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

[Ville Syrjälä]

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().

I used the following cocci snippet to find these:
@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