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    SubjectRe: [PATCH v4 7/8] ARM: mm: allow non-text sections to be non-executable
    On Wed, Aug 13, 2014 at 06:06:32PM +0100, Kees Cook wrote:
    > Adds CONFIG_ARM_KERNMEM_PERMS to separate the kernel memory regions
    > into section-sized areas that can have different permisions. Performs
    > the NX permission changes during free_initmem, so that init memory can be
    > reclaimed.
    >
    > This uses section size instead of PMD size to reduce memory lost to
    > padding on non-LPAE systems.
    >
    > Based on work by Brad Spengler, Larry Bassel, and Laura Abbott.

    [...]

    > diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
    > index 6f57cb94367f..a3d07ca2bbb4 100644
    > --- a/arch/arm/kernel/vmlinux.lds.S
    > +++ b/arch/arm/kernel/vmlinux.lds.S
    > @@ -8,6 +8,9 @@
    > #include <asm/thread_info.h>
    > #include <asm/memory.h>
    > #include <asm/page.h>
    > +#ifdef CONFIG_ARM_KERNMEM_PERMS
    > +#include <asm/pgtable.h>
    > +#endif
    >
    > #define PROC_INFO \
    > . = ALIGN(4); \
    > @@ -90,6 +93,11 @@ SECTIONS
    > _text = .;
    > HEAD_TEXT
    > }
    > +
    > +#ifdef CONFIG_ARM_KERNMEM_PERMS
    > + . = ALIGN(1<<SECTION_SHIFT);
    > +#endif
    > +
    > .text : { /* Real text segment */
    > _stext = .; /* Text and read-only data */
    > __exception_text_start = .;
    > @@ -145,7 +153,11 @@ SECTIONS
    > _etext = .; /* End of text and rodata section */
    >
    > #ifndef CONFIG_XIP_KERNEL
    > +# ifdef CONFIG_ARM_KERNMEM_PERMS
    > + . = ALIGN(1<<SECTION_SHIFT);
    > +# else
    > . = ALIGN(PAGE_SIZE);
    > +# endif

    This might be cleaner if we had a single macro (ALIGN_MIN?) that expanded to
    ALIGN(1 << SECTION_SHIFT) #ifdef CONFIG_ARM_KERNMEM_PERMS.

    > diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
    > index ad82c05bfc3a..ccf392ef40d4 100644
    > --- a/arch/arm/mm/init.c
    > +++ b/arch/arm/mm/init.c
    > @@ -32,6 +32,11 @@
    > #include <asm/tlb.h>
    > #include <asm/fixmap.h>
    >
    > +#ifdef CONFIG_ARM_KERNMEM_PERMS
    > +#include <asm/system_info.h>
    > +#include <asm/cp15.h>
    > +#endif

    We already #include cp15.h in this file. If you need system_info.h, I don't
    think you should bother making the include conditional.

    > +/*
    > + * Updates section permissions only for the current mm (sections are
    > + * copied into each mm). During startup, this is the init_mm.
    > + */
    > +static inline void section_update(unsigned long addr, pmdval_t mask,
    > + pmdval_t prot)
    > +{
    > + struct mm_struct *mm;
    > + pmd_t *pmd;
    > +
    > + mm = current->active_mm;
    > + pmd = pmd_offset(pud_offset(pgd_offset(mm, addr), addr), addr);
    > +
    > +#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
    > + flush_pmd_entry(pmd);
    > + local_flush_tlb_kernel_range(addr, addr + SECTION_SIZE);

    Why only a local flush? You're changing global mappings here, right?

    > +}
    > +
    > +/* Make sure extended page tables are in use. */
    > +static inline bool arch_has_strict_perms(void)
    > +{
    > + unsigned int cr;
    > +
    > + if (cpu_architecture() < CPU_ARCH_ARMv6)
    > + return false;
    > +
    > + cr = get_cr();
    > + if (!(cr & CR_XP))
    > + return false;
    > +
    > + return true;

    return !!(get_cr() & CR_XP)?

    > +}
    > +
    > +#define set_section_perms(perms, field) { \
    > + size_t i; \
    > + unsigned long addr; \
    > + \
    > + if (!arch_has_strict_perms()) \
    > + return; \
    > + \
    > + for (i = 0; i < ARRAY_SIZE(perms); i++) { \
    > + if (!IS_ALIGNED(perms[i].start, SECTION_SIZE) || \
    > + !IS_ALIGNED(perms[i].end, SECTION_SIZE)) { \
    > + pr_err("BUG: section %lx-%lx not aligned to %lx\n", \
    > + perms[i].start, perms[i].end, \
    > + SECTION_SIZE); \
    > + continue; \
    > + } \
    > + \
    > + for (addr = perms[i].start; \
    > + addr < perms[i].end; \
    > + addr += SECTION_SIZE) \
    > + section_update(addr, perms[i].mask, \
    > + perms[i].field); \
    > + } \
    > +}
    > +
    > +static inline void fix_kernmem_perms(void)
    > +{
    > + set_section_perms(nx_perms, prot);
    > +}
    > +#else
    > +static inline void fix_kernmem_perms(void) { }
    > +#endif /* CONFIG_ARM_KERNMEM_PERMS */
    > +
    > void free_initmem(void)
    > {
    > #ifdef CONFIG_HAVE_TCM
    > extern char __tcm_start, __tcm_end;
    > +#endif
    > +
    > + fix_kernmem_perms();
    >
    > +#ifdef CONFIG_HAVE_TCM

    You could avoid the double #ifdef by moving the tcm stuff into another
    function (free_tcmmem?)

    Will


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