From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-185.mta1.migadu.com (out-185.mta1.migadu.com [95.215.58.185]) (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 5545E3B7B72 for ; Tue, 11 Aug 2026 02:48:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.185 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786416541; cv=none; b=HcElZunzuL9EtwwpKSV7IaBzrdRZJyyUZNesgOwBpiuM8r2619wkAPsgOLwpQCSmFwTNoMZWO5O36SaOaZGWDId3g81cyutPB+5y06YQfEsHcoyChvrXjVP/Q+JKFogBSSSe+YMrJC1VDEN0NtgifT1tExkl/k2VCYtYnb2dvjg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786416541; c=relaxed/simple; bh=Y+3nBZ4Io5rKWs7yxMqlLuuQRcvJVuL7NTZZTl+xlqE=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=BADKFXrCh8HoAfImb0l5hxgczHJiOQeHzo/5p5HMjQ9QTfWOwdcmcnLP3wX8uXGnBMzokbko262HDmxz1y7SvrHMp/rfUzxLh7QUeVws6HU0QHC11VfTOxM7Sy3SbukQ+4YhgmbyMvmvViWtrDla2LAt7SsnOEsoF5MfsVZnDLg= 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=ZQSuEH8O; arc=none smtp.client-ip=95.215.58.185 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="ZQSuEH8O" Content-Type: text/plain; charset=us-ascii DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1786416532; 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=ZhHjo3aIwixazs38WpONPdhAQWYBalHyX4mnm9h4mLg=; b=ZQSuEH8OMzSl9+vo5G7FZYxMvdPLWoizh5Q/APLwfiqTR//gsSmDE/j2IwSnqSHtQxh5G7 g4/LuySrHj6GvQMxJaE4+ccI+7lVXUwdTNkXhGf5l5oV/vKbm9/QPLYo32tn2bTW/JNJm9 tXqq547o0NdEvY14OFAEZtWOkbBNnK8= 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 v6] 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: <20260810230844.3778931-1-souravpanda@google.com> Date: Tue, 11 Aug 2026 10:48:31 +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: <15EAC670-AA9F-4361-985C-F8ED2C322CF9@linux.dev> References: <20260810230844.3778931-1-souravpanda@google.com> To: Sourav Panda X-Migadu-Flow: FLOW_OUT > On Aug 11, 2026, at 07:08, 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. Enclose the allocation attempts within > the cpuset seqcount retry loop so that if the cpuset changes = concurrently > during allocation, the attempts are retried using the updated = nodemask. > 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 or unexpected allocation failures. >=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 v6: > - Enclosed the CMA allocation attempts within the cpuset seqcount = retry loop, > retrying allocation upon cpuset mems_allowed updates to prevent = unexpected > allocation failures as suggested by Muchun Song. > - v5: = https://lore.kernel.org/linux-mm/20260809043250.2917406-1-souravpanda@goog= le.com/ > - 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 | 23 ++++++++++++++++++----- > 1 file changed, 18 insertions(+), 5 deletions(-) >=20 > diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c > index 39344d6c78d8..9debf033d4fd 100644 > --- a/mm/hugetlb_cma.c > +++ b/mm/hugetlb_cma.c > @@ -3,6 +3,7 @@ > #include > #include > #include > +#include > #include >=20 > #include > @@ -30,15 +31,27 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int = order, gfp_t gfp_mask, > int node; > struct folio *folio; > struct page *page =3D NULL; > + const nodemask_t *nmask; > + nodemask_t local_node_mask; > + unsigned int cpuset_mems_cookie; >=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; I don't think we need a local_node_mask, nmask =3D = &cpuset_current_mems_allowed is enough. nodemask_t could be a huge array, so it might consume a lot = of memory on the stack. I don't think we need to do that unless we see a clear = necessity. Otherwise, LGTM. Thanks. > + } 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 > @@ -48,8 +61,12 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int = order, gfp_t gfp_mask, > } > } >=20 > - if (!page) > + if (!page) { > + if (!nodemask && > + = unlikely(read_mems_allowed_retry(cpuset_mems_cookie))) > + goto retry_cpuset; > return NULL; > + } >=20 > folio =3D page_folio(page); > folio_set_hugetlb_cma(folio); > --=20 > 2.55.0