On Fri, Oct 17, 2014 at 1:15 AM, Hillf Danton <[email protected]> wrote:
> Hey Kees
>
>> From: Kees Cook <[email protected]>
>> To: [email protected]
>> Cc: Kees Cook <[email protected]>, Will Deacon <[email protected]>,
>> Rabin Vincent <[email protected]>, Laura Abbott <[email protected]>, Rob
>> Herring <[email protected]>, Leif Lindholm <[email protected]>, Mark
>> Salter <[email protected]>, Liu hua <
>> Subject: [PATCH v6 8/8] ARM: mm: allow text and rodata sections to be
>> read-only
>> Date: Thu, 18 Sep 2014 12:19:09 -0700
>>
>> This introduces CONFIG_DEBUG_RODATA, making kernel text and rodata
>> read-only. Additionally, this splits rodata from text so that rodata can
>> also be NX, which may lead to wasted memory when aligning to SECTION_SIZE.
>> The read-only areas are made writable during ftrace updates and kexec.
>>
>> Signed-off-by: Kees Cook <[email protected]>
>> Tested-by: Laura Abbott <[email protected]>
>> Acked-by: Nicolas Pitre <[email protected]>
>> ---
>> [...]
>> +#ifdef CONFIG_DEBUG_RODATA
>> +static struct section_perm ro_perms[] = {
>> + /* Make kernel code and rodata RX (set RO). */
>> + {
>> + .start = (unsigned long)_stext,
>> + .end = (unsigned long)__init_begin,
>> +#ifdef CONFIG_ARM_LPAE
>> + .mask = ~PMD_SECT_RDONLY,
>> + .prot = PMD_SECT_RDONLY,
>> +#else
>> + .mask = ~(PMD_SECT_APX | PMD_SECT_AP_WRITE),
>> + .prot = PMD_SECT_APX | PMD_SECT_AP_WRITE,
>> + .clear = PMD_SECT_AP_WRITE,
>
> [1] after set_kernel_text_ro, PMD_SECT_AP_WRITE is set
> in corresponding PMD.
>
> [2] after set_kernel_text_rw, PMD_SECT_AP_WRITE is also set
> in corresponding PMD.
>
> as shown in [7/8] in this patch series.
>
> +#ifdef CONFIG_ARM_LPAE
> + pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot);
> +#else
> + if (addr & SECTION_SIZE)
> + pmd[1] = __pmd((pmd_val(pmd[1]) & mask) | prot);
> + else
> + pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot);
> +#endif
>
> [3] As far as I could understand,
> I wonder if PMD_SECT_AP_WRITE should be cleared
> when setting RO.
The bit names are confusing. PMD_SECT_APX is what actually controls
the ROness. As in, the region is read-only when PMD_SECT_APX=1 and
PMD_SECT_AP_WRITE=1, and writable when PMD_SECT_APX=0 and
PMD_SECT_AP_WRITE=1.
> btw, would you please shed light on PMD_SECT_APX,
> since it is not included in both MT_MEMORY_RWX and
> MT_MEMORY_RW mm types.
>
> [MT_MEMORY_RWX] = {
> .prot_pte = L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY,
> .prot_l1 = PMD_TYPE_TABLE,
> .prot_sect = PMD_TYPE_SECT | PMD_SECT_AP_WRITE,
> .domain = DOMAIN_KERNEL,
> },
> [MT_MEMORY_RW] = {
> .prot_pte = L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY |
> L_PTE_XN,
> .prot_l1 = PMD_TYPE_TABLE,
> .prot_sect = PMD_TYPE_SECT | PMD_SECT_AP_WRITE,
> .domain = DOMAIN_KERNEL,
> },
Correct, both of those are read-write, so PMD_SECT_APX is not set. If
there were an MT_MEMORY_RO type, it would have PMD_SECT_APX set. For
example, see that MT_ROM gets it set when extended page tables are
available (which the ro-nx code similarly checks via
arch_has_strict_perms()).
I hope that helps!
-Kees
--
Kees Cook
Chrome OS Security
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