From c148dc8e2fa403be501612ee409db866eeed35c0 Mon Sep 17 00:00:00 2001 From: Claudio Imbrenda Date: Fri, 28 Apr 2023 11:27:53 +0200 Subject: [PATCH] KVM: s390: fix race in gmap_make_secure() Fix a potential race in gmap_make_secure() and remove the last user of follow_page() without FOLL_GET. The old code is locking something it doesn't have a reference to, and as explained by Jason and David in this discussion: https://lore.kernel.org/linux-mm/Y9J4P%2FRNvY1Ztn0Q@nvidia.com/ it can lead to all kind of bad things, including the page getting unmapped (MADV_DONTNEED), freed, reallocated as a larger folio and the unlock_page() would target the wrong bit. There is also another race with the FOLL_WRITE, which could race between the follow_page() and the get_locked_pte(). The main point is to remove the last use of follow_page() without FOLL_GET or FOLL_PIN, removing the races can be considered a nice bonus. Link: https://lore.kernel.org/linux-mm/Y9J4P%2FRNvY1Ztn0Q@nvidia.com/ Suggested-by: Jason Gunthorpe Fixes: 214d9bbcd3a6 ("s390/mm: provide memory management functions for protected KVM guests") Reviewed-by: Jason Gunthorpe Signed-off-by: Claudio Imbrenda Message-Id: <20230428092753.27913-2-imbrenda@linux.ibm.com> --- arch/s390/kernel/uv.c | 32 +++++++++++--------------------- 1 file changed, 11 insertions(+), 21 deletions(-) diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c index 9f18a4af9c13..cb2ee06df286 100644 --- a/arch/s390/kernel/uv.c +++ b/arch/s390/kernel/uv.c @@ -192,21 +192,10 @@ static int expected_page_refs(struct page *page) return res; } -static int make_secure_pte(pte_t *ptep, unsigned long addr, - struct page *exp_page, struct uv_cb_header *uvcb) +static int make_page_secure(struct page *page, struct uv_cb_header *uvcb) { - pte_t entry = READ_ONCE(*ptep); - struct page *page; int expected, cc = 0; - if (!pte_present(entry)) - return -ENXIO; - if (pte_val(entry) & _PAGE_INVALID) - return -ENXIO; - - page = pte_page(entry); - if (page != exp_page) - return -ENXIO; if (PageWriteback(page)) return -EAGAIN; expected = expected_page_refs(page); @@ -304,17 +293,18 @@ again: goto out; rc = -ENXIO; - page = follow_page(vma, uaddr, FOLL_WRITE); - if (IS_ERR_OR_NULL(page)) - goto out; - - lock_page(page); ptep = get_locked_pte(gmap->mm, uaddr, &ptelock); - if (should_export_before_import(uvcb, gmap->mm)) - uv_convert_from_secure(page_to_phys(page)); - rc = make_secure_pte(ptep, uaddr, page, uvcb); + if (pte_present(*ptep) && !(pte_val(*ptep) & _PAGE_INVALID) && pte_write(*ptep)) { + page = pte_page(*ptep); + rc = -EAGAIN; + if (trylock_page(page)) { + if (should_export_before_import(uvcb, gmap->mm)) + uv_convert_from_secure(page_to_phys(page)); + rc = make_page_secure(page, uvcb); + unlock_page(page); + } + } pte_unmap_unlock(ptep, ptelock); - unlock_page(page); out: mmap_read_unlock(gmap->mm); -- 2.39.5