Re: [PATCH v6] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Sourav Panda]

On Mon, Aug 10, 2026 at 10:26 PM Anshuman Khandual
<anshuman.khandual@xxxxxxx> wrote:
>
> On Tue, Aug 11, 2026 at 10:48:31AM +0800, Muchun Song wrote:
> >
> >
> > > On Aug 11, 2026, at 07:08, Sourav Panda <souravpanda@xxxxxxxxxx> 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. 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.
> > >
> > > 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@xxxxxxxxxxxxxxx
> > > Signed-off-by: Sourav Panda <souravpanda@xxxxxxxxxx>
> > > ---
> > > 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@xxxxxxxxxx/
> > > - v4: https://lore.kernel.org/linux-mm/20260726072935.3513996-1-souravpanda@xxxxxxxxxx/
> > > - v3: https://lore.kernel.org/linux-mm/20260705175119.440599-1-souravpanda@xxxxxxxxxx/
> > > - v2: https://lore.kernel.org/linux-mm/20260704174930.2885785-1-souravpanda@xxxxxxxxxx/
> > > - v1: https://lore.kernel.org/linux-mm/20260702215713.627941-1-souravpanda@xxxxxxxxxx/
> > >
> > > mm/hugetlb_cma.c | 23 ++++++++++++++++++-----
> > > 1 file changed, 18 insertions(+), 5 deletions(-)
> > >
> > > 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 <linux/mm.h>
> > > #include <linux/cma.h>
> > > #include <linux/compiler.h>
> > > +#include <linux/cpuset.h>
> > > #include <linux/mm_inline.h>
> > >
> > > #include <asm/page.h>
> > > @@ -30,15 +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;
> >
> > I don't think we need a local_node_mask, nmask = &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.
>
> But does not local_node_mask capture current snapshot for cpuset_current_mems_allowed
> after first taking a reference via read_mems_allowed_begin() which ensures that nmask
> has a constant value there after. Otherwise nmask could just change value by the time
> it actually gets used.
>
Hi,

Apologies for the poor netiquette of rushing out v7 so quickly (I just
saw your message after sending v7)! I am heading out on a long
vacation starting this Thursday, so I wanted to get an updated version
out for review before leaving.

Regarding local_node_mask: I thought the seqcount mechanism
(read_mems_allowed_begin() / read_mems_allowed_retry()) guarantees
correctness here without needing a stack copy. If
current->mems_allowed changes at any point while nmask is being used
(even during allocation) and the allocation fails,
read_mems_allowed_retry() detects the sequence change and jumps back
to retry_cpuset to retry with the updated nodemask.

I've incorporated this into v7 (Sorry again - for sending it so fast):
https://lore.kernel.org/linux-mm/20260810230844.3778931-1-souravpanda@xxxxxxxxxx/

Thanks,
Sourav

> >
> > Otherwise, LGTM.
> >
> > Thanks.
> >
> > > +   } 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;
> > > +   }
> > >
> > >     folio = page_folio(page);
> > >     folio_set_hugetlb_cma(folio);
> > > --
> > > 2.55.0
> >