Re: [PATCH 1/5] arm64: vdso: don't free unallocated pages

Mark Rutland
2020-04-14T15:12:55Z

On Tue, Apr 14, 2020 at 03:53:45PM +0100, Vincenzo Frascino wrote:
>
> On 4/14/20 2:27 PM, Mark Rutland wrote:
> > On Tue, Apr 14, 2020 at 01:50:38PM +0100, Vincenzo Frascino wrote:
> >> Hi Mark,
> >>
> >> On 4/14/20 11:42 AM, Mark Rutland wrote:
> >>> The aarch32_vdso_pages[] array never has entries allocated in the C_VVAR
> >>> or C_VDSO slots, and as the array is zero initialized these contain
> >>> NULL.
> >>>
> >>> However in __aarch32_alloc_vdso_pages() when
> >>> aarch32_alloc_kuser_vdso_page() fails we attempt to free the page whose
> >>> struct page is at NULL, which is obviously nonsensical.
> >>
> >> Could you please explain why do you think that free(NULL) is "nonsensical"?
> >
> > Regardless of the below, can you please explain why it is sensical? I'm
> > struggling to follow your argument here.
>
> free(NULL) is a no-operation ("no action occurs") according to the C standard
> (ISO-IEC 9899 paragraph 7.20.3.2). Hence this should not cause any bug if the
> allocator is correctly implemented.

This is true, but irrelevant. The C standard does not define
free_page(), which is a Linnux kernel internal function, and does not
have the same semantics as free().

> > * It serves no legitimate purpose. One cannot free a page without a
> > corresponding struct page.
> >
> > * It is redundant. Removing the code does not detract from the utility
> > of the remainging code, or make that remaing code more complex.

> > * free_page(x) calls free_pages(x, 0), which checks virt_addr_valid(x).
> > As page_to_virt(NULL) is not a valid linear map address, this can
> > trigger a VM_BUG_ON()
> >
>
> free_pages(x, 0) checks virt_addr_valid(x) only if "addr != 0" (as per C
> standard) which makes me infer what I stated above. But maybe I am missing
> something.

Regardless, this is all academic unless you disagree with the first two
bullets above.

You don't randomly sprinkle a program with free(NULL) for the fun of it.
Similarly, and regardless of how obfuscated, one should not do the same
here.

Thanks,
Mark.