On 11/29/2017 07:42 AM, Michal Hocko wrote:
From: Michal Hocko <mho...@suse.com>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 <abdha...@linux.vnet.ibm.com> Cc: Joel Stanley <j...@jms.id.au> Acked-by: Kees Cook <keesc...@chromium.org> Signed-off-by: Michal Hocko <mho...@suse.com> ---
Reviewed-by: Khalid Aziz <khalid.a...@oracle.com>
