Re: [PATCH] drm/i915: Fix "mitigations" parsing if i915 is builtin

[Jisheng Zhang]

Hi Ville,

On Tue, 13 Apr 2021 19:59:34 +0300 Ville Syrjälä wrote:


> 
> 
> On Tue, Apr 13, 2021 at 05:02:40PM +0800, Jisheng Zhang wrote:
> > I met below error during boot with i915 builtin if pass
> > "i915.mitigations=off":
> > [    0.015589] Booting kernel: `off' invalid for parameter `i915.mitigations'
> >
> > The reason is slab subsystem isn't ready at that time, so kstrdup()
> > returns NULL. Fix this issue by using stack var instead of kstrdup().
> >
> > Fixes: 984cadea032b ("drm/i915: Allow the sysadmin to override security mitigations")
> > Signed-off-by: Jisheng Zhang <Jisheng.Zhang@xxxxxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/i915/i915_mitigations.c | 7 ++-----
> >  1 file changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_mitigations.c b/drivers/gpu/drm/i915/i915_mitigations.c
> > index 84f12598d145..7dadf41064e0 100644
> > --- a/drivers/gpu/drm/i915/i915_mitigations.c
> > +++ b/drivers/gpu/drm/i915/i915_mitigations.c
> > @@ -29,15 +29,13 @@ bool i915_mitigate_clear_residuals(void)
> >  static int mitigations_set(const char *val, const struct kernel_param *kp)
> >  {
> >       unsigned long new = ~0UL;
> > -     char *str, *sep, *tok;
> > +     char str[64], *sep, *tok;
> >       bool first = true;
> >       int err = 0;
> >
> >       BUILD_BUG_ON(ARRAY_SIZE(names) >= BITS_PER_TYPE(mitigations));
> >
> > -     str = kstrdup(val, GFP_KERNEL);
> > -     if (!str)
> > -             return -ENOMEM;
> > +     strncpy(str, val, sizeof(str) - 1);  
> 
> I don't think strncpy() guarantees that the string is properly
> terminated.
> 
> Also commit b1b6bed3b503 ("usb: core: fix quirks_param_set() writing to
> a const pointer") looks broken as well given your findings, and
> arch/um/drivers/virtio_uml.c seems to suffer from this as well.
> kernel/params.c itself seems to have some slab_is_available() magic
> around kmalloc().

Just tried the "usbcore.quirks" with usb builtin, I can't reproduce the
issue. Futher investigation shows that device_param_cb() macro is the
key, or the "6" in __level_param_cb(name, ops, arg, perm, 6) is the key.
While i915.mitigations uses module_param_cb_unsafe(), in which the level
will be "-1"

arch/um/drivers/virtio_uml.c also makes use of device_param_cb()

thanks