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Date: Tue, 10 Apr 2018 09:57:46 +0200
From: Arnd Bergmann <arnd@...db.de>
To: Laura Abbott <labbott@...hat.com>
Cc: Alex Dubov <oakad@...oo.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] misc: tifm: Remove VLA

On Mon, Apr 9, 2018 at 11:07 PM, Laura Abbott <labbott@...hat.com> wrote:
> There's an ongoing effort to remove VLAs[1] from the kernel to eventually
> turn on -Wvla. The single VLA can either take a value of 2 or 4 so switch
> to the upper bound.
>
> [1] https://lkml.org/lkml/2018/3/7/621
>
> Signed-off-by: Laura Abbott <labbott@...hat.com>
> ---
>  drivers/misc/tifm_7xx1.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/misc/tifm_7xx1.c b/drivers/misc/tifm_7xx1.c
> index e5f108713dd8..690eaaea5ce4 100644
> --- a/drivers/misc/tifm_7xx1.c
> +++ b/drivers/misc/tifm_7xx1.c
> @@ -239,7 +239,8 @@ static int tifm_7xx1_resume(struct pci_dev *dev)
>         unsigned long timeout;
>         unsigned int good_sockets = 0, bad_sockets = 0;
>         unsigned long flags;
> -       unsigned char new_ids[fm->num_sockets];
> +       /* Maximum number of entries is 4 */
> +       unsigned char new_ids[4];
>         DECLARE_COMPLETION_ONSTACK(finish_resume);
>

I like the idea of removing all the VLAs, but this one appears to make
the code less robust rather than more: In case of an unexpected
fm->num_sockets value, we now not only overflow the kernel stack
area but also the local variable into the neighboring stack slots.

Maybe add an extra overflow check?

       Arnd

diff --git a/drivers/misc/tifm_7xx1.c b/drivers/misc/tifm_7xx1.c
index e5f108713dd8..c702cd92d396 100644
--- a/drivers/misc/tifm_7xx1.c
+++ b/drivers/misc/tifm_7xx1.c
@@ -239,9 +239,12 @@ static int tifm_7xx1_resume(struct pci_dev *dev)
        unsigned long timeout;
        unsigned int good_sockets = 0, bad_sockets = 0;
        unsigned long flags;
-       unsigned char new_ids[fm->num_sockets];
+      /* Maximum number of entries is 4 */
+       unsigned char new_ids[4];
        DECLARE_COMPLETION_ONSTACK(finish_resume);

+       if (WARN_ON(fm->num_sockets > ARRAY_SIZE(new_ids)))
+               return -ENXIO;
+
        pci_set_power_state(dev, PCI_D0);
        pci_restore_state(dev);
        rc = pci_enable_device(dev);

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