[PATCH v2] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[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.

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>
Reported-by: Anthropic
Assisted-by: gkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
v2: - drop huge comment
    - rework error return value based on David's suggestion (heck,
      pretty much the full patch was written by him now)
    Link to v1: https://lore.kernel.org/r/2026042334-acutely-unadorned-e05c@gregkh

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

diff --git a/mm/gup.c b/mm/gup.c
index ad9ded39609c..2f6f95a167af 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)
@@ -2019,8 +2020,14 @@ 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 +2038,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
 		*locked = 0;
 	}
 
-	return i ? : -EFAULT;
+	return i ? : err;
 }
 #endif /* !CONFIG_MMU */
 
-- 
2.54.0