[PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Sourav Panda]

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().

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.

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.

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.

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:
   <TASK>
   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 <souravpanda@google.com>
---
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@google.com/
- 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 | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

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 <linux/mm.h>
 #include <linux/cma.h>
 #include <linux/compiler.h>
+#include <linux/cpuset.h>
 #include <linux/mm_inline.h>
 
 #include <asm/page.h>
@@ -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 = NULL;
+	nodemask_t local_node_mask;
 
 	if (!hugetlb_cma_size)
 		return NULL;
 
-	if (hugetlb_cma[nid])
+	if (!nodemask) {
+		unsigned int cpuset_mems_cookie;
+
+		do {
+			cpuset_mems_cookie = read_mems_allowed_begin();
+			local_node_mask = cpuset_current_mems_allowed;
+		} while (read_mems_allowed_retry(cpuset_mems_cookie));
+
+		nodemask = &local_node_mask;
+	}
+
+	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

Re: [PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Muchun Song]

> On Aug 9, 2026, at 12:32, Sourav Panda <souravpanda@google.com> 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().
> 
> 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.
> 
> 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.
> 
> 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.
> 
> 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:
>   <TASK>
>   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 <souravpanda@google.com>
> ---
> 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@google.com/
> - 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 | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
> 
> 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 <linux/mm.h>
> #include <linux/cma.h>
> #include <linux/compiler.h>
> +#include <linux/cpuset.h>
> #include <linux/mm_inline.h>
> 
> #include <asm/page.h>
> @@ -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 = NULL;
> + 	nodemask_t local_node_mask;
> 
> 	if (!hugetlb_cma_size)
> 		return NULL;
> 
> - 	if (hugetlb_cma[nid])
> + 	if (!nodemask) {
> + 		unsigned int cpuset_mems_cookie;
> +
> + 		do {
> + 			cpuset_mems_cookie = read_mems_allowed_begin();
> + 			local_node_mask = cpuset_current_mems_allowed;

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.

Muchun,
Thanks.

> + 		} while (read_mems_allowed_retry(cpuset_mems_cookie));
> +
> + 		nodemask = &local_node_mask;
> + 	}
> +
> + 	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

Re: [PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Sourav Panda]

On Sat, Aug 8, 2026 at 11:58 PM Muchun Song <muchun.song@linux.dev> wrote:
>
>
>
> > On Aug 9, 2026, at 12:32, Sourav Panda <souravpanda@google.com> 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().
> >
> > 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.
> >
> > 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.
> >
> > 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.
> >
> > 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:
> >   <TASK>
> >   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 <souravpanda@google.com>
> > ---
> > 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@google.com/
> > - 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 | 15 ++++++++++++++-
> > 1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > 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 <linux/mm.h>
> > #include <linux/cma.h>
> > #include <linux/compiler.h>
> > +#include <linux/cpuset.h>
> > #include <linux/mm_inline.h>
> >
> > #include <asm/page.h>
> > @@ -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 = NULL;
> > +     nodemask_t local_node_mask;
> >
> >       if (!hugetlb_cma_size)
> >               return NULL;
> >
> > -     if (hugetlb_cma[nid])
> > +     if (!nodemask) {
> > +             unsigned int cpuset_mems_cookie;
> > +
> > +             do {
> > +                     cpuset_mems_cookie = read_mems_allowed_begin();
> > +                     local_node_mask = cpuset_current_mems_allowed;
>
> 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.
>
> Muchun,
> Thanks.

Hi Muchun!

What do you think of the following diff? This way
cma_alloc_frozen_compound is also protected from stale nodemasks.

         if (!hugetlb_cma_size)
             return NULL;

    -    if (hugetlb_cma[nid])
    +retry_cpuset:
    +    if (!nodemask) {
    +        cpuset_mems_cookie = read_mems_allowed_begin();
    +        local_node_mask = cpuset_current_mems_allowed;
    +        nmask = &local_node_mask;
    +    } else {
    +        nmask = nodemask;
    +    }
    +
    +    if (hugetlb_cma[nid] && node_isset(nid, *nmask))
             page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);

         if (!page && !(gfp_mask & __GFP_THISNODE)) {
    -        for_each_node_mask(node, *nodemask) {
    +        for_each_node_mask(node, *nmask) {
                 if (node == nid || !hugetlb_cma[node])
                     continue;

    @@ -48,6 +61,9 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int
order, gfp_t gfp_mask,
             }
         }

    +    if (!page && !nodemask &&
unlikely(read_mems_allowed_retry(cpuset_mems_cookie)))
    +        goto retry_cpuset;
    +
         if (!page)
             return NULL;

If this looks good to you, I'll send out v6 with this change.

