Re: [PATCH v2] gpio: cdev: fix kernel stack leak to user-space in error path

[Kent Gibson]

On Mon, Sep 21, 2026 at 10:51:42AM -0400, Bartosz Golaszewski wrote:
> On Mon, 21 Sep 2026 15:35:05 +0200, Kent Gibson <warthog618@xxxxxxxxx> said:
> > On Mon, Sep 21, 2026 at 02:56:58PM +0200, Bartosz Golaszewski wrote:
> >> If we fail to acquire the GPIO chip guard in gpio_desc_to_lineinfo(), we
> >> return immediately before zeroing the info struct we'll end up passing
> >> to the user-space later in lineinfo_get_v1(). This may leak the kernel
> >> stack contents. Make gpio_desc_to_lineinfo() return int so that the
> >> -ENODEV returned on failure to acquire the guard can be propagated to
> >> the callers.
> >>
> >> While not strictly necessary: move the memset() before trying to acquire
> >> the SRCU read lock too for good measure.
> >>
> >> Fixes: d83cee3d2bb1 ("gpio: protect the pointer to gpio_chip in gpio_device with SRCU")
> >> Cc: stable@xxxxxxxxxxxxxxx
> >> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> >> Closes: https://sashiko.dev/#/patchset/20260912123529.7951-1-tzungbi%40kernel.org?part=3
> >> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxxxxxxxx>
> >> ---
> >> Changes in v2:
> >> - Make gpio_desc_to_lineinfo() return an integer return value so that
> >>   the error code returned in case of a failure to acquire the guard can
> >>   be propagated down the stack
> >> - Link to v1: https://patch.msgid.link/20260917-gpio-cdev-stack-leak-fixes-v1-1-2bb52879e49c@xxxxxxxxxxxxxxxx
> >
> >> @@ -2521,7 +2533,14 @@ static int lineinfo_changed_notify(struct notifier_block *nb,
> >>
> >>  	ctx->chg.event_type = action;
> >>  	ctx->chg.timestamp_ns = ktime_get_ns();
> >> -	gpio_desc_to_lineinfo(desc, &ctx->chg.info, true);
> >> +
> >> +	ret = gpio_desc_to_lineinfo(desc, &ctx->chg.info, true);
> >> +	if (ret) {
> >> +		fput(fp);
> >> +		pr_err("GPIO controller that generated the line info notification was removed\n");
> >> +		return NOTIFY_DONE;
> >> +	}
> >> +
> >
> > Does resolving a race warrant an error?
> >
> 
> I guess not, we can just not do anything in this case.
> 

You could log a debug rather than an error, but I'm not sure that adds anything
- I have no problem with quietly discarding the notify if the chip has been
removed.

> > Also, compared to the kzalloc_obj() error path earlier in the function, here
> > you flip the order of the fput() and pr_err(). Is that significant?
> >
> 
> No, if anything this version is more correct as we drop the no-longer-needed
> resource before printing a log. But with the above it's irrelevant anyway.
>

I was thinking the same, and wondering if we should flip the other case, but
small potatoes and not part of this patch.

Cheers,
Kent,