From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta0.migadu.com (out-177.mta0.migadu.com [91.218.175.177]) (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 24BEB330317 for ; Mon, 10 Aug 2026 02:24:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786328660; cv=none; b=I1vDCKY0FBKYUYNq9UAo9i76leBgUZhsCnQfZY91rMW3QbnUFMjYzoqKtz+uceF7wJExpkUjuJuDJuQneo1/pfc97OOJB54rwA0UvdrlGCDXgNaA/0vMR9L3XKpIWGjv+bSXoWVEdvJWHu59qRx8XQl/IqDKCYZsMXLU7WcyJ1s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786328660; c=relaxed/simple; bh=YMFVhgiqOuynwsLHwI4jEe3s6hqPZNfdjTxLHmpqvqw=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=oT29nmd/svC4UTfv6kyPQgjzQzR8UJOju65ySOVuFFVZn34CbNY9T/Ydxdl+u9tyZ4il3zoVvi1SnpjKMRh8oBUeyWXswAj4XqmGzhunjufk8C0dt1H6lCFNP4xTeCwKmKBJpRFfm4z9y+C1D1q/CKgurc7bf65ghEi1Laj2QeM= 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=ILCFU8nT; arc=none smtp.client-ip=91.218.175.177 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="ILCFU8nT" Content-Type: text/plain; charset=utf-8 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1786328655; 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=ABX3O1pHOpPNXhdDN7NESgAwKT5nM7Ptyk6UPNmiDP0=; b=ILCFU8nT+DUlUTjwXVxPFLSNlOurmhZvUKZN4Lw+5JeiqBoOtBpThpuBUPQGnpdx+RaNPc VmC62bKa7Fae4RTFvm4gZ1sy2yquG3e/IjSfwV+VJgiV9IIblW2eH27I22cnh8IR/iWjc8 ZD+gF/vJODJ+Lup7E5aI57V9NoxJiOQ= Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.600.51.1.1\)) Subject: Re: [PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: Date: Mon, 10 Aug 2026 10:23:54 +0800 Cc: osalvador@suse.de, akpm@linux-foundation.org, usama.arif@linux.dev, shakeel.butt@linux.dev, wangkefeng.wang@huawei.com, anshuman.khandual@arm.com, david@kernel.org, surenb@google.com, fvdl@google.com, gthelen@google.com, hannes@cmpxchg.org, riel@surriel.com, sj@kernel.org, vbabka@suse.cz, mhocko@suse.com, bjackman@google.com, zi.yan@sent.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <60440F8D-5EE8-4254-9057-5FAD09B5D1FF@linux.dev> References: <20260809043250.2917406-1-souravpanda@google.com> To: Sourav Panda X-Migadu-Flow: FLOW_OUT > On Aug 10, 2026, at 04:11, Sourav Panda = wrote: >=20 > On Sat, Aug 8, 2026 at 11:58=E2=80=AFPM Muchun Song = wrote: >>=20 >>=20 >>=20 >>> On Aug 9, 2026, at 12:32, 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() via >>> alloc_gigantic_frozen_folio(). >>>=20 >>> Additionally, hugetlb_cma_alloc_frozen_folio() previously attempted >>> allocation on hugetlb_cma[nid] without verifying if nid is included = in >>> the caller's nodemask. Adding a node_isset(nid, *nodemask) check = ensures >>> the initial preferred node allocation honors the memory policy / = nodemask. >>>=20 >>> However, hugetlb_cma_alloc_frozen_folio() dereferences the nodemask = in >>> node_isset(nid, *nodemask) and for_each_node_mask(node, *nodemask), >>> leading to a null pointer dereference kernel panic when nodemask is = NULL. >>>=20 >>> Fix this by checking if nodemask is NULL in >>> hugetlb_cma_alloc_frozen_folio() and defaulting it to >>> cpuset_current_mems_allowed (safely read using a seqcount retry = loop). >>> This ensures that the initial node check and fallback loop safely = honor >>> the task's cpuset without violating cpuset constraints or causing = NULL >>> pointer dereferences. >>>=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 >>> Fixes: eb02f14c4a2b ("mm/hugetlb: allow overcommitting gigantic = hugepages") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Sourav Panda >>> --- >>> Changes in v5: >>> - Replaced defaulting nodemask to &node_states[N_MEMORY] with safely = reading >>> cpuset_current_mems_allowed using a seqcount retry loop, ensuring = fallback >>> allocations comply with task hardwall cpusets as suggested by Usama = Arif. >>> - v4: = https://lore.kernel.org/linux-mm/20260726072935.3513996-1-souravpanda@goog= le.com/ >>> - v3: = https://lore.kernel.org/linux-mm/20260705175119.440599-1-souravpanda@googl= e.com/ >>> - 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/ >>>=20 >>> mm/hugetlb_cma.c | 15 ++++++++++++++- >>> 1 file changed, 14 insertions(+), 1 deletion(-) >>>=20 >>> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c >>> index 39344d6c78d8..3ae9347078e9 100644 >>> --- a/mm/hugetlb_cma.c >>> +++ b/mm/hugetlb_cma.c >>> @@ -3,6 +3,7 @@ >>> #include >>> #include >>> #include >>> +#include >>> #include >>>=20 >>> #include >>> @@ -30,11 +31,23 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int = order, gfp_t gfp_mask, >>> int node; >>> struct folio *folio; >>> struct page *page =3D NULL; >>> + nodemask_t local_node_mask; >>>=20 >>> if (!hugetlb_cma_size) >>> return NULL; >>>=20 >>> - if (hugetlb_cma[nid]) >>> + if (!nodemask) { >>> + unsigned int cpuset_mems_cookie; >>> + >>> + do { >>> + cpuset_mems_cookie =3D = read_mems_allowed_begin(); >>> + local_node_mask =3D = cpuset_current_mems_allowed; >>=20 >> I think it only makes sense to move cma_alloc_frozen_compound() = inside the >> loop. Otherwise, during subsequent memory allocations, we could still = face >> unexpected allocation failures. >>=20 >> Muchun, >> Thanks. >=20 > Hi Muchun! Hi, >=20 > What do you think of the following diff? This way > cma_alloc_frozen_compound is also protected from stale nodemasks. >=20 > if (!hugetlb_cma_size) > return NULL; >=20 > - if (hugetlb_cma[nid]) > +retry_cpuset: > + if (!nodemask) { > + cpuset_mems_cookie =3D read_mems_allowed_begin(); > + local_node_mask =3D cpuset_current_mems_allowed; > + nmask =3D &local_node_mask; > + } else { > + nmask =3D nodemask; > + } > + > + if (hugetlb_cma[nid] && node_isset(nid, *nmask)) > page =3D cma_alloc_frozen_compound(hugetlb_cma[nid], = order); >=20 > if (!page && !(gfp_mask & __GFP_THISNODE)) { > - for_each_node_mask(node, *nodemask) { > + for_each_node_mask(node, *nmask) { > if (node =3D=3D nid || !hugetlb_cma[node]) > continue; >=20 > @@ -48,6 +61,9 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int > order, gfp_t gfp_mask, > } > } >=20 > + if (!page && !nodemask && > unlikely(read_mems_allowed_retry(cpuset_mems_cookie))) > + goto retry_cpuset; > + > if (!page) > return NULL; The overall looks good to me. But can we change it to this here? if (!page) { if (!nodemask && = unlikely(read_mems_allowed_retry(cpuset_mems_cookie))) goto retry_cpuset; return NULL; } That way, we can remove the 'if (!page)' check here. Muchun, Thanks. >=20 > If this looks good to you, I'll send out v6 with this change. >=20 > With regards, > Sourav Panda >=20 >>=20 >>> + } while (read_mems_allowed_retry(cpuset_mems_cookie)); >>> + >>> + nodemask =3D &local_node_mask; >>> + } >>> + >>> + 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)) { >>> -- >>> 2.55.0