From nobody Sat Feb 7 08:43:22 2026 Received: from mail-oo1-f71.google.com (mail-oo1-f71.google.com [209.85.161.71]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EDA1F378816 for ; Mon, 2 Feb 2026 15:11:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.161.71 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770045114; cv=none; b=k246NaKCFx/vUDs/Lk2HcHkiGHAwog3gdQd0nZ7AI/V04G+8vfj0+BWICn1BDgVLRcOvABwTCWYngks4zVA3ElQ2ezE/+TFhNLBMxPl8o91TZS9TQfuq+a3eX8GJuLXTJ8p50u/yk1fPOwnVbTW1cTE2iUfhrb/ZGxHGuSIwgVA= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770045114; c=relaxed/simple; bh=2wOW7xa3fsBSjt/9vajstfXVG1DpV2i16jYH255Tn/E=; h=MIME-Version:Date:In-Reply-To:Message-ID:Subject:From:To: Content-Type; b=RLWlmVovk3syOhwCmMCeJNBLd2F8BBzSD4IMfaVkZpu8q15FApPV6QkZTHQJwYK8nsnnjO2ikehRgX5NoeIQZNXeVU3XcFXwZjiuD6+JDVyO//N+6GePKLVb/jSLcMuEXhA9df+wrT+KY45S0ffh/HkqwYQhxRNYIBC2ou3tL+U= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=syzkaller.appspotmail.com; spf=pass smtp.mailfrom=M3KW2WVRGUFZ5GODRSRYTGD7.apphosting.bounces.google.com; arc=none smtp.client-ip=209.85.161.71 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=syzkaller.appspotmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=M3KW2WVRGUFZ5GODRSRYTGD7.apphosting.bounces.google.com Received: by mail-oo1-f71.google.com with SMTP id 006d021491bc7-66497f2cd6dso6387248eaf.2 for ; Mon, 02 Feb 2026 07:11:52 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1770045112; x=1770649912; h=to:from:subject:message-id:in-reply-to:date:mime-version :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=SnvP88T/yIKHoq9MV4l0CNS6wi1B9TQ+h6eUslQjGRQ=; b=Bx6civ5EGzehEtvdXE3Vql4oWLb1cEc6fQYdN3z6llaiBvJMGb7X/gcOyvZ5Cm4g/1 K/Zp1IfvMfVf+3CSOvvXzZ+mPfqP1PYgUi+gAOCOaEBAR8hirrsFr7zCLp10QjGYlGp/ 6K06bUh2RKVZYTmwCAX0p3gKPifI5M+otPSB8l3fvdZy9ISkX5Y51roHizxAs+/2OGiT a4PSVvRVF5aAga92wQLavx/h2qAgMXyu3HPSqCa04iWwwe7STObxW2BpzwwKrfARQ4jv 0E8u2jniG8WBR7IauarhalI0iM1YzV7nnmiQuVd9eoU8DEtTxxgeGXN7I8gHp64+WKRF F8WQ== X-Gm-Message-State: AOJu0YzMZT4Z1HQc7hJsQvJcmOA2HD0fkEyomLtaNEsWcbaxQyzuxkiO y2uD7qjeU0kE44GLduy8Fte3iFdlWjTz7fY1i2k4OznbpzemzznkYq4RACtx27jAlMpyKv4r038 O2tZgdVmFWd2REeCldRpdghEwbr0D8eqlXXol7gHNkNH+SpqN46amjZQyxRM= Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Received: by 2002:a05:6820:4512:b0:662:f898:2382 with SMTP id 006d021491bc7-6630f3a66bfmr4053410eaf.63.1770045111793; Mon, 02 Feb 2026 07:11:51 -0800 (PST) Date: Mon, 02 Feb 2026 07:11:51 -0800 In-Reply-To: <697d115a.050a0220.1d61ec.0004.GAE@google.com> X-Google-Appengine-App-Id: s~syzkaller X-Google-Appengine-App-Id-Alias: syzkaller Message-ID: <6980beb7.a00a0220.488d7.000c.GAE@google.com> Subject: Forwarded: [PATCH] KVM: guest_memfd: Reject large folios until support is implemented From: syzbot To: linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" For archival purposes, forwarding an incoming command email to linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com. *** Subject: [PATCH] KVM: guest_memfd: Reject large folios until support is imp= lemented Author: kartikey406@gmail.com #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git= master Large folios are not yet supported in guest_memfd (see TODO comment in kvm_gmem_get_folio()), but can still be allocated if userspace uses madvise(MADV_HUGEPAGE), which overrides the folio order restrictions set by mapping_set_folio_order_range(). When a large folio is allocated, it triggers WARN_ON_ONCE() at line 416 in kvm_gmem_fault_user_mapping(), causing a kernel panic if panic_on_warn is enabled. Add mapping_set_folio_order_range(0, 0) as defense in depth, and actively check for large folios in kvm_gmem_get_folio() on both the fast-path (existing folio) and slow-path (newly created folio). If a large folio is found, unlock it, drop the reference, and return -E2BIG to prevent the WARNING from triggering. This avoids kernel panics when panic_on_warn is enabled. Reported-by: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com Signed-off-by: Deepanshu Kartikey --- virt/kvm/guest_memfd.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index fdaea3422c30..ee5bcf238f98 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -143,13 +143,29 @@ static struct folio *kvm_gmem_get_folio(struct inode = *inode, pgoff_t index) folio =3D __filemap_get_folio(inode->i_mapping, index, FGP_LOCK | FGP_ACCESSED, 0); if (!IS_ERR(folio)) - return folio; + goto check_folio; =20 policy =3D mpol_shared_policy_lookup(&GMEM_I(inode)->policy, index); folio =3D __filemap_get_folio_mpol(inode->i_mapping, index, FGP_LOCK | FGP_ACCESSED | FGP_CREAT, mapping_gfp_mask(inode->i_mapping), policy); mpol_cond_put(policy); + if (IS_ERR(folio)) + return folio; +check_folio: + /* + * Large folios are not supported yet. This can still happen + * despite mapping_set_folio_order_range() if userspace uses + * madvise(MADV_HUGEPAGE) which can override the folio order + * restrictions. Reject the large folio and remove it from + * the page cache so the next fault can allocate a order-0 + * page instead. + */ + if (folio_test_large(folio)) { + folio_unlock(folio); + folio_put(folio); + return ERR_PTR(-E2BIG); + } =20 return folio; } @@ -596,6 +612,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t si= ze, u64 flags) inode->i_mode |=3D S_IFREG; inode->i_size =3D size; mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER); + mapping_set_folio_order_range(inode->i_mapping, 0, 0); mapping_set_inaccessible(inode->i_mapping); /* Unmovable mappings are supposed to be marked unevictable as well. */ WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping)); --=20 2.43.0