[PATCH v3] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[PATCH v3] 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>
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>
---
I added a best-guess fixes tag. Given that this has been around for a
while ... and it's nommu, I think this is fine just going into the next
merge window. In that case, I will pick this up myself. If we want this
as a hotfix, Andrew please pick it up.
---
 mm/gup.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index eb898ea1ee22e..1681bea8eed7e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1659,6 +1659,10 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
 	if (!nr_pages)
 		return 0;
 
+	/* See the MMU variant: support the traditional behavior. */
+	if (pages && !(flags & FOLL_PIN))
+		flags |= FOLL_GET;
+
 	/*
 	 * The internal caller expects GUP to manage the lock internally and the
 	 * lock must be released when this returns.
@@ -1983,6 +1987,7 @@ 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 +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

Best regards,
--  
David Hildenbrand (Arm) <david@kernel.org>

Re: [PATCH v3] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[Andrew Morton]

On Sat, 19 Sep 2026 12:04:36 +0200 "David Hildenbrand (arm)" <david@kernel.org> wrote:

> I added a best-guess fixes tag. Given that this has been around for a
> while ... and it's nommu, I think this is fine just going into the next
> merge window. In that case, I will pick this up myself. If we want this
> as a hotfix, Andrew please pick it up.

go for it

Re: [PATCH v3] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[Greg Kroah-Hartman]

On Sat, Sep 19, 2026 at 12:04:36PM +0200, 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>
> 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>
> ---
> I added a best-guess fixes tag. Given that this has been around for a
> while ... and it's nommu, I think this is fine just going into the next
> merge window. In that case, I will pick this up myself. If we want this
> as a hotfix, Andrew please pick it up.

Which ever you feel is fine, as it's nommu, obviously not many care
about it, including me :)

thanks,

greg k-h

Re: [PATCH v3] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()

[David Hildenbrand]

On Sat, 19 Sep 2026 12:04:36 +0200, David Hildenbrand (arm) wrote:
> 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.
> 
> [...]

Applied, thanks!

tree: mm/core.git
branch: gup-7.4.misc

Applied patches will appear soon in mm-next, followed by linux-next.

Please report any outstanding bugs that were missed during review, so we
can either drop the patch series or decide how to fix it up.

It is encouraged to provide Acked-bys, Reviewed-bys, and Tested-bys even
though the patches have already been applied. If possible, patch trailers
will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. Further, this is not a guarantee that the
patches will go upstream. If in doubt, please check the listed branch.

[1/1] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked()
      https://git.kernel.org/mm/core/c/8ad0454e

-- 
Cheers,

David