From: Zizhi Wo Date: Sat, 18 Apr 2026 06:06:34 +0000 (+0800) Subject: fs: aio: reject partial mremap to avoid Null-pointer-dereference error X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=3adf7ae18bf42601246031002287c103a27df307;p=thirdparty%2Fkernel%2Fstable.git fs: aio: reject partial mremap to avoid Null-pointer-dereference error [BUG] Recently, our internal syzkaller testing uncovered a null pointer dereference issue: BUG: kernel NULL pointer dereference, address: 0000000000000000 ... [ 51.111664] filemap_read_folio+0x25/0xe0 [ 51.112410] filemap_fault+0xad7/0x1250 [ 51.113112] __do_fault+0x4b/0x460 [ 51.113699] do_pte_missing+0x5bc/0x1db0 [ 51.114250] ? __pte_offset_map+0x23/0x170 [ 51.114822] __handle_mm_fault+0x9f8/0x1680 ... Crash analysis showed the file involved was an AIO ring file. The phenomenon triggered is the same as the issue described in [1]. [CAUSE] Consider the following scenario: userspace sets up an AIO context via io_setup(), which creates a VMA covering the entire ring buffer. Then userspace calls mremap() with the AIO ring address as the source, a smaller old_len (less than the full ring size), MREMAP_MAYMOVE set, and without MREMAP_DONTUNMAP. The kernel will relocate the requested portion to a new destination address. During this move, __split_vma() splits the original AIO ring VMA. The requested portion is unmapped from the source and re-established at the destination, while the remainder stays at the original source address as an orphan VMA. The aio_ring_mremap() callback fires on the new destination VMA, updating ctx->mmap_base to the destination address. But the callback is unaware that only a partial region was moved and that an orphan VMA still exists at the source: source(AIO): +-------------------+---------------------+ | moved to dest | orphan VMA (AIO) | +-------------------+---------------------+ A A+partial_len A+ctx->mmap_size dest: +-------------------+ | moved VMA (AIO) | +-------------------+ B B+partial_len Later, io_destroy() calls vm_munmap(ctx->mmap_base, ctx->mmap_size), which unmaps the destination. This not only fails to unmap the orphan VMA at the source, but also overshoots the destination VMA and may unmap unrelated mappings adjacent to it! After put_aio_ring_file() calls truncate_setsize() to remove all pages from the pagecache, any subsequent access to the orphan VMA triggers filemap_fault(), which calls a_ops->read_folio(). Since aio does not implement read_folio, this results in a NULL pointer dereference. [FIX] Note that expanding mremap (new_len > old_len) is already rejected because AIO ring VMAs are created with VM_DONTEXPAND. The only problematic case is a partial move where "old_len == new_len" but both are smaller than the full ring size. Fix this by checking in aio_ring_mremap() that the new VMA covers the entire ring. This ensures the AIO ring is always moved as a whole, preventing orphan VMAs and the subsequent crash. [1]: https://lore.kernel.org/all/20260413010814.548568-1-wozizhi@huawei.com/ Signed-off-by: Zizhi Wo Link: https://patch.msgid.link/20260418060634.3713620-1-wozizhi@huaweicloud.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner --- diff --git a/fs/aio.c b/fs/aio.c index d7910c7c93a6..722476560848 100644 --- a/fs/aio.c +++ b/fs/aio.c @@ -422,7 +422,8 @@ static int aio_ring_mremap(struct vm_area_struct *vma) ctx = rcu_dereference(table->table[i]); if (ctx && ctx->aio_ring_file == file) { - if (!atomic_read(&ctx->dead)) { + if (!atomic_read(&ctx->dead) && + (ctx->mmap_size == (vma->vm_end - vma->vm_start))) { ctx->user_id = ctx->mmap_base = vma->vm_start; res = 0; }