Commit eab09532 authored by Kees Cook's avatar Kees Cook Committed by Linus Torvalds
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binfmt_elf: use ELF_ET_DYN_BASE only for PIE

The ELF_ET_DYN_BASE position was originally intended to keep loaders
away from ET_EXEC binaries.  (For example, running "/lib/
/bin/cat" might cause the subsequent load of /bin/cat into where the
loader had been loaded.)

With the advent of PIE (ET_DYN binaries with an INTERP Program Header),
ELF_ET_DYN_BASE continued to be used since the kernel was only looking
at ET_DYN.  However, since ELF_ET_DYN_BASE is traditionally set at the
top 1/3rd of the TASK_SIZE, a substantial portion of the address space
is unused.

For 32-bit tasks when RLIMIT_STACK is set to RLIM_INFINITY, programs are
loaded above the mmap region.  This means they can be made to collide
(CVE-2017-1000370) or nearly collide (CVE-2017-1000371) with
pathological stack regions.

Lowering ELF_ET_DYN_BASE solves both by moving programs below the mmap
region in all cases, and will now additionally avoid programs falling
back to the mmap region by enforcing MAP_FIXED for program loads (i.e.
if it would have collided with the stack, now it will fail to load
instead of falling back to the mmap region).

To allow for a lower ELF_ET_DYN_BASE, loaders (ET_DYN without INTERP)
are loaded into the mmap region, leaving space available for either an
ET_EXEC binary with a fixed location or PIE being loaded into mmap by
the loader.  Only PIE programs are loaded offset from ELF_ET_DYN_BASE,
which means architectures can now safely lower their values without risk
of loaders colliding with their subsequently loaded programs.

For 64-bit, ELF_ET_DYN_BASE is best set to 4GB to allow runtimes to use
the entire 32-bit address space for 32-bit pointers.

Thanks to PaX Team, Daniel Micay, and Rik van Riel for inspiration and
suggestions on how to implement this solution.

Fixes: d1fd836d ("mm: split ET_DYN ASLR from mmap ASLR")

Signed-off-by: default avatarKees Cook <>
Acked-by: default avatarRik van Riel <>
Cc: Daniel Micay <>
Cc: Qualys Security Advisory <>
Cc: Thomas Gleixner <>
Cc: Ingo Molnar <>
Cc: "H. Peter Anvin" <>
Cc: Alexander Viro <>
Cc: Dmitry Safonov <>
Cc: Andy Lutomirski <>
Cc: Grzegorz Andrejczuk <>
Cc: Masahiro Yamada <>
Cc: Benjamin Herrenschmidt <>
Cc: Catalin Marinas <>
Cc: Heiko Carstens <>
Cc: James Hogan <>
Cc: Martin Schwidefsky <>
Cc: Michael Ellerman <>
Cc: Paul Mackerras <>
Cc: Pratyush Anand <>
Cc: Russell King <>
Cc: Will Deacon <>
Cc: <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent c257a340
......@@ -245,12 +245,13 @@ extern int force_personality32;
#define ELF_EXEC_PAGESIZE 4096
/* This is the location that an ET_DYN program is loaded if exec'ed. Typical
use of this is to invoke "./ someprog" to test out a new version of
the loader. We need to make sure that it is out of the way of the program
that it will "exec", and that there is sufficient room for the brk. */
#define ELF_ET_DYN_BASE (TASK_SIZE / 3 * 2)
* This is the base location for PIE (ET_DYN with INTERP) loads. On
* 64-bit, this is raised to 4GB to leave the entire 32-bit address
* space open for things that want to use the area for 32-bit pointers.
#define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x000400000UL : \
/* This yields a mask that user programs can use to figure out what
instruction set this CPU supports. This could be done in user space,
......@@ -927,17 +927,60 @@ static int load_elf_binary(struct linux_binprm *bprm)
vaddr = elf_ppnt->p_vaddr;
* If we are loading ET_EXEC or we have already performed
* the ET_DYN load_addr calculations, proceed normally.
if (loc->elf_ex.e_type == ET_EXEC || load_addr_set) {
elf_flags |= MAP_FIXED;
} else if (loc->elf_ex.e_type == ET_DYN) {
/* Try and get dynamic programs out of the way of the
* default mmap base, as well as whatever program they
* might try to exec. This is because the brk will
* follow the loader, and is not movable. */
load_bias = ELF_ET_DYN_BASE - vaddr;
if (current->flags & PF_RANDOMIZE)
load_bias += arch_mmap_rnd();
load_bias = ELF_PAGESTART(load_bias);
* This logic is run once for the first LOAD Program
* Header for ET_DYN binaries to calculate the
* randomization (load_bias) for all the LOAD
* Program Headers, and to calculate the entire
* size of the ELF mapping (total_size). (Note that
* load_addr_set is set to true later once the
* initial mapping is performed.)
* There are effectively two types of ET_DYN
* binaries: programs (i.e. PIE: ET_DYN with INTERP)
* and loaders (ET_DYN without INTERP, since they
* _are_ the ELF interpreter). The loaders must
* be loaded away from programs since the program
* may otherwise collide with the loader (especially
* for ET_EXEC which does not have a randomized
* position). For example to handle invocations of
* "./ someprog" to test out a new version of
* the loader, the subsequent program that the
* loader loads must avoid the loader itself, so
* they cannot share the same load range. Sufficient
* room for the brk must be allocated with the
* loader as well, since brk must be available with
* the loader.
* Therefore, programs are loaded offset from
* ELF_ET_DYN_BASE and loaders are loaded into the
* independently randomized mmap region (0 load_bias
* without MAP_FIXED).
if (elf_interpreter) {
load_bias = ELF_ET_DYN_BASE;
if (current->flags & PF_RANDOMIZE)
load_bias += arch_mmap_rnd();
elf_flags |= MAP_FIXED;
} else
load_bias = 0;
* Since load_bias is used for all subsequent loading
* calculations, we must lower it by the first vaddr
* so that the remaining calculations based on the
* ELF vaddrs will be correctly offset. The result
* is then page aligned.
load_bias = ELF_PAGESTART(load_bias - vaddr);
total_size = total_mapping_size(elf_phdata,
if (!total_size) {
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