From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 E6A89265CC0 for ; Tue, 30 Sep 2025 06:45:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759214757; cv=none; b=MpNFhbwZj6Aa/NUaNbYI4Jd4JgTI8xogl7HIaWcwJOXIHcRpxWy3bxoHvYsuF4TJTh+pKZx4ZFuatz5KKkHywikM3bXh+ASRxbkp1+OIKIXnxDnBJNUEbLrPU5rr4+IbipyVb7+E3m3pWViJKy+Td1VgOGo7FZ7Lx/R4d1VHcKg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759214757; c=relaxed/simple; bh=JoBHMz2B+sjw9rguNZd8esTdi3gTDyQjuYPnndRxhgQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tScCS5z4SLCl+PbauLANIUi4YX5IEN8B7uEqNa2TKFxlLwEp1Zr8GqlvltQkrkcf3ntCxtU0T94FckTdCPSb8IJM21TinuEejxYfWnaOpXP7SsBn1yYozkXeDXbJAjHxCjOJXomaMXYxp1xA0KDdoH8wqTEqlvZJNr5ZFT2F1OY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Xibjcj7C; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Xibjcj7C" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1759214754; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=8AIMq+OlhpyT2npgIFmZrEOK/CbKwyfphzbxq2DUajU=; b=Xibjcj7CHeBLRgtTVaUSMBuClYACz5rRf8o0x2mJUi4vR9R9mhdPBwSLr4nk4BoIZwN0yn l0crbBv8OWf0Y/5K692MzKkR5PdL3Oaju4S52H/9YkgQnC/12wforUuoyxtR5dXKX0ZiK0 4e9ewBBV4nya0fYAR4gDU0YuaHwJSKQ= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-465-tRZ0hcLjMYOXLaMcWo_2bg-1; Tue, 30 Sep 2025 02:45:52 -0400 X-MC-Unique: tRZ0hcLjMYOXLaMcWo_2bg-1 X-Mimecast-MFC-AGG-ID: tRZ0hcLjMYOXLaMcWo_2bg_1759214752 Received: by mail-wr1-f69.google.com with SMTP id ffacd0b85a97d-3ee13e43dd9so2675681f8f.1 for ; Mon, 29 Sep 2025 23:45:52 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1759214752; x=1759819552; h=content-transfer-encoding:in-reply-to:autocrypt:content-language :from:references:cc:to:subject:user-agent:mime-version:date :message-id:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=8AIMq+OlhpyT2npgIFmZrEOK/CbKwyfphzbxq2DUajU=; b=r1aM1W8Xkokp4rkfTwPPPzWp18DKUvAoiNDk+lJLCUTJ6k45rgg6B4i/4Me8BODOYE rK4e63KaI00BIOtcqbHR/1TB2xh0guCyEono8bWX41mOSfwY/j3panwun7DYkc59wJBW 2HvrE0wQQCHqEh3DTK9QyFVj1pgoFT6wfumK2QEXNVKJ6qWWqI0eaymnbQs/LF69zF0z PeCNDP7IQobdlGRJDoqJdV+RgGdKGn7DUqvXJVE44R93fP8ch8QwtFBsBI7Boq2girz8 CdhEcyntYgzSjWjVshrnNjso7RcOb73/LCK0Q1dCh1a9NhZIlMGEgz09ULOJrffDJmoY 0aIw== X-Forwarded-Encrypted: i=1; AJvYcCVsV9cUXFFSfsLmr2dQqlFk8pgRmyl4z1TvfGIktGAIdkrxxzi8h/P74UTDhkOYEPMwvCM2KCwg0OJeRtw=@vger.kernel.org X-Gm-Message-State: AOJu0YzUV1aqFNmay0HzbfWp5umg4TAhRXxkYric6MLwECaXjsPKlXdz g5wSakBCvPVZ2yV8sm0Xu/msLN4tFelEF67Q1hxcw6YMyC1CW230SQzjbJCDcp79iv5ZNlY0YHs hsj8cVCp7pVZhZTU4tvZRUnDf+QOQXMcZ5wnyVW5zCgN5Y3jVDZyILpakvVGAdcFHVg== X-Gm-Gg: ASbGncuPo6cRYS6bLYuBjHdsDlvwP80YyVQh8jS3xy3R5ZSCTh8zgK0EEkl7sXXc9md in1f6z/w5n9qjbDXeAEaNeAGywCWAsjpd1dr9kzz20YqI2/371vpnIIoAotbb4XppeUM0FAzisv L7a9hfbccjNBohIe3KX7+NOVOL7LfvVO3hSMW/XDkpzUcorif12ZHMEFmy3pZgEgPXC15PlaFhT Wvp0Ptoon2Qc6HAleg86Uz8genwmrLo1gmDjTcCa0DOXZF0Xi5N/Nc9L+16txO3u2bhBTxuHW30 qO/lN2QTajxzOnAaNrA+gZONLRSdmppNi8vQWkLDZdNe/orGExrCip8H7p3jtyjQBQepw4iZfKM