From f471e8b060798f26a7fc339c6152f82f22a7b33d Mon Sep 17 00:00:00 2001 From: Arnout Engelen Date: Sun, 8 May 2022 17:32:22 +0200 Subject: [PATCH] hw/usb/hcd-ehci: fix writeback order The 'active' bit passes control over a qTD between the guest and the controller: set to 1 by guest to enable execution by the controller, and the controller sets it to '0' to hand back control to the guest. ehci_state_writeback write two dwords to main memory using DMA: the third dword of the qTD (containing dt, total bytes to transfer, cpage, cerr and status) and the fourth dword of the qTD (containing the offset). This commit makes sure the fourth dword is written before the third, avoiding a race condition where a new offset written into the qTD by the guest after it observed the status going to go to '0' gets overwritten by a 'late' DMA writeback of the previous offset. This race condition could lead to 'cpage out of range (5)' errors, and reproduced by: ./qemu-system-x86_64 -enable-kvm -bios $SEABIOS/bios.bin -m 4096 -device usb-ehci -blockdev driver=file,read-only=on,filename=/home/aengelen/Downloads/openSUSE-Tumbleweed-DVD-i586-Snapshot20220428-Media.iso,node-name=iso -device usb-storage,drive=iso,bootindex=0 -chardev pipe,id=shell,path=/tmp/pipe -device virtio-serial -device virtconsole,chardev=shell -device virtio-rng-pci -serial mon:stdio -nographic (press a key, select 'Installation' (2), and accept the default values. On my machine the 'cpage out of range' is reproduced while loading the Linux Kernel about once per 7 attempts. With the fix in this commit it no longer fails) This problem was previously reported as a seabios problem in https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/OUTHT5ISSQJGXPNTUPY3O5E5EPZJCHM3/ and as a nixos CI build failure in https://github.com/NixOS/nixpkgs/issues/170803 Signed-off-by: Arnout Engelen Signed-off-by: Gerd Hoffmann --- hw/usb/hcd-ehci.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/hw/usb/hcd-ehci.c b/hw/usb/hcd-ehci.c index 33a8a377bd..d4da8dcb8d 100644 --- a/hw/usb/hcd-ehci.c +++ b/hw/usb/hcd-ehci.c @@ -2011,7 +2011,10 @@ static int ehci_state_writeback(EHCIQueue *q) ehci_trace_qtd(q, NLPTR_GET(p->qtdaddr), (EHCIqtd *) &q->qh.next_qtd); qtd = (uint32_t *) &q->qh.next_qtd; addr = NLPTR_GET(p->qtdaddr); - put_dwords(q->ehci, addr + 2 * sizeof(uint32_t), qtd + 2, 2); + /* First write back the offset */ + put_dwords(q->ehci, addr + 3 * sizeof(uint32_t), qtd + 3, 1); + /* Then write back the token, clearing the 'active' bit */ + put_dwords(q->ehci, addr + 2 * sizeof(uint32_t), qtd + 2, 1); ehci_free_packet(p); /*