[PATCH v6] powerpc: Do not make the entire heap executable
Kees Cook
keescook at chromium.org
Tue Oct 4 08:51:28 AEDT 2016
On Mon, Oct 3, 2016 at 9:13 AM, Denys Vlasenko <dvlasenk at redhat.com> 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 at obsidianresearch.com>
> Signed-off-by: Denys Vlasenko <dvlasenk at redhat.com>
> Acked-by: Kees Cook <keescook at chromium.org>
> Acked-by: Michael Ellerman <mpe at ellerman.id.au>
> CC: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> CC: Paul Mackerras <paulus at samba.org>
> CC: "Aneesh Kumar K.V" <aneesh.kumar at linux.vnet.ibm.com>
> CC: Kees Cook <keescook at chromium.org>
> CC: Oleg Nesterov <oleg at redhat.com>
> CC: Michael Ellerman <mpe at ellerman.id.au>
> CC: Florian Weimer <fweimer at redhat.com>
> CC: linux-mm at kvack.org
> CC: linuxppc-dev at lists.ozlabs.org
> CC: linux-kernel at vger.kernel.org
> ---
> 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
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