From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (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 CB8CD44E64F for ; Thu, 6 Aug 2026 11:07:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786014471; cv=none; b=HPf2xNGPB3kUV0hAMPoqZqeaTjzL1yuWsCz0WAo6h1gK8jBBHIIvZw43lJucTAwdV2EJGiZ7U1tfrCJ2Qtcc87aq+gbtIQW3RkviLSLzNWglezWg3gAWezNv+mMnddnBtg4vaBeqfPKxyvD52ADN/Kfi/NKiehoJRx0zJJdui1g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786014471; c=relaxed/simple; bh=qToY0viEFyai/5FBTS6zZUbeeyFd4524M/CmBVrd+Sg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QAUg30aRn5lgICfj2bhjxCksTcRtj3oBtpWL1xadQbPDRxTnSMx/A802Sk8BnPvj1jl0IqBbb26B7kbvBobxke5lYl+jASQQlvRQxUlVuDYTYi6xruMWdz+9kkmG0oy5DT7iMatEljZE02PBaWrfwUtqey6yPurWAQWiNjCxTZA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=hcW/YmTW; arc=none smtp.client-ip=91.218.175.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="hcW/YmTW" Message-ID: <007cdb8b-01e3-4d73-9ac9-1a52ab5798dd@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1786014459; 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; bh=umeg7T32naGcJC9MzeautcojuTNjpQjQcwcgWxX/wig=; b=hcW/YmTWu8rMYvyiIgd5FnpBnnsvyKdtYB13BU82KR0gatM1/3wXRw1pCBQ9MyKsf8DUPC kiAoFeH5CBh018Gv1n0EQF01ytl1jmY7fOS2XskqCUwiDFl0NPqQcaOtQL1pQYf3ZS7NZB hujwJYjhy6eveLPkHXkJV/3IohB8Boc= Date: Thu, 6 Aug 2026 12:07:36 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v4] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio To: Sourav Panda , Anshuman Khandual Cc: muchun.song@linux.dev, osalvador@suse.de, akpm@linux-foundation.org, david@kernel.org, surenb@google.com, fvdl@google.com, gthelen@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260726072935.3513996-1-souravpanda@google.com> <20260805202444.1573218-1-usama.arif@linux.dev> <2qq6r5zvegrk5bbx7qbkscwqmccda7mxtzuvd3wln24yjm2qhx@663fzadsgmg3> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 06/08/2026 06:33, Sourav Panda wrote: > On Wed, Aug 5, 2026 at 7:47 PM Anshuman Khandual > wrote: >> >> On Wed, Aug 05, 2026 at 01:24:42PM -0700, Usama Arif wrote: >>> On Sun, 26 Jul 2026 07:29:34 +0000 Sourav Panda wrote: >>> >>>> alloc_buddy_hugetlb_folio_with_mpol() can pass a NULL nodemask to >>>> alloc_fresh_hugetlb_folio() as a fallback to allocate from all >>>> nodes. If order is gigantic, alloc_fresh_hugetlb_folio() propagates >>>> the NULL nodemask down to hugetlb_cma_alloc_frozen_folio() via >>>> alloc_gigantic_frozen_folio(). >>>> >>>> hugetlb_cma_alloc_frozen_folio() blindly dereferences the nodemask in >>>> node_isset(nid, *nodemask) and for_each_node_mask(node, *nodemask), >>>> leading to a null pointer dereference kernel panic. >>>> >>>> Fix this by checking if nodemask is NULL in >>>> hugetlb_cma_alloc_frozen_folio() and defaulting it to >>>> node_states[N_MEMORY]. This allows hugetlb_cma allocations to fall >>>> back to any node with memory, keeping behavior consistent with >>>> alloc_contig_frozen_pages() and alloc_buddy_frozen_folio(). >>>> >>>> >From a userspace perspective, this bug allows an unprivileged user to >>>> crash the kernel (trigger a panic) by requesting a gigantic hugepage >>>> allocation with MPOL_PREFERRED_MANY on a system where CMA is only >>>> configured on a subset of NUMA nodes. >>>> >>>> This can be reproduced by booting a VM with two NUMA nodes, restricting >>>> CMA to Node 1 (e.g., hugetlb_cma=1:1G default_hugepagesz=1G >>>> hugepagesz=1G hugepages=0), and running a program that allocates a >>>> 1GB hugepage area without reserving, restricts allocation to Node 0 >>>> using mbind() with MPOL_PREFERRED_MANY, and triggers a page fault: >>>> >>>> void *ptr = mmap(NULL, 1UL << 30, PROT_READ | PROT_WRITE, >>>> MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB | >>>> MAP_HUGE_1GB | MAP_NORESERVE, -1, 0); >>>> unsigned long nodemask = 1; /* Node 0 */ >>>> mbind(ptr, 1UL << 