From nobody Wed Oct 1 22:31:24 2025 Received: from pdx-out-008.esa.us-west-2.outbound.mail-perimeter.amazon.com (pdx-out-008.esa.us-west-2.outbound.mail-perimeter.amazon.com [52.42.203.116]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B9D062DAFDD; Wed, 1 Oct 2025 09:04:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=52.42.203.116 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759309467; cv=none; b=QUrNKbz22zx/rT8TcFVtbxWjpzi2w8stLGZ8USJ49eS0rcdYAIULiH5UH+oLSz3IsgCMCkPWx2/JwLun9LKwdyChrxXeeXBB6xAk6bl49Sg39snbRKh4ueFdUdKx2aTVsDaTx3MxEUvlMLZX7HrBl8DrxRf1MsnKVqnffVHxEsY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759309467; c=relaxed/simple; bh=7HSxwXm+a/QSLPKWcep56Ih/v+0i2+KazMsze1uGMZ8=; h=From:To:CC:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=HgnK/7ca0qyas8z742E8Qk8epg7nsEZcaJ86cK/Eb/OC7EoSJMDtEQZZZnM2jr/iqbH4zGvtnCrfI1Be8G8IrRZwzC1zoA2ghPx5tHN2HlKK0LJYM1KkrJ9Mb6ZSKg3tkcDv0pdtRZLMtu5XyXDIeXuJ9CvKYWPKkZQYFFewx7o= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.de; spf=pass smtp.mailfrom=amazon.de; dkim=pass (2048-bit key) header.d=amazon.de header.i=@amazon.de header.b=P7Kexn1g; arc=none smtp.client-ip=52.42.203.116 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=amazon.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=amazon.de header.i=@amazon.de header.b="P7Kexn1g" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.de; i=@amazon.de; q=dns/txt; s=amazoncorp2; t=1759309465; x=1790845465; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=OzPLc+IFhXU2hCT/ga3y8QABI/mqNeOmZhS6AUI98bs=; b=P7Kexn1gutVrcl+bBNfXrbUMeg66qNUqO77t4iMoO42bl8CoDRoxikuI 6CHIMeFUdwjS2GfvRUQiNbgnRUIEXKsS/TQyJamZCuFnvRG+wYzWb5ViR psvT5StyoNzFVjTxWMKJMdtHxHUo8pCTE4BINvd+QXrZn9uwsZQjMDJA0 ZpXTds+W2jDtmtlflKKKdKXQQmY3RtJqWs0S2WKLRR102IPOsFOrztzAT +IpynbmXoIhcLVSuHernbgeRxcFfkDBrVEApl9Am4tJyQ9p2VpNS/Xcsy Y/0dNYLGE2nzGA7VK0XtspcxgqKjBKGcYeMQLfR7IKqunS3+TL3g5mhsc A==; X-CSE-ConnectionGUID: FGEcwYeHToCCs/kvg08eTA== X-CSE-MsgGUID: fHqXDOd5TZmmQ05DQ2mzkQ== X-IronPort-AV: E=Sophos;i="6.18,306,1751241600"; d="scan'208";a="4058017" Received: from ip-10-5-0-115.us-west-2.compute.internal (HELO smtpout.naws.us-west-2.prod.farcaster.email.amazon.dev) ([10.5.0.115]) by internal-pdx-out-008.esa.us-west-2.outbound.mail-perimeter.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Oct 2025 09:04:25 +0000 Received: from EX19MTAUWC002.ant.amazon.com [10.0.21.151:30255] by smtpin.naws.us-west-2.prod.farcaster.email.amazon.dev [10.0.32.190:2525] with esmtp (Farcaster) id e9f24429-472f-496a-a05a-30cd931969d7; Wed, 1 Oct 2025 09:04:25 +0000 (UTC) X-Farcaster-Flow-ID: e9f24429-472f-496a-a05a-30cd931969d7 Received: from EX19D001UWA001.ant.amazon.com (10.13.138.214) by EX19MTAUWC002.ant.amazon.com (10.250.64.143) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.20; Wed, 1 Oct 2025 09:04:25 +0000 Received: from dev-dsk-acsjakub-1b-6f9934e2.eu-west-1.amazon.com (172.19.75.107) by EX19D001UWA001.ant.amazon.com (10.13.138.214) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.20; Wed, 1 Oct 2025 09:04:23 +0000 From: Jakub Acs To: CC: , , , , , , , , Subject: [PATCH v3 1/2] mm/ksm: fix flag-dropping behavior in ksm_madvise Date: Wed, 1 Oct 2025 09:03:52 +0000 Message-ID: <20251001090353.57523-2-acsjakub@amazon.de> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20251001090353.57523-1-acsjakub@amazon.de> References: <20251001090353.57523-1-acsjakub@amazon.