From 1dea8e081ec3833e5150250b9fa3afd0dc8768db Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Mon, 10 Aug 2026 20:10:01 +0200 Subject: [PATCH] mm/page-writeback: document folio_mark_dirty() locking more explicitly We have had bugs where set_page_dirty() was used on a page from GUP without appropriate locking, leading to UAF, in: - KVM, see https://lore.kernel.org/r/20260810-x86-kvm-setpagedirty-v1-1-85f180892d4f@google.com - i915, see commit 0d4bbe3d407f ("drm/i915/userptr: Try to acquire the page lock around set_page_dirty()"). - VMCI, see commit 5a16c535409f ("VMCI: Use set_page_dirty_lock() when unregistering guest memory") - kpc2000 staging driver, see commit b6d13bd9f2c1 ("staging: kpc2000: kpc_dma: Convert set_page_dirty() --> set_page_dirty_lock()") I think set_page_dirty() and folio_mark_dirty() need more explicit documentation on how they should be used with pages from GUP; so add a comment on top of set_page_dirty() and make the comment above folio_mark_dirty() more explicit. Link: https://lore.kernel.org/20260810-set-page-dirty-warnings-v2-1-1bd40fadfacd@google.com Signed-off-by: Jann Horn Reviewed-by: Jan Kara Reviewed-by: Christoph Hellwig Cc: Matthew Wilcox (Oracle) Signed-off-by: Andrew Morton --- mm/folio-compat.c | 1 + mm/page-writeback.c | 5 +++++ 2 files changed, 6 insertions(+) diff --git a/mm/folio-compat.c b/mm/folio-compat.c index a02179a0bded..6212fdd6761a 100644 --- a/mm/folio-compat.c +++ b/mm/folio-compat.c @@ -41,6 +41,7 @@ void set_page_writeback(struct page *page) } EXPORT_SYMBOL(set_page_writeback); +/* Read the comment above folio_mark_dirty() regarding required locks! */ bool set_page_dirty(struct page *page) { return folio_mark_dirty(page_folio(page)); diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 47495be68598..3a03e04b640c 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -2763,6 +2763,11 @@ EXPORT_SYMBOL(folio_redirty_for_writepage); * in this folio. Truncation will block on the page table lock as it * unmaps pages before removing the folio from its mapping. * + * .. DANGER:: + * Do not use this on a folio obtained from a function like + * get_user_pages_fast() without holding appropriate locks; you might want to + * use set_page_dirty_lock() or folio_mark_dirty_lock() instead. + * * Return: True if the folio was newly dirtied, false if it was already dirty. */ bool folio_mark_dirty(struct folio *folio)