Re: [PATCH] mm/sl[aou]b: make kfree() aware of error pointers

[Andrey Ryabinin]

2014-09-10 18:24 GMT+04:00 Jiri Kosina <jkosina@xxxxxxx>:
> On Wed, 10 Sep 2014, Theodore Ts'o wrote:
>
>> So I wouldn't be so sure that we don't have these sorts of bugs hiding
>> somewhere; and it's extremely easy for them to sneak in.  That being
>> said, I'm not in favor of making changes to kfree; I'd much rather
>> depending on better testing and static checkers to fix them, since
>> kfree *is* a hot path.
>
> I of course have no objections to this check being added to whatever
> static checker, that would be very welcome improvement.
>
> Still, I believe that kernel shouldn't be just ignoring kfree(ERR_PTR)
> happening. Would something like the below be more acceptable?
>
>
>
> From: Jiri Kosina <jkosina@xxxxxxx>
> Subject: [PATCH] mm/sl[aou]b: make kfree() aware of error pointers
>
> Freeing if ERR_PTR is not covered by ZERO_OR_NULL_PTR() check already
> present in kfree(), but it happens in the wild and has disastrous effects.
>
> Issue a warning and don't proceed trying to free the memory if
> CONFIG_DEBUG_SLAB is set.
>

This won't work cause CONFIG_DEBUG_SLAB  is only for CONFIG_SLAB=y

How about just VM_BUG_ON(IS_ERR(ptr)); ?


> Inspired by a9cfcd63e8d ("ext4: avoid trying to kfree an ERR_PTR pointer").
>
> Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> ---
>  mm/slab.c | 6 ++++++
>  mm/slob.c | 7 ++++++-
>  mm/slub.c | 7 ++++++-
>  3 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index a467b30..6f49d6b 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3612,6 +3612,12 @@ void kfree(const void *objp)
>
>         trace_kfree(_RET_IP_, objp);
>
> +#ifdef CONFIG_DEBUG_SLAB
> +       if (unlikely(IS_ERR(objp))) {
> +                       WARN(1, "trying to free ERR_PTR\n");
> +                       return;
> +       }
> +#endif
>         if (unlikely(ZERO_OR_NULL_PTR(objp)))
>                 return;
>         local_irq_save(flags);
> diff --git a/mm/slob.c b/mm/slob.c
> index 21980e0..66422a0 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -488,7 +488,12 @@ void kfree(const void *block)
>         struct page *sp;
>
>         trace_kfree(_RET_IP_, block);
> -
> +#ifdef CONFIG_DEBUG_SLAB
> +       if (unlikely(IS_ERR(block))) {
> +               WARN(1, "trying to free ERR_PTR\n");
> +               return;
> +       }
> +#endif
>         if (unlikely(ZERO_OR_NULL_PTR(block)))
>                 return;
>         kmemleak_free(block);
> diff --git a/mm/slub.c b/mm/slub.c
> index 3e8afcc..21155ae 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3337,7 +3337,12 @@ void kfree(const void *x)
>         void *object = (void *)x;
>
>         trace_kfree(_RET_IP_, x);
> -
> +#ifdef CONFIG_DEBUG_SLAB
> +       if (unlikely(IS_ERR(x))) {
> +               WARN(1, "trying to free ERR_PTR\n");
> +               return;
> +       }
> +#endif
>         if (unlikely(ZERO_OR_NULL_PTR(x)))
>                 return;
>
>
> --
> Jiri Kosina
> SUSE Labs
>
> --
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-- 
Best regards,
Andrey Ryabinin
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