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ClientProxiedBy: EX19D038UWC002.ant.amazon.com (10.13.139.238) To EX19D001UWA001.ant.amazon.com (10.13.138.214) Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" syzkaller discovered the following crash: (kernel BUG) [ 44.607039] ------------[ cut here ]------------ [ 44.607422] kernel BUG at mm/userfaultfd.c:2067! [ 44.608148] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NO= PTI [ 44.608814] CPU: 1 UID: 0 PID: 2475 Comm: reproducer Not tainted 6.16.0-= rc6 #1 PREEMPT(none) [ 44.609635] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS = rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 [ 44.610695] RIP: 0010:userfaultfd_release_all+0x3a8/0x460 [ 44.617726] Call Trace: [ 44.617926] [ 44.619284] userfaultfd_release+0xef/0x1b0 [ 44.620976] __fput+0x3f9/0xb60 [ 44.621240] fput_close_sync+0x110/0x210 [ 44.622222] __x64_sys_close+0x8f/0x120 [ 44.622530] do_syscall_64+0x5b/0x2f0 [ 44.622840] entry_SYSCALL_64_after_hwframe+0x76/0x7e [ 44.623244] RIP: 0033:0x7f365bb3f227 Kernel panics because it detects UFFD inconsistency during userfaultfd_release_all(). Specifically, a VMA which has a valid pointer to vma->vm_userfaultfd_ctx, but no UFFD flags in vma->vm_flags. The inconsistency is caused in ksm_madvise(): when user calls madvise() with MADV_UNMEARGEABLE on a VMA that is registered for UFFD in MINOR mode, it accidentally clears all flags stored in the upper 32 bits of vma->vm_flags. Assuming x86_64 kernel build, unsigned long is 64-bit and unsigned int and int are 32-bit wide. This setup causes the following mishap during the &=3D ~VM_MERGEABLE assignment. VM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000'0000. After ~ is applied, it becomes 0x7fff'ffff unsigned int, which is then promoted to unsigned long before the & operation. This promotion fills upper 32 bits with leading 0s, as we're doing unsigned conversion (and even for a signed conversion, this wouldn't help as the leading bit is 0). & operation thus ends up AND-ing vm_flags with 0x0000'0000'7fff'ffff instead of intended 0xffff'ffff'7fff'ffff and hence accidentally clears the upper 32-bits of its value. Fix it by changing `VM_MERGEABLE` constant to unsigned long, using the BIT() macro. Note: other VM_* flags are not affected: This only happens to the VM_MERGEABLE flag, as the other VM_* flags are all constants of type int and after ~ operation, they end up with leading 1 and are thus converted to unsigned long with leading 1s. Note 2: After commit 31defc3b01d9 ("userfaultfd: remove (VM_)BUG_ON()s"), this is no longer a kernel BUG, but a WARNING at the same place: [ 45.595973] WARNING: CPU: 1 PID: 2474 at mm/userfaultfd.c:2067 but the root-cause (flag-drop) remains the same. Fixes: 7677f7fd8be76 ("userfaultfd: add minor fault registration mode") Signed-off-by: Jakub Acs Cc: Andrew Morton Cc: David Hildenbrand Cc: Xu Xin Cc: Chengming Zhou Cc: Peter Xu Cc: Axel Rasmussen Cc: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org Acked-by: David Hildenbrand Acked-by: SeongJae Park --- include/linux/mm.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 1ae97a0b8ec7..c6794d0e24eb 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -296,7 +296,7 @@ extern unsigned int kobjsize(const void *objp); #define VM_MIXEDMAP 0x10000000 /* Can contain "struct page" and pure PFN p= ages */ #define VM_HUGEPAGE 0x20000000 /* MADV_HUGEPAGE marked this vma */ #define VM_NOHUGEPAGE 0x40000000 /* MADV_NOHUGEPAGE marked this vma */ -#define VM_MERGEABLE 0x80000000 /* KSM may merge identical pages */ +#define VM_MERGEABLE BIT(31) /* KSM may merge identical pages */ =20 #ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS #define VM_HIGH_ARCH_BIT_0 32 /* bit only usable on 64-bit architectures */ --=20 2.47.3 Amazon Web Services Development Center Germany GmbH Tamara-Danz-Str. 13 10243 Berlin Geschaeftsfuehrung: Christian Schlaeger Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B Sitz: Berlin Ust-ID: DE 365 538 597