m0pBF3hO4 X-Received: by 2002:a05:6000:1a89:b0:3ed:a43d:8eba with SMTP id ffacd0b85a97d-40e4b389211mr18844302f8f.52.1759214751625; Mon, 29 Sep 2025 23:45:51 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFg/W6e/I7hHziArdv6ifYggaC+j0tYvxahMU2MCeZ7zKwkNHwGQwxT9opm5ct5RbwIhmPfXQ== X-Received: by 2002:a05:6000:1a89:b0:3ed:a43d:8eba with SMTP id ffacd0b85a97d-40e4b389211mr18844269f8f.52.1759214751197; Mon, 29 Sep 2025 23:45:51 -0700 (PDT) Received: from ?IPV6:2a01:599:901:4a65:f2e2:845:f3d2:404d? ([2a01:599:901:4a65:f2e2:845:f3d2:404d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e5c06a9ffsm7025105e9.0.2025.09.29.23.45.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Sep 2025 23:45:50 -0700 (PDT) Message-ID: Date: Tue, 30 Sep 2025 08:45:48 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/ksm: fix flag-dropping behavior in ksm_madvise To: Jakub Acs , linux-mm@kvack.org Cc: Andrew Morton , Xu Xin , Chengming Zhou , Peter Xu , Axel Rasmussen , Mike Kravetz , linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20250930063921.62354-1-acsjakub@amazon.de> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20250930063921.62354-1-acsjakub@amazon.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.09.25 08:39, Jakub Acs wrote: > 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 casting `VM_MERGEABLE` constant to unsigned long to preserve > the upper 32 bits, in case it's needed. > > 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: Mike Kravetz > Cc: linux-mm@kvack.org > Cc: linux-kernel@vger.kernel.org > Cc: stable@vger.kernel.org > --- > > I looked around the kernel and found one more flag that might be > causing similar issues: "IORESOURCE_BUSY" - as its inverted version is > bit-anded to unsigned long fields. However, it seems those fields don't > actually use any bits from upper 32-bits as flags (yet?). > > I also considered changing the constant definition by adding ULL, but am > not sure where else that could blow up, plus it would likely call to > define all the related constants as ULL for consistency. If you'd prefer > that fix, let me know. > > > mm/ksm.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 160787bb121c..c24137a1eeb7 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -2871,7 +2871,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start, > return err; > } > > - *vm_flags &= ~VM_MERGEABLE; > + *vm_flags &= ~((unsigned long) VM_MERGEABLE); > break; > } > Wouldn't it be better to just do diff --git a/include/linux/mm.h b/include/linux/mm.h index 1ae97a0b8ec75..0eaf8af153f98 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 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 0x80000000ul /* 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 */ And for consistency doing it to all other flags as well? After all we have typedef unsigned long vm_flags_t; -- Cheers David / dhildenb