From d6c355c7dabcd753a75bc77d150d36328a355267 Mon Sep 17 00:00:00 2001 From: Benjamin LaHaise Date: Mon, 9 Sep 2013 11:57:59 -0400 Subject: aio: fix race in ring buffer page lookup introduced by page migration support Prior to the introduction of page migration support in "fs/aio: Add support to aio ring pages migration" / 36bc08cc01709b4a9bb563b35aa530241ddc63e3, mapping of the ring buffer pages was done via get_user_pages() while retaining mmap_sem held for write. This avoided possible races with userland racing an munmap() or mremap(). The page migration patch, however, switched to using mm_populate() to prime the page mapping. mm_populate() cannot be called with mmap_sem held. Instead of dropping the mmap_sem, revert to the old behaviour and simply drop the use of mm_populate() since get_user_pages() will cause the pages to get mapped anyways. Thanks to Al Viro for spotting this issue. Signed-off-by: Benjamin LaHaise --- fs/aio.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) (limited to 'fs') diff --git a/fs/aio.c b/fs/aio.c index 6e267553604c..f4a27af6c9ac 100644 --- a/fs/aio.c +++ b/fs/aio.c @@ -307,16 +307,25 @@ static int aio_setup_ring(struct kioctx *ctx) aio_free_ring(ctx); return -EAGAIN; } - up_write(&mm->mmap_sem); - - mm_populate(ctx->mmap_base, populate); pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base); + + /* We must do this while still holding mmap_sem for write, as we + * need to be protected against userspace attempting to mremap() + * or munmap() the ring buffer. + */ ctx->nr_pages = get_user_pages(current, mm, ctx->mmap_base, nr_pages, 1, 0, ctx->ring_pages, NULL); + + /* Dropping the reference here is safe as the page cache will hold + * onto the pages for us. It is also required so that page migration + * can unmap the pages and get the right reference count. + */ for (i = 0; i < ctx->nr_pages; i++) put_page(ctx->ring_pages[i]); + up_write(&mm->mmap_sem); + if (unlikely(ctx->nr_pages != nr_pages)) { aio_free_ring(ctx); return -EAGAIN; -- cgit v1.2.3