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From: Muchun Song In-Reply-To: <20260705175119.440599-1-souravpanda@google.com> Date: Mon, 6 Jul 2026 14:17:39 +0800 Cc: 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, Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan Content-Transfer-Encoding: quoted-printable Message-Id: References: <20260705175119.440599-1-souravpanda@google.com> To: Sourav Panda X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 16916140003 X-Stat-Signature: ebuybz7r7paqjwdxxm5bksfrqmty7mz4 X-HE-Tag: 1783318708-981640 X-HE-Meta: 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 MEjNDgGP M5gboec8IZSB809JxzdwoRDCFEpHd/IGoQ7Q8MrFmyDYTpdHY4N/K/bO2Qrjr1EbRwoP6J0PeP4h2EHCbSxaPPDRXUS37QZOv58ow53d5uoliDYZsdC/q2pjF1PgQl3otZiEIdk9jVwAF5t2lIXxdrpfGX1g5oKhT7fE8dOmJWskzjEhC9P8KqtJh/1Z+vlfQFmx9Cuj99t97xgRw0np9XVjq8Lcv3AhAlrD0XCoIDG4B7qOP280bYEOEWpI8k+RUQNx+Or+cgx/k6PRWcEkLe4vWN1XQDPee1cdodwMBD7JCsptmJ5iMlpSyzxTQXhuRnYYZVQMn66UA8Io= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Jul 6, 2026, at 01:51, Sourav Panda wrote: >=20 > 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() which = blindly > dereferences it in for_each_node_mask(), leading to a null pointer > dereference. >=20 > Similarly, if the CMA allocation fails, the fallback > alloc_contig_frozen_pages() is also called with a NULL nodemask, > which may cause issues. >=20 > Fix this by explicitly checking if nodemask is NULL in > alloc_fresh_hugetlb_folio() and defaulting to > cpuset_current_mems_allowed. This ensures that both the CMA and > contiguous allocators receive a valid nodemask safely using a seqcount > loop to prevent torn reads. >=20 > =46rom 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. >=20 > This can be reproduced by booting a VM with two NUMA nodes, = restricting > CMA to Node 1 (e.g., hugetlb_cma=3D1:1G default_hugepagesz=3D1G = hugepagesz=3D1G > hugepages=3D0), 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: >=20 > void *ptr =3D mmap(NULL, 1UL << 30, PROT_READ | PROT_WRITE, > MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB | > MAP_HUGE_1GB | MAP_NORESERVE, -1, 0); > unsigned long nodemask =3D 1; /* Node 0 */ > mbind(ptr, 1UL << 30, MPOL_PREFERRED_MANY, &nodemask, > sizeof(nodemask) * 8, 0); > memset(ptr, 0, 1UL << 30); /* Trigger fault */ >=20 > This results in a NULL pointer dereference: >=20 > 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 >=20 > Additionally, this patch adds a missing node_isset(nid, *nodemask) = check > in hugetlb_cma_alloc_frozen_folio() to ensure the initial node = allocation > attempt respects the memory policy. >=20 > Fixes: eb02f14c4a2b ("mm/hugetlb: allow overcommitting gigantic = hugepages") > Cc: stable@vger.kernel.org > Signed-off-by: Sourav Panda > --- > Changes in v3: > - Condensed the reproducer and QEMU setup from v1 discussion into a = readable summary per Andrew Morton. > - Safely read cpuset_current_mems_allowed using a seqcount loop to = prevent torn reads. > - v2: = https://lore.kernel.org/linux-mm/20260704174930.2885785-1-souravpanda@goog= le.com/ > - v1: = https://lore.kernel.org/linux-mm/20260702215713.627941-1-souravpanda@googl= e.com/ > mm/hugetlb.c | 12 ++++++++++++ > mm/hugetlb_cma.c | 2 +- > 2 files changed, 13 insertions(+), 1 deletion(-) >=20 > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 571212b80835..ee67ea29c003 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -1864,6 +1864,18 @@ static struct folio = *alloc_fresh_hugetlb_folio(struct hstate *h, > gfp_t gfp_mask, int nid, nodemask_t *nmask) > { > struct folio *folio; > + nodemask_t local_node_mask; > + > + if (!nmask) { > + unsigned int cpuset_mems_cookie; > + > + do { > + cpuset_mems_cookie =3D = read_mems_allowed_begin(); > + local_node_mask =3D cpuset_current_mems_allowed; > + } while (read_mems_allowed_retry(cpuset_mems_cookie)); > + > + nmask =3D &local_node_mask; > + } As Andrew pointed it out, it is not helpful to prevent any = false-positive allocation failures. We need something like the following: seq =3D read_mems_allowed_begin(); alloc_nodemask =3D nodemask; if (cpusets_enabled() && !alloc_nodemask) alloc_nodemask =3D &cpuset_current_mems_allowed; for_each_node_mask(node, alloc_nodemask) { if (cpusets_enabled() && nodemask && !cpuset_zone_allowed(zone, gfp_mask)) continue; ... } if (!page && read_mems_allowed_retry(seq)) goto retry; After further consideration, I believe we only need to fix alloc_gigantic_frozen_folio(), as alloc_buddy_frozen_folio() already handles the NULL nodemask properly via prepare_alloc_pages(). To address the issue in alloc_gigantic_frozen_folio(), we have two potential options: - Option 1: Fix it directly within alloc_gigantic_frozen_folio(). This would centrally cover both of its underlying allocation paths: hugetlb_cma_alloc_frozen_folio() and alloc_contig_frozen_pages(). - Option 2: Fix hugetlb_cma_alloc_frozen_folio() and = alloc_contig_frozen_pages() separately. The rationale here is that since the buddy allocator already handles this case internally, and alloc_contig_frozen_pages() is also a low-level allocator, it would be better to keep their = behaviors consistent. Both approaches are viable from my perspective. I'm CCing more MM = maintainers and reviewers to gather your insights and decide on the best path = forward. Muchun, Thanks. >=20 > folio =3D only_alloc_fresh_hugetlb_folio(h, gfp_mask, nid, = nmask, NULL); > if (folio) > diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c > index 39344d6c78d8..79dbd0baafa3 100644 > --- a/mm/hugetlb_cma.c > +++ b/mm/hugetlb_cma.c > @@ -34,7 +34,7 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int = order, gfp_t gfp_mask, > if (!hugetlb_cma_size) > return NULL; >=20 > - if (hugetlb_cma[nid]) > + if (hugetlb_cma[nid] && node_isset(nid, *nodemask)) > page =3D cma_alloc_frozen_compound(hugetlb_cma[nid], = order); >=20 > if (!page && !(gfp_mask & __GFP_THISNODE)) { > --=20 > 2.55.0.rc0.799.gd6f94ed593-goog >=20