io_uring/kbuf: don't allow registered buffer rings on highmem pages

syzbot reports that registering a mapped buffer ring on arm32 can
trigger an OOPS. Registered buffer rings have two modes, one of them
is the application passing in the memory that the buffer ring should
reside in. Once those pages are mapped, we use page_address() to get
a virtual address. This will obviously fail on highmem pages, which
aren't mapped.

Add a check if we have any highmem pages after mapping, and fail the
attempt to register a provided buffer ring if we do. This will return
the same error as kernels that don't support provided buffer rings to
begin with.

Link: https://lore.kernel.org/io-uring/000000000000af635c0606bcb889@google.com/
Fixes: c56e022 ("io_uring: add support for user mapped provided buffer ring")
Cc: stable@vger.kernel.org
Reported-by: syzbot+2113e61b8848fa7951d8@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/kbuf.c

@@ -477,14 +477,25 @@ static int io_pin_pbuf_ring(struct io_uring_buf_reg *reg,
 {
 	struct io_uring_buf_ring *br;
 	struct page **pages;
-	int nr_pages;
+	int i, nr_pages;
 
 	pages = io_pin_pages(reg->ring_addr,
 			     flex_array_size(br, bufs, reg->ring_entries),
 			     &nr_pages);
 	if (IS_ERR(pages))
 		return PTR_ERR(pages);
 
+	/*
+	 * Apparently some 32-bit boxes (ARM) will return highmem pages,
+	 * which then need to be mapped. We could support that, but it'd
+	 * complicate the code and slowdown the common cases quite a bit.
+	 * So just error out, returning -EINVAL just like we did on kernels
+	 * that didn't support mapped buffer rings.
+	 */
+	for (i = 0; i < nr_pages; i++)
+		if (PageHighMem(pages[i]))
+			goto error_unpin;
+
 	br = page_address(pages[0]);
 #ifdef SHM_COLOUR
 	/*
@@ -496,20 +507,20 @@ static int io_pin_pbuf_ring(struct io_uring_buf_reg *reg,
 	 * should use IOU_PBUF_RING_MMAP instead, and liburing will handle
 	 * this transparently.
 	 */
-	if ((reg->ring_addr | (unsigned long) br) & (SHM_COLOUR - 1)) {
-		int i;
-
-		for (i = 0; i < nr_pages; i++)
-			unpin_user_page(pages[i]);
-		return -EINVAL;
-	}
+	if ((reg->ring_addr | (unsigned long) br) & (SHM_COLOUR - 1))
+		goto error_unpin;
 #endif
 	bl->buf_pages = pages;
 	bl->buf_nr_pages = nr_pages;
 	bl->buf_ring = br;
 	bl->is_mapped = 1;
 	bl->is_mmap = 0;
 	return 0;
+error_unpin:
+	for (i = 0; i < nr_pages; i++)
+		unpin_user_page(pages[i]);
+	kvfree(pages);
+	return -EINVAL;
 }
 
 static int io_alloc_pbuf_ring(struct io_uring_buf_reg *reg,