Re: [PATCH 2/2] fs, elf: drop MAP_FIXED usage from elf_map

[Kees Cook]

On Thu, Nov 16, 2017 at 2:19 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> Both load_elf_interp and load_elf_binary rely on elf_map to map segments
> on a controlled address and they use MAP_FIXED to enforce that. This is
> however dangerous thing prone to silent data corruption which can be
> even exploitable. Let's take CVE-2017-1000253 as an example. At the time
> (before eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE"))
> ELF_ET_DYN_BASE was at TASK_SIZE / 3 * 2 which is not that far away from
> the stack top on 32b (legacy) memory layout (only 1GB away). Therefore
> we could end up mapping over the existing stack with some luck.
>
> The issue has been fixed since then (a87938b2e246 ("fs/binfmt_elf.c:
> fix bug in loading of PIE binaries")), ELF_ET_DYN_BASE moved moved much
> further from the stack (eab09532d400 and later by c715b72c1ba4 ("mm:
> revert x86_64 and arm64 ELF_ET_DYN_BASE base changes")) and excessive
> stack consumption early during execve fully stopped by da029c11e6b1
> ("exec: Limit arg stack to at most 75% of _STK_LIM"). So we should be
> safe and any attack should be impractical. On the other hand this is
> just too subtle assumption so it can break quite easily and hard to
> spot.
>
> I believe that the MAP_FIXED usage in load_elf_binary (et. al) is still
> fundamentally dangerous. Moreover it shouldn't be even needed. We are
> at the early process stage and so there shouldn't be unrelated mappings
> (except for stack and loader) existing so mmap for a given address
> should succeed even without MAP_FIXED. Something is terribly wrong if
> this is not the case and we should rather fail than silently corrupt the
> underlying mapping.
>
> Address this issue by changing MAP_FIXED to the newly added
> MAP_FIXED_SAFE. This will mean that mmap will fail if there is an
> existing mapping clashing with the requested one without clobbering it.
>
> Cc: Abdul Haleem <abdhalee@xxxxxxxxxxxxxxxxxx>
> Cc: Joel Stanley <joel@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Once (if?) the name gets settled, this looks good to me:

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/metag/kernel/process.c |  6 +++++-
>  fs/binfmt_elf.c             | 12 ++++++++----
>  2 files changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/arch/metag/kernel/process.c b/arch/metag/kernel/process.c
> index c4606ce743d2..2286140e54e0 100644
> --- a/arch/metag/kernel/process.c
> +++ b/arch/metag/kernel/process.c
> @@ -398,7 +398,7 @@ unsigned long __metag_elf_map(struct file *filep, unsigned long addr,
>         tcm_tag = tcm_lookup_tag(addr);
>
>         if (tcm_tag != TCM_INVALID_TAG)
> -               type &= ~MAP_FIXED;
> +               type &= ~(MAP_FIXED | MAP_FIXED_SAFE);
>
>         /*
>         * total_size is the size of the ELF (interpreter) image.
> @@ -416,6 +416,10 @@ unsigned long __metag_elf_map(struct file *filep, unsigned long addr,
>         } else
>                 map_addr = vm_mmap(filep, addr, size, prot, type, off);
>
> +       if ((type & MAP_FIXED_SAFE) && BAD_ADDR(map_addr))
> +               pr_info("%d (%s): Uhuuh, elf segement at %p requested but the memory is mapped already\n",
> +                               task_pid_nr(current), tsk->comm, (void*)addr);
> +
>         if (!BAD_ADDR(map_addr) && tcm_tag != TCM_INVALID_TAG) {
>                 struct tcm_allocation *tcm;
>                 unsigned long tcm_addr;
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 6466153f2bf0..12b21942ccde 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -372,6 +372,10 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
>         } else
>                 map_addr = vm_mmap(filep, addr, size, prot, type, off);
>
> +       if ((type & MAP_FIXED_SAFE) && BAD_ADDR(map_addr))
> +               pr_info("%d (%s): Uhuuh, elf segement at %p requested but the memory is mapped already\n",
> +                               task_pid_nr(current), current->comm, (void*)addr);
> +
>         return(map_addr);
>  }
>
> @@ -569,7 +573,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>                                 elf_prot |= PROT_EXEC;
>                         vaddr = eppnt->p_vaddr;
>                         if (interp_elf_ex->e_type == ET_EXEC || load_addr_set)
> -                               elf_type |= MAP_FIXED;
> +                               elf_type |= MAP_FIXED_SAFE;
>                         else if (no_base && interp_elf_ex->e_type == ET_DYN)
>                                 load_addr = -vaddr;
>
> @@ -929,7 +933,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>                  * the ET_DYN load_addr calculations, proceed normally.
>                  */
>                 if (loc->elf_ex.e_type == ET_EXEC || load_addr_set) {
> -                       elf_flags |= MAP_FIXED;
> +                       elf_flags |= MAP_FIXED_SAFE;
>                 } else if (loc->elf_ex.e_type == ET_DYN) {
>                         /*
>                          * This logic is run once for the first LOAD Program
> @@ -965,7 +969,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>                                 load_bias = ELF_ET_DYN_BASE;
>                                 if (current->flags & PF_RANDOMIZE)
>                                         load_bias += arch_mmap_rnd();
> -                               elf_flags |= MAP_FIXED;
> +                               elf_flags |= MAP_FIXED_SAFE;
>                         } else
>                                 load_bias = 0;
>
> @@ -1220,7 +1224,7 @@ static int load_elf_library(struct file *file)
>                         (eppnt->p_filesz +
>                          ELF_PAGEOFFSET(eppnt->p_vaddr)),
>                         PROT_READ | PROT_WRITE | PROT_EXEC,
> -                       MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE,
> +                       MAP_FIXED_SAFE | MAP_PRIVATE | MAP_DENYWRITE,
>                         (eppnt->p_offset -
>                          ELF_PAGEOFFSET(eppnt->p_vaddr)));
>         if (error != ELF_PAGESTART(eppnt->p_vaddr))
> --
> 2.15.0
>



-- 
Kees Cook
Pixel Security