNED(next, PUD_SIZE)) { > pud_clear(pud); > @@ -430,6 +479,9 @@ static void kasan_remove_p4d_table(p4d_t *p4d, unsign= ed long addr, > continue; > =20 > if (kasan_pud_table(*p4d)) { > + if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_P4D_MODIFIED) > + continue; > + > if (IS_ALIGNED(addr, P4D_SIZE) && > IS_ALIGNED(next, P4D_SIZE)) { > p4d_clear(p4d); > @@ -464,6 +516,9 @@ void kasan_remove_zero_shadow(void *start, unsigned l= ong size) > continue; > =20 > if (kasan_p4d_table(*pgd)) { > + if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PGD_MODIFIED) > + continue; > + > if (IS_ALIGNED(addr, PGDIR_SIZE) && > IS_ALIGNED(next, PGDIR_SIZE)) { > pgd_clear(pgd); [Severity: High] Can skipping synchronized kasan_early_shadow_{p4d,pud,pmd,pte} entries here prevent kasan_remove_zero_shadow() from unmapping the zero shadow at all? When kasan_add_zero_shadow() -> kasan_populate_early_shadow() runs at runti= me (with slab_is_available() =3D=3D true) for a range covering an aligned PGDI= R_SIZE, P4D_SIZE, PUD_SIZE, or PMD_SIZE boundary at a level included in ARCH_PAGE_TABLE_SYNC_MASK (for example, PGTBL_P4D_MODIFIED on 4-level x86_6= 4, PGTBL_PGD_MODIFIED on 5-level x86_64, or PGTBL_PMD_MODIFIED on 32-bit x86/A= RM), it installs the shared static kasan_early_shadow_{p4d,pud,pmd,pte} table directly into the synchronized entry. When kasan_remove_zero_shadow() is later called to remove that shadow mappi= ng, hitting this continue leaves the entry pointing to kasan_early_shadow_*. Unlike a dynamically allocated page table (where kasan_remove_*_table() descends and clears all child entries before kasan_free_*() retains only the empty top-level table), kasan_early_shadow_* is a shared static table whose entries cannot be cleared and remain permanently mapped to the read-only kasan_early_shadow_page. If normal System RAM is subsequently hot-added and onlined in that address range, kasan_mem_notifier() in mm/kasan/shadow.c checks shadow_mapped(): mm/kasan/shadow.c:kasan_mem_notifier() { ... case MEM_GOING_ONLINE: { ... if (shadow_mapped(shadow_start)) return NOTIFY_OK; } Because the read-only kasan_early_shadow_page PTE is still present, shadow_mapped() returns true and kasan_mem_notifier() skips allocating writable shadow memory, causing subsequent KASAN shadow writes to fault on = the read-only zero shadow page. In addition, if a smaller or unaligned range within that region is later passed to kasan_add_zero_shadow(), kasan_populate_early_shadow() sees !p*d_none(*p*d) and descends into and modifies the shared global kasan_early_shadow_* tables. Should kasan_populate_early_shadow() avoid installing the static kasan_early_shadow_{p4d,pud,pmd,pte} tables at page table levels that are in ARCH_PAGE_TABLE_SYNC_MASK when slab_is_available() is true, and instead allocate a normal page table at that synchronized level so its entries can = be cleared during kasan_remove_zero_shadow()? --=20 Sashiko AI review =C2=B7 https://sashiko.dev/#/patchset/20261018043501.3257= 632-1-songmuchun@bytedance.com?part=3D1