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

[Dmitry Fomichev]

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.fomic...@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);
                        list_del_init(&cmd->queue_entry);
                        if (err_level == 1) {
                                /*
-- 
2.21.0