With regards,
Sourav Panda

>
> > +             } while (read_mems_allowed_retry(cpuset_mems_cookie));
> > +
> > +             nodemask = &local_node_mask;
> > +     }
> > +
> > +     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
>

Re: [PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Muchun Song]

> On Aug 10, 2026, at 04:11, Sourav Panda <souravpanda@google.com> wrote:
> 
> On Sat, Aug 8, 2026 at 11:58 PM Muchun Song <muchun.song@linux.dev> wrote:
>> 
>> 
>> 
>>> On Aug 9, 2026, at 12:32, Sourav Panda <souravpanda@google.com> 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().
>>> 
>>> 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.
>>> 
>>> 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.
>>> 
>>> 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.
>>> 
>>> 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:
>>>  <TASK>
>>>  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 <souravpanda@google.com>
>>> ---
>>> 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@google.com/
>>> - 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 | 15 ++++++++++++++-
>>> 1 file changed, 14 insertions(+), 1 deletion(-)
>>> 
>>> 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 <linux/mm.h>
>>> #include <linux/cma.h>
>>> #include <linux/compiler.h>
>>> +#include <linux/cpuset.h>
>>> #include <linux/mm_inline.h>
>>> 
>>> #include <asm/page.h>
>>> @@ -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 = NULL;
>>> +     nodemask_t local_node_mask;
>>> 
>>>      if (!hugetlb_cma_size)
>>>              return NULL;
>>> 
>>> -     if (hugetlb_cma[nid])
>>> +     if (!nodemask) {
>>> +             unsigned int cpuset_mems_cookie;
>>> +
>>> +             do {
>>> +                     cpuset_mems_cookie = read_mems_allowed_begin();
>>> +                     local_node_mask = cpuset_current_mems_allowed;
>> 
>> 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.
>> 
>> Muchun,
>> Thanks.
> 
> Hi Muchun!

Hi,

> 
> What do you think of the following diff? This way
> cma_alloc_frozen_compound is also protected from stale nodemasks.
> 
>         if (!hugetlb_cma_size)
>             return NULL;
> 
>    -    if (hugetlb_cma[nid])
>    +retry_cpuset:
>    +    if (!nodemask) {
>    +        cpuset_mems_cookie = read_mems_allowed_begin();
>    +        local_node_mask = cpuset_current_mems_allowed;
>    +        nmask = &local_node_mask;
>    +    } else {
>    +        nmask = nodemask;
>    +    }
>    +
>    +    if (hugetlb_cma[nid] && node_isset(nid, *nmask))
>             page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);
> 
>         if (!page && !(gfp_mask & __GFP_THISNODE)) {
>    -        for_each_node_mask(node, *nodemask) {
>    +        for_each_node_mask(node, *nmask) {
>                 if (node == nid || !hugetlb_cma[node])
>                     continue;
> 
>    @@ -48,6 +61,9 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int
> order, gfp_t gfp_mask,
>             }
>         }
> 
>    +    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.

> 
> If this looks good to you, I'll send out v6 with this change.
> 
> With regards,
> Sourav Panda
> 
>> 
>>> +             } while (read_mems_allowed_retry(cpuset_mems_cookie));
>>> +
>>> +             nodemask = &local_node_mask;
>>> +     }
>>> +
>>> +     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

Re: [PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Sourav Panda]

