On Mon, Apr 07, 2014 at 08:15:09PM -0700, 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.
>
> Signed-off-by: Kees Cook <keesc...@chromium.org>
> ---
[ ... ]
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 2a77ba8796ae..66a7283583cd 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -31,6 +31,11 @@
> #include <asm/tlb.h>
> #include <asm/fixmap.h>
>
> +#ifdef CONFIG_ARM_KERNMEM_PERMS
> +#include <asm/system_info.h>
> +#include <asm/cp15.h>
> +#endif
> +
> #include <asm/mach/arch.h>
> #include <asm/mach/map.h>
>
> @@ -623,11 +628,112 @@ void __init mem_init(void)
> }
> }
>
> +#ifdef CONFIG_ARM_KERNMEM_PERMS
> +struct section_perm {
> + unsigned long start;
> + unsigned long end;
> + pmdval_t mask;
> + pmdval_t prot;
> +};
> +
> +struct section_perm nx_perms[] = {
> + /* Make pages tables, etc before _stext RW (set NX). */
> + {
> + .start = PAGE_OFFSET,
> + .end = (unsigned long)_stext,
> + .mask = ~PMD_SECT_XN,
> + .prot = PMD_SECT_XN,
> + },
> + /* Make init RW (set NX). */
> + {
> + .start = (unsigned long)__init_begin,
> + .end = (unsigned long)_sdata,
> + .mask = ~PMD_SECT_XN,
> + .prot = PMD_SECT_XN,
> + },
> +};
> +
> +/*
> + * 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);
> +}
> +
> +/* 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;
> +}
> +
> +#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();
If it's practical to allow kprobes to modify the underlying .text
without changing the section mappings; then I think it would be lot
cleaner to put down read only .text sections from the beginning, and
not have any section modification code.
Cheers,
--
Steve
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