30, MPOL_PREFERRED_MANY, &nodemask, >>>> sizeof(nodemask) * 8, 0); >>>> memset(ptr, 0, 1UL << 30); /* Trigger fault */ >>>> >>>> This results in a NULL pointer dereference: >>>> >>>> BUG: kernel NULL pointer dereference, address: 0000000000000000 >>>> #PF: supervisor read access in kernel mode >>>> #PF: error_code(0x0000) - not-present page >>>> Oops: Oops: 0000 [#1] SMP NOPTI >>>> RIP: 0010:hugetlb_cma_alloc_frozen_folio+0x75/0x120 >>>> Call Trace: >>>> >>>> only_alloc_fresh_hugetlb_folio.isra.0+0x2c/0x160 >>>> alloc_surplus_hugetlb_folio+0x6d/0x100 >>>> alloc_hugetlb_folio+0x3c5/0x660 >>>> hugetlb_no_page+0x3d9/0x650 >>>> >>>> Fixes: eb02f14c4a2b ("mm/hugetlb: allow overcommitting gigantic hugepages") >>>> Cc: stable@vger.kernel.org >>>> Signed-off-by: Sourav Panda >>>> --- >>>> Changes in v4: >>>> - Reverted the alloc_fresh_hugetlb_folio() cpuset snapshot approach from v3. >>>> As Muchun Song pointed out, snapshotting cpuset_current_mems_allowed does >>>> not prevent false-positive allocation failures without complex retry loops, >>>> and alloc_contig_frozen_pages() / alloc_buddy_frozen_folio() already handle >>>> NULL nodemasks safely internally. >>>> - Handled NULL nodemask directly inside hugetlb_cma_alloc_frozen_folio() >>>> by defaulting nodemask to node_states[N_MEMORY] (Option 2), keeping >>>> HugeTLB allocators clean and consistent. >>>> - v3: https://lore.kernel.org/linux-mm/20260705175119.440599-1-souravpanda@google.com/ >>>> - v2: https://lore.kernel.org/linux-mm/20260704174930.2885785-1-souravpanda@google.com/ >>>> - v1: https://lore.kernel.org/linux-mm/20260702215713.627941-1-souravpanda@google.com/ >>>> >>>> mm/hugetlb_cma.c | 5 ++++- >>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c >>>> index 39344d6c78d8..5744de0ceeb7 100644 >>>> --- a/mm/hugetlb_cma.c >>>> +++ b/mm/hugetlb_cma.c >>>> @@ -34,7 +34,10 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask, >>>> if (!hugetlb_cma_size) >>>> return NULL; >>>> >>>> - if (hugetlb_cma[nid]) >>>> + if (!nodemask) >>>> + nodemask = &node_states[N_MEMORY]; >>> >>> Hi Sourav, >>> >>> hmm what if cpuset.mems only allows allocation from node 0, and hugetlb CMA >>> is only available on node 1. This will no now allow allocating a gigantic page >>> on node 1 when its explicitly not allowed? >> >> That's fair point but should not the user be also responsible in provding a right >> nodemask containing CMA memory if it prefers avoiding node_states[N_MEMORY] based >> fallback mechanism in kernel ? >> I don't think the task can be made responsible for providing a CMA-aware fallback mask here. Userspace supplies the MPOL_PREFERRED_MANY preference, but the kernel itself changes the second attempt to a NULL nodemask. That permits fallback outside the preferred policy nodes; it does not permit fallback outside the task's hardwall cpuset. > > We have precedence for three options, which makes selecting one difficult :) > > 1) nodemask = &cpuset_current_mems_allowed is currently being used in: > only_alloc_fresh_hugetlb_folio --> alloc_buddy_frozen_folio --> > __alloc_frozen_pages_noprof --> prepare_alloc_pages. Here I think using &cpuset_current_mems_allowed is part of the page allocator's wider cpuset handling. > 2) Some places use cookies with cpuset_current_mems_allowed to prevent > torn writes. But this adds complexity. > An example would be dequeue_hugetlb_folio_nodemask() > 3) node_states[N_MEMORY] is sprinkled all over the kernel (simplest). > I would prefer option 2. A NULL mempolicy mask should fall back to cpuset_current_mems_allowed, not all N_MEMORY nodes. >>> >>> Would it not be better to check cpuset as well? >>> >>> Thanks, >>> Usama >>> >>>> + >>>> + if (hugetlb_cma[nid] && node_isset(nid, *nodemask)) >>>> page = cma_alloc_frozen_compound(hugetlb_cma[nid], order); >>>> >>>> if (!page && !(gfp_mask & __GFP_THISNODE)) { >>>> -- >>>> 2.55.0 >>>>