Re: [RFC PATCH] memory: Fix dma-reentrancy issues at the MMIO level

[Klaus Jensen]

On Dec 17 06:27, Qiuhao Li wrote:
> Thanks Alex. It seems this patch sets and checks if the destination device is 
> busy. But how about the data transfers not triggered directly by PMIO/MMIO 
> handlers? For example:
> 
> 1. Device A Timer's callback -> Device A MMIO handler
> 2. Device A BH's callback -> Device A MMIO handler
> 
> In these situations, when A launches a DMA to itself, the 
> dev->engaged_in_direct_io is not set, so the operation is allowed. Maybe we 
> should log the source and check the destination when we launch data 
> transfers. Is there a way to do that?
> 
> Below is a reproducer in NVMe which triggers DMA in a timer's callback 
> (nvme_process_sq). I can still trigger use-after-free exception with this 
> patch on qemu-6.1.0:
> 
> cat << EOF | ./qemu-system-x86_64 -display none -machine accel=qtest \
> -machine q35 -nodefaults -drive file=null-co://,if=none,format=raw,id=disk0 \
> -device nvme,drive=disk0,serial=1 -qtest stdio \
> 
> outl 0xcf8 0x80000810               /* MLBAR (BAR0) – Memory Register Base 
> Address, lower 32-bits */
> outl 0xcfc 0xe0000000               /* MMIO Base Address = 0xe0000000 */
> outl 0xcf8 0x80000804               /* CMD - Command */
> outw 0xcfc 0x06                     /* Bus Master Enable, Memory Space Enable 
> */
> write 0xe0000024 0x4 0x02000200     /* [3] 3.1.8, Admin Queue Attributes */
> write 0xe0000028 0x4 0x00100000     /* asq = 0x1000 */
> write 0xe0000030 0x4 0x00200000     /* acq = 0x2000 */
> write 0xe0000014 0x4 0x01004600     /* [3] 3.1.5, Controller Configuration, 
> start ctrl */
> write 0xe0001000 0x1 0x01           /* [3] 3.1.24, SQyTDBL – Submission Queue 
> y Tail Doorbell */
> write 0x1000 0x1 0x02               /* cmd->opcode, 
> NVME_ADM_CMD_GET_LOG_PAGE, nvme_get_log() */
> write 0x1018 0x4 0x140000e0         /* prp1 = 0xe0000014, NVME_REG_CC, 
> nvme_ctrl_reset() */
> write 0x1028 0x4 0x03000004         /* cmd->cdw10, lid = 3 
> NVME_LOG_FW_SLOT_INFO, nvme_fw_log_info, buf_len = 4 */
> write 0x1030 0x4 0xfc010000         /* cmd->cdw12 = 0x1fc, Log Page Offset, 
> trans_len = sizeof(fw_log) - 0x1fc = 4 */
> clock_step
> EOF
> 
> CC: Mauro Matteo Cascella and Philippe Mathieu-Daudé. Should we put the 
> reproducer above to https://gitlab.com/qemu-project/qemu/-/issues/556?
> 

This is a good reproducer. Does it still work if you do the `write
0xe0001000 0x1 0x01` at the end instead? It looks weird that you ring
the doorbell prior to writing the command in the queue.

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