linux-kernel - Re: [PATCH v2 13/16] mm/debug_vm_pgtable: Remove uninitialized

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Date:   Mon, 22 Jun 2020 10:27:26 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Andy Whitcroft <apw@...onical.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        drbd-dev@...ts.linbit.com, linux-block@...r.kernel.org,
        b43-dev@...ts.infradead.org,
        Network Development <netdev@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        linux-ide@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-spi@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2 13/16] mm/debug_vm_pgtable: Remove uninitialized_var() usage

On Fri, Jun 19, 2020 at 8:30 PM Kees Cook <keescook@...omium.org> wrote:
>
> Using uninitialized_var() is dangerous as it papers over real bugs[1]
> (or can in the future), and suppresses unrelated compiler warnings (e.g.
> "unused variable"). If the compiler thinks it is uninitialized, either
> simply initialize the variable or make compiler changes. As a precursor
> to removing[2] this[3] macro[4], just initialize this variable to NULL.
>
> [1] https://lore.kernel.org/lkml/20200603174714.192027-1-glider@google.com/
> [2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=1TGqCR5vQkCzWJ0QxK6CernOU6eedsudAixw@mail.gmail.com/
> [3] https://lore.kernel.org/lkml/CA+55aFwgbgqhbp1fkxvRKEpzyR5J8n1vKT1VZdz9knmPuXhOeg@mail.gmail.com/
> [4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=yVJu65TpLgN_ybYNv0VEOKA@mail.gmail.com/
>
> Fixes: 399145f9eb6c ("mm/debug: add tests validating architecture page table helpers")
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  mm/debug_vm_pgtable.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index e45623016aea..83c9e88a052a 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -307,7 +307,7 @@ static int __init debug_vm_pgtable(void)
>         phys_addr_t paddr;
>         unsigned long vaddr, pte_aligned, pmd_aligned;
>         unsigned long pud_aligned, p4d_aligned, pgd_aligned;
> -       spinlock_t *uninitialized_var(ptl);
> +       spinlock_t *ptl = NULL;

It looks like the address of ptl is passed to pte_alloc_map_lock.  It
looks like pte_offset_map_lock unconditionally assigns through that
pointer before reading.  So this could be left uninitialized, but
initializing it doesn't hurt.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
>         pr_info("Validating architecture page table helpers\n");
>         prot = vm_get_page_prot(VMFLAGS);
> --

-- 
Thanks,
~Nick Desaulniers