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

[PATCH] 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.fomichev@wdc.com>
---
 drivers/target/target_core_user.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
index 04eda111920e..a0231491fa36 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);
-- 
2.21.0

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

[Mike Christie]

On 08/10/2019 04:19 PM, 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 | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
> index 04eda111920e..a0231491fa36 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);

If you are adding a warn here, I think you want to also add a warn in
tcmu_reset_ring where there is another code path we do the same sequence
of operations where we check the bit then access the se_cmd after if not
set.

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

[Dmitry Fomichev]

On Sat, 2019-08-10 at 18:55 -0500, Mike Christie wrote:
> On 08/10/2019 04:19 PM, 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 | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
> > index 04eda111920e..a0231491fa36 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);
> 
> If you are adding a warn here, I think you want to also add a warn in
> tcmu_reset_ring where there is another code path we do the same sequence
> of operations where we check the bit then access the se_cmd after if not
> set.
> 
Mike,
Good point, thanks. I am going to send out the updated version of the patch.

Dmitry