From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pdx-out-011.esa.us-west-2.outbound.mail-perimeter.amazon.com (pdx-out-011.esa.us-west-2.outbound.mail-perimeter.amazon.com [52.35.192.45]) (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 2C7A1227599 for ; Fri, 7 Nov 2025 12:19:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=52.35.192.45 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762517954; cv=none; b=cYJ/OW2miIReCVxbucW0aZnnzJJ09SFaFvhVCK2qNrDEystN9+ucHebWcxCYbwna/5LGkE4DGzKyl3jeOWd9Zqy4afs71vYaJXiPetkg0LyyxBr6Wa9Dm3JqJkhroMUKuOQKIKU3tXQltl9ylIwXt2Ne/db59mV3x+c2cIYPw2o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762517954; c=relaxed/simple; bh=IEEsvSLnLdLR3HzK6awpeGFZ9L1+bCdiZOEVaUTpEx8=; h=From:To:CC:Subject:Date:Message-ID:MIME-Version:Content-Type; b=r5SJqsraNR21MhhnoKtvDnUL3mRfo8Nk34Qi4gurwtqNf5KMSYMI5+6xJz+uKLcRdWLOGZz0pnwY2Jb6F3I3r2ZKrmgYRLsyQpkKFAa9bGfgL0Fh8lfUBnUsARnXdRPQcTg7HWTW8JP3qhJxqE1L8PPURMm58PaW0YAyMYTH14Q= 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=fQB6xAKU; arc=none smtp.client-ip=52.35.192.45 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="fQB6xAKU" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.de; i=@amazon.de; q=dns/txt; s=amazoncorp2; t=1762517952; x=1794053952; h=from:to:cc:subject:date:message-id:mime-version: content-transfer-encoding; bh=iC4VHAV1x9DMp6YRQbTpETxeQ3IKsuhEXkpQWnJpA6U=; b=fQB6xAKU+XRYV9KwJYlZeNM4vrfA/mwCIPMAUBbrBPXRrrKK54NWmiUA mK7TIaZLxw213tzk8mlv41dEBJzJq/Ubuma4/cbuRybuPBbE91OR0TOYX yj3NxnUxTZIADEGfFVsOYDNn//wCkwiM4XvmpnbP5NzaRWvTRY+tkiyza Y7EcMEQq5lX56H8AUpsmcBa6tYOFtKqR0/G9dh3HzOX0Y8uZgMpPBQlKD cQ/Ak1KUcKTaInP5/rjuZJlF9VqPW+v85lr8M3brt7Aft8hHWcaY7n0wk CM5FZ8ZuxLn1W5hm586jLfxipV7l+RXU6OAUD4Ld36gXn1VvmIcImJdPf A==; X-CSE-ConnectionGUID: FQsDeWY7SBm0jZKBGjrMsw== X-CSE-MsgGUID: Za46B6qtQN6kNoPwDQ45Tg== X-IronPort-AV: E=Sophos;i="6.19,286,1754956800"; d="scan'208";a="6397449" 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-011.esa.us-west-2.outbound.mail-perimeter.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Nov 2025 12:19:06 +0000 Received: from EX19MTAUWB002.ant.amazon.com [205.251.233.111:18560] by smtpin.naws.us-west-2.prod.farcaster.email.amazon.dev [10.0.33.235:2525] with esmtp (Farcaster) id 8268f991-8120-4265-ba79-439932378916; Fri, 7 Nov 2025 12:19:05 +0000 (UTC) X-Farcaster-Flow-ID: 8268f991-8120-4265-ba79-439932378916 Received: from EX19D001UWA001.ant.amazon.com (10.13.138.214) by EX19MTAUWB002.ant.amazon.com (10.250.64.231) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.29; Fri, 7 Nov 2025 12:18:59 +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 15.2.2562.29; Fri, 7 Nov 2025 12:18:57 +0000 From: Jakub Acs To: CC: , , Miguel Ojeda , David Hildenbrand , SeongJae Park , Alice Ryhl , Xu Xin , Chengming Zhou , Peter Xu , Axel Rasmussen , Andrew Morton Subject: [PATCH 5.15.y] mm/ksm: fix flag-dropping behavior in ksm_madvise Date: Fri, 7 Nov 2025 12:17:55 +0000 Message-ID: <20251107121755.14479-1-acsjakub@amazon.de> X-Mailer: git-send-email 2.47.3 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ClientProxiedBy: EX19D041UWA003.ant.amazon.com (10.13.139.105) To EX19D001UWA001.ant.amazon.com (10.13.138.214) Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit [ Upstream commit f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93 ] 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 NOPTI [ 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 &= ~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. [akpm@linux-foundation.org: rust bindgen wasn't able to handle BIT(), from Miguel] Link: https://lore.kernel.org/oe-kbuild-all/202510030449.VfSaAjvd-lkp@intel.com/ Link: https://lkml.kernel.org/r/20251001090353.57523-2-acsjakub@amazon.de Fixes: 7677f7fd8be7 ("userfaultfd: add minor fault registration mode") Signed-off-by: Jakub Acs Signed-off-by: Miguel Ojeda Acked-by: David Hildenbrand Acked-by: SeongJae Park Tested-by: Alice Ryhl Tested-by: Miguel Ojeda Cc: Xu Xin Cc: Chengming Zhou Cc: Peter Xu Cc: Axel Rasmussen Cc: Signed-off-by: Andrew Morton [ acsjakub: drop rust-compatibility change (no rust in 5.15) ] Signed-off-by: Jakub Acs --- Why sending to stable version from before "fixes"? In the original patch, I set fixes tag to the change that allows the panic to manifest, not to the one that is real root-cause of the problem. The change that introduced the root-cause of the problem is: f8af4da3b4c1 ("ksm: the mm interface to ksm"), as pointed out by Vlastimil in [1]. Hence, as the older kernels can be affected by the flag-drop as well, backport to older kernels. [1]: https://lore.kernel.org/all/13c7242e-3a40-469b-9e99-8a65a21449bb@suse.cz/ 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 3598925561b1..071dd864a7b2 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -310,7 +310,7 @@ extern unsigned int kobjsize(const void *objp); #define VM_MIXEDMAP 0x10000000 /* Can contain "struct page" and pure PFN pages */ #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 */ #ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS #define VM_HIGH_ARCH_BIT_0 32 /* bit only usable on 64-bit architectures */ -- 2.47.3 Amazon Web Services Development Center Germany GmbH Tamara-Danz-Str. 13 10243 Berlin Geschaeftsfuehrung: Christian Schlaeger, Christof Hellmis Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B Sitz: Berlin Ust-ID: DE 365 538 597