FAILED: patch "[PATCH] mm/secretmem: fix GUP-fast succeeding on secretmem folios" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 65291dcfcf8936e1b23cfd7718fdfde7cfaf7706
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024040819-elf-bamboo-00f6@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

65291dcfcf89 ("mm/secretmem: fix GUP-fast succeeding on secretmem folios") 8f9ff2deb8b9 ("secretmem: convert page_is_secretmem() to folio_is_secretmem()")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 65291dcfcf8936e1b23cfd7718fdfde7cfaf7706 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <[email protected]>
Date: Tue, 26 Mar 2024 15:32:08 +0100
Subject: [PATCH] mm/secretmem: fix GUP-fast succeeding on secretmem folios

folio_is_secretmem() currently relies on secretmem folios being LRU folios, to save some cycles.

However, folios might reside in a folio batch without the LRU flag set, or temporarily have their LRU flag cleared. Consequently, the LRU flag is unreliable for this purpose.

In particular, this is the case when secretmem_fault() allocates a fresh page and calls filemap_add_folio()->folio_add_lru(). The folio might be added to the per-cpu folio batch and won't get the LRU flag set until the batch was drained using e.g., lru_add_drain().

Consequently, folio_is_secretmem() might not detect secretmem folios and GUP-fast can succeed in grabbing a secretmem folio, crashing the kernel when we would later try reading/writing to the folio, because the folio has been unmapped from the directmap.

Fix it by removing that unreliable check.

index 35f3a4a8ceb1..acf7e1a3f3de 100644
@@ -13,10 +13,10 @@ static inline bool folio_is_secretmem(struct folio *folio)
 	/*
 	 * Using folio_mapping() is quite slow because of the actual call
 	 * instruction.
-	 * We know that secretmem pages are not compound and LRU so we can
+	 * We know that secretmem pages are not compound, so we can
 	 * save a couple of cycles here.
 	 */
-	if (folio_test_large(folio) || !folio_test_lru(folio))
+	if (folio_test_large(folio))
 		return false;
 
 	mapping = (struct address_space *)

folio_is_secretmem() currently relies on secretmem folios being LRU folios, to save some cycles.

However, folios might reside in a folio batch without the LRU flag set, or temporarily have their LRU flag cleared. Consequently, the LRU flag is unreliable for this purpose.

In particular, this is the case when secretmem_fault() allocates a fresh page and calls filemap_add_folio()->folio_add_lru(). The folio might be added to the per-cpu folio batch and won't get the LRU flag set until the batch was drained using e.g., lru_add_drain().

Consequently, folio_is_secretmem() might not detect secretmem folios and GUP-fast can succeed in grabbing a secretmem folio, crashing the kernel when we would later try reading/writing to the folio, because the folio has been unmapped from the directmap.

Fix it by removing that unreliable check.

---
 include/linux/secretmem.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 988528b5da43..48ffe325184c 100644
@@ -14,10 +14,10 @@ static inline bool page_is_secretmem(struct page *page)
 	 * Using page_mapping() is quite slow because of the actual call
 	 * instruction and repeated compound_head(page) inside the
 	 * page_mapping() function.
-	 * We know that secretmem pages are not compound and LRU so we can
+	 * We know that secretmem pages are not compound, so we can
 	 * save a couple of cycles here.
 	 */
-	if (PageCompound(page) || !PageLRU(page))
+	if (PageCompound(page))
 		return false;
 
 	mapping = (struct address_space *)

2.44.0

Forgot to add when cherry-picking

Cheers,

David / dhildenb

Now queued up, thanks.

greg k-h

Likely we want:
git cherry-pick -sx

And then

(a) If cherry-picking failed
# <resolve conflicts, build, test, etc.>
git cherry-pick --continue

(b) If cherry-picking succeeded
# <resolve issues, build, test, etc.>
git commit --amend

Cheers,

David / dhildenb