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A concurrent bpf_arena_free_pages() call on the same pgoff can return the physical page to the allocator between these two operations. arena_vm_fault() then inserts a stale or already-freed page into the user PTE, resulting in a SIGSEGV on next access or a silent use-after-free. Fix: acquire arena->lock before vmalloc_to_page() and hold it through range_tree_clear(), making the check-and-claim atomic with respect to concurrent allocators and free operations. arena->lock is a mutex already used by arena_vm_open() and arena_vm_close() for vma_list serialization. Reusing it here is consistent with the existing locking model and avoids introducing a new lock. arena_vm_fault() runs in page fault context with mmap_read_lock held and is sleepable, so taking a mutex is safe. The pte_none() check inside apply_range_set_cb() is not a sufficient guard: it prevents double-mapping but does not prevent the range tree desynchronization that occurs when the race is lost, leaving pgoff marked free while the PTE remains populated. Fixes: a7d032218a53 ("bpf: Introduce bpf_arena") Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: stable@vger.kernel.org Signed-off-by: Andrii Kuchmenko --- kernel/bpf/arena.c | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-) diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c index a1b2c3d..e4f5c6d 100644 --- a/kernel/bpf/arena.c +++ b/kernel/bpf/arena.c @@ -XXX,7 +XXX,7 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf) struct bpf_map *map =3D vmf->vma->vm_file->private_data; struct bpf_arena *arena =3D container_of(map, struct bpf_arena, map); struct page *page; - long kbase, kaddr; + long kbase, kaddr; int ret; =20 kbase =3D bpf_arena_get_kern_vm_start(arena); @@ -XXX,12 +XXX,24 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf) kbase =3D bpf_arena_get_kern_vm_start(arena); kaddr =3D kbase + (u32)(vmf->address); =20 + /* + * Acquire arena->lock before vmalloc_to_page() and hold it through + * range_tree_clear() to close the TOCTOU window. + * + * Without this lock, a concurrent bpf_arena_free_pages() on the + * same pgoff can run between vmalloc_to_page() returning NULL and + * range_tree_clear() completing: + * + * arena_vm_fault() bpf_arena_free_pages() + * vmalloc_to_page() =3D NULL + * [window] page freed, PTE zeroed in kern vma + * range_tree_clear(pgoff) + * alloc_page() + vm_insert_page() -> stale PTE in user vma + * + * The user VMA then holds a reference to a freed physical page. + * Next access produces SIGSEGV or silent use-after-free. + */ + guard(mutex)(&arena->lock); + page =3D vmalloc_to_page((void *)kaddr); if (page) goto out; - ret =3D range_tree_clear(&arena->rt, vmf->pgoff, 1); if (ret) return VM_FAULT_SIGBUS; --=20 2.39.0