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From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 78EA340011 X-Rspam-User: X-Stat-Signature: 9qiep3ejzhbg5f5d4tgrsc7b6fq1ekmd X-HE-Tag: 1786014460-244575 X-HE-Meta: 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 KPR0wfOw njvgvqvVUXzDAr9cwYyqYuEj8bONGSeNrISVuqK0cFzQTaPYyNpmU6U7Txz2jQueCF1J6C+IG6CCtqsDAJMg146t6LHvhOb00AS0nqF0RH8mqfs5IsP59UsHCYRQ/FNvCxsjh0739OHlMwCGBdy7Sl6kGjejsR5Kw1IQcDvi67wlxvoUHRcdc2VHS8bnWVCL53qOfx4sPLRC155fxwbYFutb64FEP5x6JNxleDeWLvB5BGTqoiZDHMJ8JpZASU5QXtgC6SXENer4FcfqhcSmGtLhEHCipcFdCJQHQTpus+7TB2MYhaTjM2yhZIFcyKgU9UyTTRdU83dAUQaKYEr0OtWfQ0Y3jJke2FUe8QjNsuW8gQWapurj9yMjz8wUy3o25Z8LQX35dOzShvSk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 >>>>