A race condition is possible in stable_page_flags() where user-space is
reading /proc/kpageflags concurrently to a folio split. This may lead to
oopses or BUG_ON()s being triggered.

To fix this, this commit uses snapshot_page() in stable_page_flags() so
that stable_page_flags() works with a stable page and folio snapshots
instead.

Note that stable_page_flags() makes use of some functions that require
the original page or folio pointer to work properly (eg.
is_free_budy_page() and folio_test_idle()). Since those functions can't
be used on the page snapshot, we replace their usage with flags that
were set by snapshot_page() for this purpose.

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 fs/proc/page.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 936f8bbe5a6f..52e4f49c0638 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -147,6 +147,7 @@ static inline u64 kpf_copy_bit(u64 kflags, int ubit, int kbit)
 u64 stable_page_flags(const struct page *page)
 {
 	const struct folio *folio;
+	struct page_snapshot ps;
 	unsigned long k;
 	unsigned long mapping;
 	bool is_anon;
@@ -158,7 +159,9 @@ u64 stable_page_flags(const struct page *page)
 	 */
 	if (!page)
 		return 1 << KPF_NOPAGE;
-	folio = page_folio(page);
+
+	snapshot_page(&ps, page);
+	folio = &ps.folio_snapshot;
 
 	k = folio->flags;
 	mapping = (unsigned long)folio->mapping;
@@ -167,7 +170,7 @@ u64 stable_page_flags(const struct page *page)
 	/*
 	 * pseudo flags for the well known (anonymous) memory mapped pages
 	 */
-	if (page_mapped(page))
+	if (folio_mapped(folio))
 		u |= 1 << KPF_MMAP;
 	if (is_anon) {
 		u |= 1 << KPF_ANON;
@@ -179,7 +182,7 @@ u64 stable_page_flags(const struct page *page)
 	 * compound pages: export both head/tail info
 	 * they together define a compound page's start/end pos and order
 	 */
-	if (page == &folio->page)
+	if (ps.idx == 0)
 		u |= kpf_copy_bit(k, KPF_COMPOUND_HEAD, PG_head);
 	else
 		u |= 1 << KPF_COMPOUND_TAIL;
@@ -189,25 +192,19 @@ u64 stable_page_flags(const struct page *page)
 	         folio_test_large_rmappable(folio)) {
 		/* Note: we indicate any THPs here, not just PMD-sized ones */
 		u |= 1 << KPF_THP;
-	} else if (is_huge_zero_folio(folio)) {
+	} else if (is_huge_zero_pfn(ps.pfn)) {
 		u |= 1 << KPF_ZERO_PAGE;
 		u |= 1 << KPF_THP;
-	} else if (is_zero_folio(folio)) {
+	} else if (is_zero_pfn(ps.pfn)) {
 		u |= 1 << KPF_ZERO_PAGE;
 	}
 
-	/*
-	 * Caveats on high order pages: PG_buddy and PG_slab will only be set
-	 * on the head page.
-	 */
-	if (PageBuddy(page))
-		u |= 1 << KPF_BUDDY;
-	else if (page_count(page) == 0 && is_free_buddy_page(page))
+	if (ps.flags & PAGE_SNAPSHOT_PG_BUDDY)
 		u |= 1 << KPF_BUDDY;
 
-	if (PageOffline(page))
+	if (folio_test_offline(folio))
 		u |= 1 << KPF_OFFLINE;
-	if (PageTable(page))
+	if (folio_test_pgtable(folio))
 		u |= 1 << KPF_PGTABLE;
 	if (folio_test_slab(folio))
 		u |= 1 << KPF_SLAB;
@@ -215,7 +212,7 @@ u64 stable_page_flags(const struct page *page)
 #if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
 	u |= kpf_copy_bit(k, KPF_IDLE,          PG_idle);
 #else
-	if (folio_test_idle(folio))
+	if (ps.flags & PAGE_SNAPSHOT_PG_IDLE)
 		u |= 1 << KPF_IDLE;
 #endif
 
@@ -241,7 +238,7 @@ u64 stable_page_flags(const struct page *page)
 	if (u & (1 << KPF_HUGE))
 		u |= kpf_copy_bit(k, KPF_HWPOISON,	PG_hwpoison);
 	else
-		u |= kpf_copy_bit(page->flags, KPF_HWPOISON,	PG_hwpoison);
+		u |= kpf_copy_bit(ps.page_snapshot.flags, KPF_HWPOISON, PG_hwpoison);
 #endif
 
 	u |= kpf_copy_bit(k, KPF_RESERVED,	PG_reserved);
-- 
2.50.0

On 14.07.25 15:16, Luiz Capitulino wrote:
> A race condition is possible in stable_page_flags() where user-space is
> reading /proc/kpageflags concurrently to a folio split. This may lead to
> oopses or BUG_ON()s being triggered.
> 
> To fix this, this commit uses snapshot_page() in stable_page_flags() so
> that stable_page_flags() works with a stable page and folio snapshots
> instead.
> 
> Note that stable_page_flags() makes use of some functions that require
> the original page or folio pointer to work properly (eg.
> is_free_budy_page() and folio_test_idle()). Since those functions can't
> be used on the page snapshot, we replace their usage with flags that
> were set by snapshot_page() for this purpose.
> 
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>   fs/proc/page.c | 29 +++++++++++++----------------
>   1 file changed, 13 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 936f8bbe5a6f..52e4f49c0638 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -147,6 +147,7 @@ static inline u64 kpf_copy_bit(u64 kflags, int ubit, int kbit)
>   u64 stable_page_flags(const struct page *page)
>   {
>   	const struct folio *folio;
> +	struct page_snapshot ps;
>   	unsigned long k;
>   	unsigned long mapping;
>   	bool is_anon;
> @@ -158,7 +159,9 @@ u64 stable_page_flags(const struct page *page)
>   	 */
>   	if (!page)
>   		return 1 << KPF_NOPAGE;
> -	folio = page_folio(page);
> +
> +	snapshot_page(&ps, page);
> +	folio = &ps.folio_snapshot;

Not sure what to do here if the snapshot is not faithful. Fortunately, 
it shouldn't happen often :)

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb