Re: KASAN: use-after-free Read in idr_for_each (2)

[Matthew Wilcox]

On Sun, Nov 29, 2020 at 07:34:29PM +0800, Hillf Danton wrote:
> >  radix_tree_next_slot include/linux/radix-tree.h:422 [inline]
> >  idr_for_each+0x206/0x220 lib/idr.c:202
> >  io_destroy_buffers fs/io_uring.c:8275 [inline]
> 
> Matthew, can you shed any light on the link between the use of idr
> routines and the UAF reported?

I presume it's some misuse of IDR by io_uring.  I'd rather io_uring
didn't use the IDR at all.  This compiles; I promise no more than that.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ef3cd7fe4416..2fcf196bb3c3 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -344,7 +344,7 @@ struct io_ring_ctx {
        struct socket           *ring_sock;
 #endif
 
-       struct idr              io_buffer_idr;
+       struct xarray           io_buffers;
 
        struct idr              personality_idr;
 
@@ -1298,7 +1298,7 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct 
io_uring_params *p)
        INIT_LIST_HEAD(&ctx->cq_overflow_list);
        init_completion(&ctx->ref_comp);
        init_completion(&ctx->sq_thread_comp);
-       idr_init(&ctx->io_buffer_idr);
+       xa_init(&ctx->io_buffers);
        idr_init(&ctx->personality_idr);
        mutex_init(&ctx->uring_lock);
        init_waitqueue_head(&ctx->wait);
@@ -3042,7 +3042,7 @@ static struct io_buffer *io_buffer_select(struct io_kiocb 
*req, size_t *len,
 
        lockdep_assert_held(&req->ctx->uring_lock);
 
-       head = idr_find(&req->ctx->io_buffer_idr, bgid);
+       head = xa_load(&req->ctx->io_buffers, bgid);
        if (head) {
                if (!list_empty(&head->list)) {
                        kbuf = list_last_entry(&head->list, struct io_buffer,
@@ -3050,7 +3050,7 @@ static struct io_buffer *io_buffer_select(struct io_kiocb 
*req, size_t *len,
                        list_del(&kbuf->list);
                } else {
                        kbuf = head;
-                       idr_remove(&req->ctx->io_buffer_idr, bgid);
+                       xa_erase(&req->ctx->io_buffers, bgid);
                }
                if (*len > kbuf->len)
                        *len = kbuf->len;
@@ -4130,7 +4130,8 @@ static int __io_remove_buffers(struct io_ring_ctx *ctx, 
struct io_buffer *buf,
        }
        i++;
        kfree(buf);
-       idr_remove(&ctx->io_buffer_idr, bgid);
+       if (nbufs != -1U)
+               xa_erase(&ctx->io_buffers, bgid);
 
        return i;
 }
@@ -4148,7 +4149,7 @@ static int io_remove_buffers(struct io_kiocb *req, bool 
force_nonblock,
        lockdep_assert_held(&ctx->uring_lock);
 
        ret = -ENOENT;
-       head = idr_find(&ctx->io_buffer_idr, p->bgid);
+       head = xa_load(&ctx->io_buffers, p->bgid);
        if (head)
                ret = __io_remove_buffers(ctx, head, p->bgid, p->nbufs);
 
@@ -4225,15 +4226,15 @@ static int io_provide_buffers(struct io_kiocb *req, 
bool force_nonblock,
 
        lockdep_assert_held(&ctx->uring_lock);
 
-       list = head = idr_find(&ctx->io_buffer_idr, p->bgid);
+       list = head = xa_load(&ctx->io_buffers, p->bgid);
 
        ret = io_add_buffers(p, &head);
        if (ret < 0)
                goto out;
 
        if (!list) {
-               ret = idr_alloc(&ctx->io_buffer_idr, head, p->bgid, p->bgid + 1,
-                                       GFP_KERNEL);
+               ret = xa_err(xa_store(&ctx->io_buffers, p->bgid, head,
+                                       GFP_KERNEL));
                if (ret < 0) {
                        __io_remove_buffers(ctx, head, p->bgid, -1U);
                        goto out;
@@ -8468,19 +8469,15 @@ static int io_eventfd_unregister(struct io_ring_ctx 
*ctx)
        return -ENXIO;
 }
 
-static int __io_destroy_buffers(int id, void *p, void *data)
-{
-       struct io_ring_ctx *ctx = data;
-       struct io_buffer *buf = p;
-
-       __io_remove_buffers(ctx, buf, id, -1U);
-       return 0;
-}
-
 static void io_destroy_buffers(struct io_ring_ctx *ctx)
 {
-       idr_for_each(&ctx->io_buffer_idr, __io_destroy_buffers, ctx);
-       idr_destroy(&ctx->io_buffer_idr);
+       unsigned long pgid;
+       struct io_buffer *buf;
+
+       xa_for_each(&ctx->io_buffers, pgid, buf) {
+               xa_erase(&ctx->io_buffers, pgid);
+               __io_remove_buffers(ctx, buf, pgid, -1U);
+       }
 }
 
 static void io_ring_ctx_free(struct io_ring_ctx *ctx)