6.17-stable review patch. If anyone has any objections, please let me know.
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From: Deepanshu Kartikey kartikey406@gmail.com
commit de8798965fd0d9a6c47fc2ac57767ec32de12b49 upstream.
When allocating hugetlb folios for memfd, three initialization steps are missing:
1. Folios are not zeroed, leading to kernel memory disclosure to userspace 2. Folios are not marked uptodate before adding to page cache 3. hugetlb_fault_mutex is not taken before hugetlb_add_to_page_cache()
The memfd allocation path bypasses the normal page fault handler (hugetlb_no_page) which would handle all of these initialization steps. This is problematic especially for udmabuf use cases where folios are pinned and directly accessed by userspace via DMA.
Fix by matching the initialization pattern used in hugetlb_no_page(): - Zero the folio using folio_zero_user() which is optimized for huge pages - Mark it uptodate with folio_mark_uptodate() - Take hugetlb_fault_mutex before adding to page cache to prevent races
The folio_zero_user() change also fixes a potential security issue where uninitialized kernel memory could be disclosed to userspace through read() or mmap() operations on the memfd.
Link: https://lkml.kernel.org/r/20251112145034.2320452-1-kartikey406@gmail.com Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios") Signed-off-by: Deepanshu Kartikey kartikey406@gmail.com Reported-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com Link: https://lore.kernel.org/all/20251112031631.2315651-1-kartikey406@gmail.com/ [v1] Closes: https://syzkaller.appspot.com/bug?extid=f64019ba229e3a5c411b Suggested-by: Oscar Salvador osalvador@suse.de Suggested-by: David Hildenbrand david@redhat.com Tested-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com Acked-by: Oscar Salvador osalvador@suse.de Acked-by: David Hildenbrand (Red Hat) david@kernel.org Acked-by: Hugh Dickins hughd@google.com Cc: Vivek Kasireddy vivek.kasireddy@intel.com Cc: Jason Gunthorpe jgg@nvidia.com Cc: Jason Gunthorpe jgg@nvidia.com (v2) Cc: Christoph Hellwig hch@lst.de (v6) Cc: Dave Airlie airlied@redhat.com Cc: Gerd Hoffmann kraxel@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/memfd.c | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+)
--- a/mm/memfd.c +++ b/mm/memfd.c @@ -96,9 +96,36 @@ struct folio *memfd_alloc_folio(struct f NULL, gfp_mask); if (folio) { + u32 hash; + + /* + * Zero the folio to prevent information leaks to userspace. + * Use folio_zero_user() which is optimized for huge/gigantic + * pages. Pass 0 as addr_hint since this is not a faulting path + * and we don't have a user virtual address yet. + */ + folio_zero_user(folio, 0); + + /* + * Mark the folio uptodate before adding to page cache, + * as required by filemap.c and other hugetlb paths. + */ + __folio_mark_uptodate(folio); + + /* + * Serialize hugepage allocation and instantiation to prevent + * races with concurrent allocations, as required by all other + * callers of hugetlb_add_to_page_cache(). + */ + hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx); + mutex_lock(&hugetlb_fault_mutex_table[hash]); + err = hugetlb_add_to_page_cache(folio, memfd->f_mapping, idx); + + mutex_unlock(&hugetlb_fault_mutex_table[hash]); + if (err) { folio_put(folio); goto err_unresv;