From 4ed28639519c7bad5f518e70b3284c6e0763e650 Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Tue, 10 Apr 2018 16:36:01 -0700 Subject: fs, elf: drop MAP_FIXED usage from elf_map 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 commit 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_NOREPLACE. This will mean that mmap will fail if there is an existing mapping clashing with the requested one without clobbering it. [mhocko@suse.com: fix build] [akpm@linux-foundation.org: coding-style fixes] [avagin@openvz.org: don't use the same value for MAP_FIXED_NOREPLACE and MAP_SYNC] Link: http://lkml.kernel.org/r/20171218184916.24445-1-avagin@openvz.org Link: http://lkml.kernel.org/r/20171213092550.2774-3-mhocko@kernel.org Signed-off-by: Michal Hocko Signed-off-by: Andrei Vagin Signed-off-by: Michal Hocko Reviewed-by: Khalid Aziz Acked-by: Michael Ellerman Acked-by: Kees Cook Cc: Abdul Haleem Cc: Joel Stanley Cc: Anshuman Khandual Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/uapi/asm-generic/mman-common.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'include/uapi/asm-generic') diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h index 4aa65a8d7e92..e7ee32861d51 100644 --- a/include/uapi/asm-generic/mman-common.h +++ b/include/uapi/asm-generic/mman-common.h @@ -26,7 +26,9 @@ #else # define MAP_UNINITIALIZED 0x0 /* Don't support this flag */ #endif -#define MAP_FIXED_NOREPLACE 0x80000 /* MAP_FIXED which doesn't unmap underlying mapping */ + +/* 0x0100 - 0x80000 flags are defined in asm-generic/mman.h */ +#define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */ /* * Flags for mlock -- cgit v1.2.3