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    SubjectRe: [tip:x86/security] x86: Add NX protection for kernel data
    Konrad Rzeszutek Wilk a écrit :
    > On Fri, Jan 21, 2011 at 10:41:54PM +0100, matthieu castet wrote:
    >> Konrad Rzeszutek Wilk a écrit :
    >>>> - * .data and .bss should always be writable.
    >>>> + * .data and .bss should always be writable, but xen won't like
    >>>> + * if we make page table rw (that live in .data or .bss)
    >>>> */
    >>>> +#ifdef CONFIG_X86_32
    >>>> if (within(address, (unsigned long)_sdata, (unsigned long)_edata) ||
    >>>> - within(address, (unsigned long)__bss_start, (unsigned long)__bss_stop))
    >>>> - pgprot_val(required) |= _PAGE_RW;
    >>>> + within(address, (unsigned long)__bss_start, (unsigned long)__bss_stop)) {
    >>>> + unsigned int level;
    >>>> + if (lookup_address(address, &level) && (level != PG_LEVEL_4K))
    >>>> + pgprot_val(forbidden) |= _PAGE_RW;
    >>>> + }
    >>>> +#endif
    >>>> #if defined(CONFIG_X86_64) && defined(CONFIG_DEBUG_RODATA)
    >>>>
    >>>> fyi, it does make it boot.
    >>> Hold it.. ccache is a wonderful tool but I think I've just "rebuilt" the
    >>> binaries with the .bss HPAGE_ALIGN aligment by mistake, so this path got never
    >>> taken.
    >>>
    >>>
    >> Ok,
    >>
    >> ATM I saw the following solution to solve the problem :
    >> 1) remove the data/bss check in static_protections, it was introduced by NX patches (64edc8ed). But I am not sure it
    >> is really needed anymore.
    >> 2) add ". = ALIGN(HPAGE_SIZE)" somewhere after init section. But if we want not to be allocated in image we
    >> should put it before bss. And if we want to be freed after init, we should put before .init.end.
    >> This mean moving .smp_locks (and .data_nosave when x86 will be added) before init section. I have no idea of the impact.
    >> 3) add some logic in arch/x86/xen/mmu.c, that will ignore RW page setting for the page table marked RO.
    >> 4) make static_protections take and old_prot argument, and only apply RW .data/.bss requirement if page is already RW.
    >>
    >> If possible I will go for 1).
    >
    > Sounds good. Just send me the patch and I will test it.

    Ok, what give you the attached patch.

    I don't know if I should give the printk or not.


    Matthieu
    diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
    index 8b830ca..eec93c5 100644
    --- a/arch/x86/mm/pageattr.c
    +++ b/arch/x86/mm/pageattr.c
    @@ -256,7 +256,6 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
    unsigned long pfn)
    {
    pgprot_t forbidden = __pgprot(0);
    - pgprot_t required = __pgprot(0);

    /*
    * The BIOS area between 640k and 1Mb needs to be executable for
    @@ -283,11 +282,13 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
    __pa((unsigned long)__end_rodata) >> PAGE_SHIFT))
    pgprot_val(forbidden) |= _PAGE_RW;
    /*
    - * .data and .bss should always be writable.
    + * .data and .bss should always be writable, but xen won't like
    + * if we make page table rw (that live in .data or .bss)
    */
    if (within(address, (unsigned long)_sdata, (unsigned long)_edata) ||
    within(address, (unsigned long)__bss_start, (unsigned long)__bss_stop))
    - pgprot_val(required) |= _PAGE_RW;
    + if ((pgprot_val(prot) & _PAGE_RW) == 0)
    + printk(KERN_INFO "RO page for 0x%lx in bss/data.\n", address);

    #if defined(CONFIG_X86_64) && defined(CONFIG_DEBUG_RODATA)
    /*
    @@ -327,7 +328,6 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
    #endif

    prot = __pgprot(pgprot_val(prot) & ~pgprot_val(forbidden));
    - prot = __pgprot(pgprot_val(prot) | pgprot_val(required));

    return prot;
    }
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