Re: [PATCH v6] powerpc: Do not make the entire heap executable

[Kees Cook]

On Mon, Oct 3, 2016 at 9:13 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 32-bit powerpc the ELF PLT sections of binaries (built with --bss-plt,
> or with a toolchain which defaults to it) look like this:
>
>   [17] .sbss             NOBITS          0002aff8 01aff8 000014 00  WA  0   0  4
>   [18] .plt              NOBITS          0002b00c 01aff8 000084 00 WAX  0   0  4
>   [19] .bss              NOBITS          0002b090 01aff8 0000a4 00  WA  0   0  4
>
> Which results in an ELF load header:
>
>   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
>   LOAD           0x019c70 0x00029c70 0x00029c70 0x01388 0x014c4 RWE 0x10000
>
> This is all correct, the load region containing the PLT is marked as
> executable. Note that the PLT starts at 0002b00c but the file mapping ends at
> 0002aff8, so the PLT falls in the 0 fill section described by the load header,
> and after a page boundary.
>
> Unfortunately the generic ELF loader ignores the X bit in the load headers
> when it creates the 0 filled non-file backed mappings. It assumes all of these
> mappings are RW BSS sections, which is not the case for PPC.
>
> gcc/ld has an option (--secure-plt) to not do this, this is said to incur
> a small performance penalty.
>
> Currently, to support 32-bit binaries with PLT in BSS kernel maps *entire
> brk area* with executable rights for all binaries, even --secure-plt ones.
>
> Stop doing that.
>
> Teach the ELF loader to check the X bit in the relevant load header
> and create 0 filled anonymous mappings that are executable
> if the load header requests that.
>
> The patch was originally posted in 2012 by Jason Gunthorpe
> and apparently ignored:
>
> https://lkml.org/lkml/2012/9/30/138
>
> Lightly run-tested.
>
> Signed-off-by: Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> CC: Paul Mackerras <paulus@xxxxxxxxx>
> CC: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> CC: Florian Weimer <fweimer@xxxxxxxxxx>
> CC: linux-mm@xxxxxxxxx
> CC: linuxppc-dev@xxxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
> Changes since v5:
> * made do_brk_flags() error out if any bits other than VM_EXEC are set.
>   (Kees Cook: "With this, I'd be happy to Ack.")
>   See https://patchwork.ozlabs.org/patch/661595/

Excellent, thanks for the v6! Should this go via the ppc tree or the -mm tree?

