Re: [PATCH v2] tcmu: avoid use-after-free after command timeout

Damien Le Moal
2019-08-12T18:00:26Z

On 2019/08/11 11:25, Dmitry Fomichev wrote:
> In tcmu_handle_completion() function, the variable called read_len is
> always initialized with a value taken from se_cmd structure. If this
> function is called to complete an expired (timed out) out command, the
> session command pointed by se_cmd is likely to be already deallocated by
> the target core at that moment. As the result, this access triggers a
> use-after-free warning from KASAN.
>
> This patch fixes the code not to touch se_cmd when completing timed out
> TCMU commands. It also resets the pointer to se_cmd at the time when the
> TCMU_CMD_BIT_EXPIRED flag is set because it is going to become invalid
> after calling target_complete_cmd() later in the same function,
> tcmu_check_expired_cmd().
>
> Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
> ---
> drivers/target/target_core_user.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
> index 04eda111920e..661bb9358364 100644
> --- a/drivers/target/target_core_user.c
> +++ b/drivers/target/target_core_user.c
> @@ -1132,14 +1132,16 @@ static void tcmu_handle_completion(struct tcmu_cmd *cmd, struct tcmu_cmd_entry *
> struct se_cmd *se_cmd = cmd->se_cmd;
> struct tcmu_dev *udev = cmd->tcmu_dev;
> bool read_len_valid = false;
> - uint32_t read_len = se_cmd->data_length;
> + uint32_t read_len;
>
> /*
> * cmd has been completed already from timeout, just reclaim
> * data area space and free cmd
> */
> - if (test_bit(TCMU_CMD_BIT_EXPIRED, &cmd->flags))
> + if (test_bit(TCMU_CMD_BIT_EXPIRED, &cmd->flags)) {
> + WARN_ON_ONCE(se_cmd);
> goto out;
> + }
>
> list_del_init(&cmd->queue_entry);
>
> @@ -1152,6 +1154,7 @@ static void tcmu_handle_completion(struct tcmu_cmd *cmd, struct tcmu_cmd_entry *
> goto done;
> }
>
> + read_len = se_cmd->data_length;
> if (se_cmd->data_direction == DMA_FROM_DEVICE &&
> (entry->hdr.uflags & TCMU_UFLAG_READ_LEN) && entry->rsp.read_len) {
> read_len_valid = true;
> @@ -1307,6 +1310,7 @@ static int tcmu_check_expired_cmd(int id, void *p, void *data)
> */
> scsi_status = SAM_STAT_CHECK_CONDITION;
> list_del_init(&cmd->queue_entry);
> + cmd->se_cmd = NULL;
> } else {
> list_del_init(&cmd->queue_entry);
> idr_remove(&udev->commands, id);
> @@ -2022,6 +2026,7 @@ static void tcmu_reset_ring(struct tcmu_dev *udev, u8 err_level)
>
> idr_remove(&udev->commands, i);
> if (!test_bit(TCMU_CMD_BIT_EXPIRED, &cmd->flags)) {
> + WARN_ON(!cmd->se_cmd);

May be WARN_ON_ONCE() similarly to the previous one ? No strong opinion about it
though.

> list_del_init(&cmd->queue_entry);
> if (err_level == 1) {
> /*
>

Apart from the optional nit above:

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>

--
Damien Le Moal
Western Digital Research