[PATCH v4] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[PATCH v4] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[David Hildenbrand (Arm)]

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

The !CONFIG_MMU implementation of __get_user_pages_locked() takes a bare
get_page() reference for each page regardless of foll_flags:
	if (pages[i])
		get_page(pages[i]);

This is reached from pin_user_pages*() with FOLL_PIN set.
unpin_user_page() is shared between MMU and NOMMU configurations and
unconditionally calls gup_put_folio(..., FOLL_PIN), which subtracts
GUP_PIN_COUNTING_BIAS (1024) from the folio refcount.

This means that pin adds 1, and then unpin will subtract 1024.

If a user maps a page (refcount 1), registers it 1023 times as an
io_uring fixed buffer (1023 pin_user_pages calls -> refcount 1024), then
unregisters: the first unpin_user_page subtracts 1024, refcount hits 0,
the page is freed and returned to the buddy allocator.  The remaining
1022 unpins write into whatever was reallocated, and the user's VMA
still maps the freed page (NOMMU has no MMU to invalidate it).
Reallocating the page for an io_uring pbuf_ring then lets userspace
corrupt the new owner's data through the stale mapping.

Use try_grab_folio() which adds GUP_PIN_COUNTING_BIAS for FOLL_PIN and 1
for FOLL_GET, mirroring the CONFIG_MMU path so pin and unpin are
symmetric. Keep supporting the traditional behavior where users specify
a pages array but don't set FOLL_GET.

While at it, don't return NULL pointers in the page array,
as this is really not expected for GUP users; instead, just fail and return
-EFAULT.

[ david: support traditional behavior with no FOLL_GET, extend
  description ]

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Lance Yang <lance.yang@linux.dev>
Reported-by: Anthropic
Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
Assisted-by: gkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
v3 -> v4:
* Put FOLL_GET fixup into the NOMMU __get_user_pages_locked()

I'll queue the updated version after some more review.
---
 mm/gup.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index eb898ea1ee22e..4e9b5ac2cb62e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1983,6 +1983,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
 	struct vm_area_struct *vma;
 	bool must_unlock = false;
 	vm_flags_t vm_flags;
+	int ret, err = -EFAULT;
 	long i;
 
 	if (!nr_pages)
@@ -1999,6 +2000,10 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
 		*locked = 1;
 	}
 
+	/* See the MMU variant: support the traditional behavior. */
+	if (pages && !(foll_flags & FOLL_PIN))
+		foll_flags |= FOLL_GET;
+
 	/* calculate required read or write permissions.
 	 * If FOLL_FORCE is set, we only require the "MAY" flags.
 	 */
@@ -2019,8 +2024,14 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
 
 		if (pages) {
 			pages[i] = virt_to_page((void *)start);
-			if (pages[i])
-				get_page(pages[i]);
+			if (!pages[i])
+				break;
+			ret = try_grab_folio(page_folio(pages[i]), 1, foll_flags);
+			if (unlikely(ret)) {
+				pages[i] = NULL;
+				err = ret;
+				break;
+			}
 		}
 
 		start = (start + PAGE_SIZE) & PAGE_MASK;
@@ -2031,7 +2042,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
 		*locked = 0;
 	}
 
-	return i ? : -EFAULT;
+	return i ? : err;
 }
 #endif /* !CONFIG_MMU */
 

---

base-commit: 40288c9206c17eb66a603262e06a58d300d0f279

change-id: 20260919-nommu_gup_pin-df63917c8182

--

Cheers,

David

Re: [PATCH v4] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[Lance Yang]

On 2026/9/21 17:57, David Hildenbrand (Arm) wrote:
> From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> The !CONFIG_MMU implementation of __get_user_pages_locked() takes a bare
> get_page() reference for each page regardless of foll_flags:
> 	if (pages[i])
> 		get_page(pages[i]);
> 
> This is reached from pin_user_pages*() with FOLL_PIN set.
> unpin_user_page() is shared between MMU and NOMMU configurations and
> unconditionally calls gup_put_folio(..., FOLL_PIN), which subtracts
> GUP_PIN_COUNTING_BIAS (1024) from the folio refcount.
> 
> This means that pin adds 1, and then unpin will subtract 1024.
> 
> If a user maps a page (refcount 1), registers it 1023 times as an
> io_uring fixed buffer (1023 pin_user_pages calls -> refcount 1024), then
> unregisters: the first unpin_user_page subtracts 1024, refcount hits 0,
> the page is freed and returned to the buddy allocator.  The remaining
> 1022 unpins write into whatever was reallocated, and the user's VMA
> still maps the freed page (NOMMU has no MMU to invalidate it).
> Reallocating the page for an io_uring pbuf_ring then lets userspace
> corrupt the new owner's data through the stale mapping.
> 
> Use try_grab_folio() which adds GUP_PIN_COUNTING_BIAS for FOLL_PIN and 1
> for FOLL_GET, mirroring the CONFIG_MMU path so pin and unpin are
> symmetric. Keep supporting the traditional behavior where users specify
> a pages array but don't set FOLL_GET.
> 
> While at it, don't return NULL pointers in the page array,
> as this is really not expected for GUP users; instead, just fail and return
> -EFAULT.
> 
> [ david: support traditional behavior with no FOLL_GET, extend
>    description ]
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Reported-by: Anthropic
> Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
> Assisted-by: gkh_clanker_t1000
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> ---

Tested-by: Lance Yang <lance.yang@linux.dev>