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

[Jisheng Zhang]

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.

wow thank you so much. I will send out patches to fix them as well.

> kernel/params.c itself seems to have some slab_is_available() magic
> around kmalloc().
> 
> I used the following cocci snippet to find these:

Nice cocci script.


> @find@
> identifier O, F;
> position PS;
> @@
> struct kernel_param_ops O = {
> ...,
>         .set = F@PS
> ,...
> };
> 
> @alloc@
> identifier ALLOC =~ "^k.*(alloc|dup)";
> identifier find.F;
> position PA;
> @@
> F(...) {
> <+...
> ALLOC@PA(...)
> ...+>  
> }
> 
> @script:python depends on alloc@
> ps << find.PS;
> pa << alloc.PA;
> @@
> coccilib.report.print_report(ps[0], "struct")
> coccilib.report.print_report(pa[0], "alloc")
> 
> That could of course miss a bunch more if they allocate
> via some other function I didn't consider.
> 
> --
> Ville Syrjälä
> Intel