[PATCH] n_tty: check for negative and zero space return from tty_write_room

[PATCH] n_tty: check for negative and zero space return from tty_write_room

[Colin King]

From: Colin Ian King <colin.king@canonical.com>

The return from tty_write_room could potentially be negative if
a tty write_room driver returns an error number (not that any seem
to do). Rather than just check for a zero return, also check for
a -ve return. This avoids the unsigned nr being set to a large unsigned
value on the assignment from variable space and can lead to overflowing
the buffer buf.  Better to be safe than assume all write_room
implementations in tty drivers are going to do the right thing.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 drivers/tty/n_tty.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 9cdb0fa3c4bf..66630787fbf9 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -550,7 +550,7 @@ static ssize_t process_output_block(struct tty_struct *tty,
 	mutex_lock(&ldata->output_lock);
 
 	space = tty_write_room(tty);
-	if (!space) {
+	if (space <= 0) {
 		mutex_unlock(&ldata->output_lock);
 		return 0;
 	}
-- 
2.20.1

Re: [PATCH] n_tty: check for negative and zero space return from tty_write_room

[Mukesh Ojha]

On 3/28/2019 10:40 PM, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
>
> The return from tty_write_room could potentially be negative if
> a tty write_room driver returns an error number (not that any seem
> to do). Rather than just check for a zero return, also check for
> a -ve return. This avoids the unsigned nr being set to a large unsigned
> value on the assignment from variable space and can lead to overflowing
> the buffer buf.  Better to be safe than assume all write_room
> implementations in tty drivers are going to do the right thing.
>
> Signed-off-by: Colin Ian King <colin.king@canonical.com>


Looks reasonable to me.


Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>

-Mukesh.


> ---
>   drivers/tty/n_tty.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index 9cdb0fa3c4bf..66630787fbf9 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -550,7 +550,7 @@ static ssize_t process_output_block(struct tty_struct *tty,
>   	mutex_lock(&ldata->output_lock);
>   
>   	space = tty_write_room(tty);
> -	if (!space) {
> +	if (space <= 0) {
>   		mutex_unlock(&ldata->output_lock);
>   		return 0;
>   	}

Re: [PATCH] n_tty: check for negative and zero space return from tty_write_room

[Dan Carpenter]

On Thu, Mar 28, 2019 at 05:10:05PM +0000, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> The return from tty_write_room could potentially be negative if
> a tty write_room driver returns an error number (not that any seem
> to do). Rather than just check for a zero return, also check for
> a -ve return. This avoids the unsigned nr being set to a large unsigned
> value on the assignment from variable space and can lead to overflowing
> the buffer buf.  Better to be safe than assume all write_room
> implementations in tty drivers are going to do the right thing.
> 
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  drivers/tty/n_tty.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index 9cdb0fa3c4bf..66630787fbf9 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -550,7 +550,7 @@ static ssize_t process_output_block(struct tty_struct *tty,
>  	mutex_lock(&ldata->output_lock);
>  
>  	space = tty_write_room(tty);
> -	if (!space) {
> +	if (space <= 0) {
>  		mutex_unlock(&ldata->output_lock);
>  		return 0;

Instead of zero we should probably return "space" to preserve the error
code.

regards,
dan carpenter