[PATCH for-next] io_uring/kbuf: disallow mapping a badly aligned provided ring buffer

[Jens Axboe <axboe@kernel.dk>]

On at least parisc, we have strict requirements on how we virtually map
an address that is shared between the application and the kernel. On
these platforms, IOU_PBUF_RING_MMAP should be used when setting up a
shared ring buffer for provided buffers. If the application is mapping
these pages and asking the kernel to pin+map them as well, then we have
no control over what virtual address we get in the kernel.

For that case, do a sanity check if SHM_COLOUR is defined, and disallow
the mapping request. The application must fall back to using
IOU_PBUF_RING_MMAP for this case, and liburing will do that transparently
with the set of helpers that it has.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/kbuf.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index cd1d9dddf58e..79c25459e8de 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -491,6 +491,24 @@ static int io_pin_pbuf_ring(struct io_uring_buf_reg *reg,
 		return PTR_ERR(pages);
 
 	br = page_address(pages[0]);
+#ifdef SHM_COLOUR
+	/*
+	 * On platforms that have specific aliasing requirements, SHM_COLOUR
+	 * is set and we must guarantee that the kernel and user side align
+	 * nicely. We cannot do that if IOU_PBUF_RING_MMAP isn't set and
+	 * the application mmap's the provided ring buffer. Fail the request
+	 * if we, by chance, don't end up with aligned addresses. The app
+	 * 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;
+	}
+#endif
 	bl->buf_pages = pages;
 	bl->buf_nr_pages = nr_pages;
 	bl->buf_ring = br;
-- 
2.39.2

-- 
Jens Axboe