linux-kernel - drivers/char/ppdev -

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Date:   Fri, 2 Oct 2020 15:01:33 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     sudipm.mukherjee@...il.com
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: drivers/char/ppdev -

Hi Sudip,

While looking at atomic_t usages and noticed a few potential
problem the way struct pp_struct: atomic_t irqc field

- There is inconsistencies the lock hold in this driver.

pp_do_mutex is help before pp_do_ioctl() is called.
static int pp_do_ioctl()
....

pp_do_mutex
        case PPCLRIRQ:
                 ret = atomic_read(&pp->irqc);
                 if (copy_to_user(argp, &ret, sizeof(ret)))
                         return -EFAULT;
                 atomic_sub(ret, &pp->irqc);
                 return 0;

This path seems safe as far as atomic_t overflow is concerned.

However, pp_poll(), pp_irq(), and pp_open() etc. don't hold
lock while pp_do_ioctl() path does.

Something to look into to see if this is safe. I see this
comment for pp_poll() /* No kernel lock held - fine */

Anyway, please take a look and see if this is indeed a problem.

thanks,
-- Shuah