2019-03-28 17:11:11

by Colin King

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

From: Colin Ian King <[email protected]>

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 <[email protected]>
---
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

2019-03-28 19:35:12

by Mukesh Ojha

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

On 3/28/2019 10:40 PM, Colin King wrote:
> From: Colin Ian King <[email protected]>
>
> 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 <[email protected]>


Looks reasonable to me.


Reviewed-by: Mukesh Ojha <[email protected]>

-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;
> }

2019-03-29 07:45:31

by Dan Carpenter

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

On Thu, Mar 28, 2019 at 05:10:05PM +0000, Colin King wrote:
> From: Colin Ian King <[email protected]>
>
> 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 <[email protected]>
> ---
> 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