Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider: - [High] Synchronous execution of __free_large_kmalloc_page in kfree_nolock() violates NMI wait-context and corrupts vmstat counters - [High] Deferred clearing of PageLargeKmalloc bypasses double-free protection, causing infinite loops --
--- Patch [4]: [PATCH RFC 4/5] mm/slab: handle large_kmalloc objects in kfree_nolock() --- commit 9af1d6506b418b2d9de0922f293e12ee55babdd3 Author: Vlastimil Babka (SUSE) <[email protected]> mm/slab: handle large_kmalloc objects in kfree_nolock() Large kmalloc objects is the only remaining case that kfree_nolock() cannot handle from kmalloc() allocations. Note kmalloc_nolock() does not return large kmalloc objects. > diff --git a/mm/slub.c b/mm/slub.c > index 423b5bdb910bd..3be98faa9f0de 100644 > --- a/mm/slub.c > +++ b/mm/slub.c [ ... ] > @@ -6365,6 +6367,21 @@ static void free_to_pcs_bulk(struct kmem_cache *s, > size_t size, void **p) > } > } > > +static inline void > +__free_large_kmalloc_page(struct page *page, unsigned int free_flags) > +{ > + unsigned int order = compound_order(page); > + > + mod_lruvec_page_state(page, NR_SLAB_UNRECLAIMABLE_B, > + -(PAGE_SIZE << order)); > + __ClearPageLargeKmalloc(page); > + > + if (free_flags & SLAB_FREE_NOLOCK) > + free_frozen_pages_nolock(page, order); [Severity: High] Is it safe to execute this synchronously in NMI context on PREEMPT_RT kernels? When kmemleak is disabled, kmemleak_may_need_free() returns false, which causes free_large_kmalloc() to call __free_large_kmalloc_page() synchronously from kfree_nolock(), even in NMI context. This results in free_frozen_pages_nolock() attempting to acquire the zone lock, which is an rt_mutex (sleeping lock) on PREEMPT_RT, leading to a panic when called from NMI. Additionally, mod_lruvec_page_state() uses a non-atomic read-modify-write protected by local_irq_save() on architectures without CONFIG_HAVE_CMPXCHG_LOCAL. This fails to mask NMIs and can silently corrupt vmstat counters. Would it be better to always defer freeing large kmalloc pages in kfree_nolock() to avoid these issues? > + else > + free_frozen_pages(page, order); > +} > + [ ... ] > @@ -6712,9 +6756,11 @@ size_t ksize(const void *objp) > } > EXPORT_SYMBOL(ksize); > > -static void free_large_kmalloc(struct page *page, void *object) > +static void free_large_kmalloc(struct page *page, void *object, > + unsigned int free_flags) > { > unsigned int order = compound_order(page); > + bool nolock = free_flags & SLAB_FREE_NOLOCK; > > if (WARN_ON_ONCE(!PageLargeKmalloc(page))) { > dump_page(page, "Not a kmalloc allocation"); > @@ -6724,14 +6770,16 @@ static void free_large_kmalloc(struct page *page, > void *object) > if (WARN_ON_ONCE(order == 0)) > pr_warn_once("object pointer: 0x%p\n", object); > > - kmemleak_free(object); > + if (!nolock) > + kmemleak_free(object); > + > kasan_kfree_large(object); > kmsan_kfree_large(object); > > - mod_lruvec_page_state(page, NR_SLAB_UNRECLAIMABLE_B, > - -(PAGE_SIZE << order)); > - __ClearPageLargeKmalloc(page); > - free_frozen_pages(page, order); > + if (unlikely(nolock && kmemleak_may_need_free(object))) > + defer_free_large_kmalloc(object); [Severity: High] Could moving the __ClearPageLargeKmalloc() call to the asynchronous __free_large_kmalloc_page() bypass double-free protection? When a free is deferred via defer_free_large_kmalloc(), the PageLargeKmalloc flag remains set until the IRQ work executes. If the caller has a double-free bug and kfree_nolock() is called again on the same object during this window, the WARN_ON_ONCE(!PageLargeKmalloc(page)) check at the start of free_large_kmalloc() will pass. This allows the second free to proceed and call defer_free_large_kmalloc() again. Since this uses llist_add() with the same llnode, it creates a circular linked list (llnode->next = llnode). When deferred_percpu_work_fn() later processes this list with llist_for_each_safe(), it will loop infinitely and cause a hard lockup. > + else > + __free_large_kmalloc_page(page, free_flags); > } -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=4
