memfd_luo_retrieve_folios() initialises err to -EIO, but the per-iteration
calls to mem_cgroup_charge(), shmem_add_to_page_cache() and
shmem_inode_acct_blocks() reuse and overwrite err.  Once any iteration
completes successfully, err becomes zero.

If a later iteration's kho_restore_folio() returns NULL, the failure path
jumps to put_folios without resetting err, so the function returns 0.
The caller memfd_luo_retrieve() then takes the success path, sets
args->file and reports the restore as successful, leaving userspace with
a partially populated memfd and no indication that anything went wrong.

Set err to -EIO in the kho_restore_folio() failure branch so the error
is propagated to the caller.

Signed-off-by: David Carlier <devnexen@gmail.com>
---
 mm/memfd_luo.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
index b02b503c750d..35d1247281e0 100644
--- a/mm/memfd_luo.c
+++ b/mm/memfd_luo.c
@@ -427,6 +427,7 @@ static int memfd_luo_retrieve_folios(struct file *file,
 		if (!folio) {
 			pr_err("Unable to restore folio at physical address: %llx\n",
 			       phys);
+			err = -EIO;
 			goto put_folios;
 		}
 		index = pfolio->index;
-- 
2.53.0

From: Mike Rapoport (Microsoft) <rppt@kernel.org>

On Wed, 15 Apr 2026 06:23:00 +0100, David Carlier wrote:
> memfd_luo_retrieve_folios() initialises err to -EIO, but the per-iteration
> calls to mem_cgroup_charge(), shmem_add_to_page_cache() and
> shmem_inode_acct_blocks() reuse and overwrite err.  Once any iteration
> completes successfully, err becomes zero.
> 
> If a later iteration's kho_restore_folio() returns NULL, the failure path
> jumps to put_folios without resetting err, so the function returns 0.
> The caller memfd_luo_retrieve() then takes the success path, sets
> args->file and reports the restore as successful, leaving userspace with
> a partially populated memfd and no indication that anything went wrong.
> 
> [...]

Applied to fixes branch of liveupdate/linux.git tree, thanks!

[1/1] mm/memfd_luo: report error when restoring a folio fails mid-loop
      commit: 0fb1daf0b78d0e23b63b6b65de56d4a3fd83bc14

tree: https://git.kernel.org/pub/scm/linux/kernel/git/liveupdate/linux
branch: fixes

--
Sincerely yours,
Mike.

On Wed, Apr 15 2026, David Carlier wrote:

> memfd_luo_retrieve_folios() initialises err to -EIO, but the per-iteration
> calls to mem_cgroup_charge(), shmem_add_to_page_cache() and
> shmem_inode_acct_blocks() reuse and overwrite err.  Once any iteration
> completes successfully, err becomes zero.
>
> If a later iteration's kho_restore_folio() returns NULL, the failure path
> jumps to put_folios without resetting err, so the function returns 0.
> The caller memfd_luo_retrieve() then takes the success path, sets
> args->file and reports the restore as successful, leaving userspace with
> a partially populated memfd and no indication that anything went wrong.
>
> Set err to -EIO in the kho_restore_folio() failure branch so the error
> is propagated to the caller.
>
> Signed-off-by: David Carlier <devnexen@gmail.com>

Reviewed-by: Pratyush Yadav <pratyush@kernel.org>

Please add these when applying:

Cc: stable@vger.kernel.org
Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")

[...]

-- 
Regards,
Pratyush Yadav