On Sun, Aug 9, 2026 at 7:24 PM Muchun Song <muchun.song@linux.dev> wrote:
>
>
>
> > On Aug 10, 2026, at 04:11, Sourav Panda <souravpanda@google.com> wrote:
> >
> > On Sat, Aug 8, 2026 at 11:58 PM Muchun Song <muchun.song@linux.dev> wrote:
> >>
> >>
> >>
> >>> On Aug 9, 2026, at 12:32, Sourav Panda <souravpanda@google.com> 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().
> >>>
> >>> 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.
> >>>
> >>> 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.
> >>>
> >>> 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.
> >>>
> >>> 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:
> >>>  <TASK>
> >>>  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 <souravpanda@google.com>
> >>> ---
> >>> 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@google.com/
> >>> - 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 | 15 ++++++++++++++-
> >>> 1 file changed, 14 insertions(+), 1 deletion(-)
> >>>
> >>> 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 <linux/mm.h>
> >>> #include <linux/cma.h>
> >>> #include <linux/compiler.h>
> >>> +#include <linux/cpuset.h>
> >>> #include <linux/mm_inline.h>
> >>>
> >>> #include <asm/page.h>
> >>> @@ -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 = NULL;
> >>> +     nodemask_t local_node_mask;
> >>>
> >>>      if (!hugetlb_cma_size)
> >>>              return NULL;
> >>>
> >>> -     if (hugetlb_cma[nid])
> >>> +     if (!nodemask) {
> >>> +             unsigned int cpuset_mems_cookie;
> >>> +
> >>> +             do {
> >>> +                     cpuset_mems_cookie = read_mems_allowed_begin();
> >>> +                     local_node_mask = cpuset_current_mems_allowed;
> >>
> >> 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.
> >>
> >> Muchun,
> >> Thanks.
> >
> > Hi Muchun!
>
> Hi,
>
> >
> > What do you think of the following diff? This way
> > cma_alloc_frozen_compound is also protected from stale nodemasks.
> >
> >         if (!hugetlb_cma_size)
> >             return NULL;
> >
> >    -    if (hugetlb_cma[nid])
> >    +retry_cpuset:
> >    +    if (!nodemask) {
> >    +        cpuset_mems_cookie = read_mems_allowed_begin();
> >    +        local_node_mask = cpuset_current_mems_allowed;
> >    +        nmask = &local_node_mask;
> >    +    } else {
> >    +        nmask = nodemask;
> >    +    }
> >    +
> >    +    if (hugetlb_cma[nid] && node_isset(nid, *nmask))
> >             page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);
> >
> >         if (!page && !(gfp_mask & __GFP_THISNODE)) {
> >    -        for_each_node_mask(node, *nodemask) {
> >    +        for_each_node_mask(node, *nmask) {
> >                 if (node == nid || !hugetlb_cma[node])
> >                     continue;
> >
> >    @@ -48,6 +61,9 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int
> > order, gfp_t gfp_mask,
> >             }
> >         }
> >
> >    +    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.

Sure thing! Thanks! I shall send this tomorrow noon (PST) since I am
OOO starting Thursday.

  @@ -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 = NULL;
  + const nodemask_t *nmask;
  + nodemask_t local_node_mask;
  + unsigned int cpuset_mems_cookie;

    if (!hugetlb_cma_size)
      return NULL;

  - if (hugetlb_cma[nid])
  +retry_cpuset:
  + if (!nodemask) {
  +   cpuset_mems_cookie = read_mems_allowed_begin();
  +   local_node_mask = cpuset_current_mems_allowed;
  +   nmask = &local_node_mask;
  + } else {
  +   nmask = nodemask;
  + }
  +
  + if (hugetlb_cma[nid] && node_isset(nid, *nmask))
      page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);

    if (!page && !(gfp_mask & __GFP_THISNODE)) {
  -   for_each_node_mask(node, *nodemask) {
  +   for_each_node_mask(node, *nmask) {
        if (node == nid || !hugetlb_cma[node])
          continue;

  @@ -48,8 +61,12 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int
order, gfp_t gfp_mask,
      }
    }

  - if (!page)
  + if (!page) {
  +   if (!nodemask &&
  +       unlikely(read_mems_allowed_retry(cpuset_mems_cookie)))
  +     goto retry_cpuset;
      return NULL;
  + }


>
> Muchun,
> Thanks.
>
> >
> > If this looks good to you, I'll send out v6 with this change.
> >
> > With regards,
> > Sourav Panda
> >
> >>
> >>> +             } while (read_mems_allowed_retry(cpuset_mems_cookie));
> >>> +
> >>> +             nodemask = &local_node_mask;
> >>> +     }
> >>> +
> >>> +     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
>
>