+ mm-gup-honour-foll_pin-in-nommu-__get_user_pages_locked.patch added to mm-new branch

+ mm-gup-honour-foll_pin-in-nommu-__get_user_pages_locked.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()
has been added to the -mm mm-new branch.  Its filename is
     mm-gup-honour-foll_pin-in-nommu-__get_user_pages_locked.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-gup-honour-foll_pin-in-nommu-__get_user_pages_locked.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()
Date: Thu, 23 Apr 2026 16:28:04 +0200

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.

Link: https://lore.kernel.org/2026042303-vendor-outright-b9d2@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.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>
Reported-by: Anthropic
Assisted-by: gkh_clanker_t1000
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/gup.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

--- a/mm/gup.c~mm-gup-honour-foll_pin-in-nommu-__get_user_pages_locked
+++ a/mm/gup.c
@@ -1983,6 +1983,7 @@ static long __get_user_pages_locked(stru
 	struct vm_area_struct *vma;
 	bool must_unlock = false;
 	vm_flags_t vm_flags;
+	int ret, err = -EFAULT;
 	long i;
 
 	if (!nr_pages)
@@ -2019,8 +2020,14 @@ static long __get_user_pages_locked(stru
 
 		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 +2038,7 @@ static long __get_user_pages_locked(stru
 		*locked = 0;
 	}
 
-	return i ? : -EFAULT;
+	return i ? : err;
 }
 #endif /* !CONFIG_MMU */
 
_

Patches currently in -mm which might be from gregkh@linuxfoundation.org are

mm-gup-honour-foll_pin-in-nommu-__get_user_pages_locked.patch