Patch "KVM: s390: fix race in gmap_make_secure()" has been added to the 6.3-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: s390: fix race in gmap_make_secure()

to the 6.3-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-s390-fix-race-in-gmap_make_secure.patch
and it can be found in the queue-6.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 55f50de6b6407890a7c8d80754540f6b6cb2c85e
Author: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Date:   Fri Apr 28 11:27:53 2023 +0200

    KVM: s390: fix race in gmap_make_secure()
    
    [ Upstream commit c148dc8e2fa403be501612ee409db866eeed35c0 ]
    
    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@xxxxxxxxxx/
    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@xxxxxxxxxx/
    Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
    Fixes: 214d9bbcd3a6 ("s390/mm: provide memory management functions for protected KVM guests")
    Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
    Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
    Message-Id: <20230428092753.27913-2-imbrenda@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index 9f18a4af9c131..cb2ee06df286c 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 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 		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);