-Kees

>
> Changes since v4:
> * if (current->personality & READ_IMPLIES_EXEC), still use VM_EXEC
>   for 32-bit executables.
>
> Changes since v3:
> * typo fix in commit message
> * rebased to current Linus tree
>
> Changes since v2:
> * moved capability to map with VM_EXEC into vm_brk_flags()
>
> Changes since v1:
> * wrapped lines to not exceed 79 chars
> * improved comment
> * expanded CC list
>
>  arch/powerpc/include/asm/page.h |  4 +++-
>  fs/binfmt_elf.c                 | 30 ++++++++++++++++++++++--------
>  include/linux/mm.h              |  1 +
>  mm/mmap.c                       | 24 +++++++++++++++++++-----
>  4 files changed, 45 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
> index 56398e7..17d3d2c 100644
> --- a/arch/powerpc/include/asm/page.h
> +++ b/arch/powerpc/include/asm/page.h
> @@ -230,7 +230,9 @@ extern long long virt_phys_offset;
>   * and needs to be executable.  This means the whole heap ends
>   * up being executable.
>   */
> -#define VM_DATA_DEFAULT_FLAGS32        (VM_READ | VM_WRITE | VM_EXEC | \
> +#define VM_DATA_DEFAULT_FLAGS32 \
> +       (((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | \
> +                                VM_READ | VM_WRITE | \
>                                  VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
>
>  #define VM_DATA_DEFAULT_FLAGS64        (VM_READ | VM_WRITE | \
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index e5495f3..12b0d19 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -91,12 +91,18 @@ static struct linux_binfmt elf_format = {
>
>  #define BAD_ADDR(x) ((unsigned long)(x) >= TASK_SIZE)
>
> -static int set_brk(unsigned long start, unsigned long end)
> +static int set_brk(unsigned long start, unsigned long end, int prot)
>  {
>         start = ELF_PAGEALIGN(start);
>         end = ELF_PAGEALIGN(end);
>         if (end > start) {
> -               int error = vm_brk(start, end - start);
> +               /*
> +                * Map the last of the bss segment.
> +                * If the header is requesting these pages to be
> +                * executable, honour that (ppc32 needs this).
> +                */
> +               int error = vm_brk_flags(start, end - start,
> +                               prot & PROT_EXEC ? VM_EXEC : 0);
>                 if (error)
>                         return error;
>         }
> @@ -524,6 +530,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>         unsigned long load_addr = 0;
>         int load_addr_set = 0;
>         unsigned long last_bss = 0, elf_bss = 0;
> +       int bss_prot = 0;
>         unsigned long error = ~0UL;
>         unsigned long total_size;
>         int i;
> @@ -606,8 +613,10 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>                          * elf_bss and last_bss is the bss section.
>                          */
>                         k = load_addr + eppnt->p_vaddr + eppnt->p_memsz;
> -                       if (k > last_bss)
> +                       if (k > last_bss) {
>                                 last_bss = k;
> +                               bss_prot = elf_prot;
> +                       }
>                 }
>         }
>
> @@ -623,13 +632,14 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>         /*
>          * Next, align both the file and mem bss up to the page size,
>          * since this is where elf_bss was just zeroed up to, and where
> -        * last_bss will end after the vm_brk() below.
> +        * last_bss will end after the vm_brk_flags() below.
>          */
>         elf_bss = ELF_PAGEALIGN(elf_bss);
>         last_bss = ELF_PAGEALIGN(last_bss);
>         /* Finally, if there is still more bss to allocate, do it. */
>         if (last_bss > elf_bss) {
> -               error = vm_brk(elf_bss, last_bss - elf_bss);
> +               error = vm_brk_flags(elf_bss, last_bss - elf_bss,
> +                               bss_prot & PROT_EXEC ? VM_EXEC : 0);
>                 if (error)
>                         goto out;
>         }
> @@ -674,6 +684,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>         unsigned long error;
>         struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
>         unsigned long elf_bss, elf_brk;
> +       int bss_prot = 0;
>         int retval, i;
>         unsigned long elf_entry;
>         unsigned long interp_load_addr = 0;
> @@ -882,7 +893,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>                            before this one. Map anonymous pages, if needed,
>                            and clear the area.  */
>                         retval = set_brk(elf_bss + load_bias,
> -                                        elf_brk + load_bias);
> +                                        elf_brk + load_bias,
> +                                        bss_prot);
>                         if (retval)
>                                 goto out_free_dentry;
>                         nbyte = ELF_PAGEOFFSET(elf_bss);
> @@ -976,8 +988,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
>                 if (end_data < k)
>                         end_data = k;
>                 k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
> -               if (k > elf_brk)
> +               if (k > elf_brk) {
> +                       bss_prot = elf_prot;
>                         elf_brk = k;
> +               }
>         }
>
>         loc->elf_ex.e_entry += load_bias;
> @@ -993,7 +1007,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>          * mapping in the interpreter, to make sure it doesn't wind
>          * up getting placed where the bss needs to go.
>          */
> -       retval = set_brk(elf_bss, elf_brk);
> +       retval = set_brk(elf_bss, elf_brk, bss_prot);
>         if (retval)
>                 goto out_free_dentry;
>         if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ef815b9..09c9133 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2059,6 +2059,7 @@ static inline void mm_populate(unsigned long addr, unsigned long len) {}
>
>  /* These take the mm semaphore themselves */
>  extern int __must_check vm_brk(unsigned long, unsigned long);
> +extern int __must_check vm_brk_flags(unsigned long, unsigned long, unsigned long);
>  extern int vm_munmap(unsigned long, size_t);
>  extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
>          unsigned long, unsigned long,
> diff --git a/mm/mmap.c b/mm/mmap.c
> index ca9d91b..307c154 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2653,11 +2653,11 @@ static inline void verify_mm_writelocked(struct mm_struct *mm)
>   *  anonymous maps.  eventually we may be able to do some
>   *  brk-specific accounting here.
>   */
> -static int do_brk(unsigned long addr, unsigned long request)
> +static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
>  {
>         struct mm_struct *mm = current->mm;
>         struct vm_area_struct *vma, *prev;
> -       unsigned long flags, len;
> +       unsigned long len;
>         struct rb_node **rb_link, *rb_parent;
>         pgoff_t pgoff = addr >> PAGE_SHIFT;
>         int error;
> @@ -2668,7 +2668,10 @@ static int do_brk(unsigned long addr, unsigned long request)
>         if (!len)
>                 return 0;
>
> -       flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> +       /* Until we need other flags, refuse anything except VM_EXEC. */
> +       if ((flags & (~VM_EXEC)) != 0)
> +               return -EINVAL;
> +       flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
>
>         error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
>         if (offset_in_page(error))
> @@ -2736,7 +2739,12 @@ out:
>         return 0;
>  }
>
> -int vm_brk(unsigned long addr, unsigned long len)
> +static int do_brk(unsigned long addr, unsigned long len)
> +{
> +       return do_brk_flags(addr, len, 0);
> +}
> +
> +int vm_brk_flags(unsigned long addr, unsigned long len, unsigned long flags)
>  {
>         struct mm_struct *mm = current->mm;
>         int ret;
> @@ -2745,13 +2753,19 @@ int vm_brk(unsigned long addr, unsigned long len)
>         if (down_write_killable(&mm->mmap_sem))
>                 return -EINTR;
>
> -       ret = do_brk(addr, len);
> +       ret = do_brk_flags(addr, len, flags);
>         populate = ((mm->def_flags & VM_LOCKED) != 0);
>         up_write(&mm->mmap_sem);
>         if (populate && !ret)
>                 mm_populate(addr, len);
>         return ret;
>  }
> +EXPORT_SYMBOL(vm_brk_flags);
> +
> +int vm_brk(unsigned long addr, unsigned long len)
> +{
> +       return vm_brk_flags(addr, len, 0);
> +}
>  EXPORT_SYMBOL(vm_brk);
>
>  /* Release all mmaps. */
> --
> 2.9.2
>



-- 
Kees Cook
Nexus Security