From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: 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/ld-linux.so.2
/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: d1fd836dcf00 ("mm: split ET_DYN ASLR from mmap ASLR")
Link: http://lkml.kernel.org/r/20170621173201.GA114489@beast
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: Daniel Micay <danielmicay@xxxxxxxxx>
Cc: Qualys Security Advisory <qsa@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Grzegorz Andrejczuk <grzegorz.andrejczuk@xxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: James Hogan <james.hogan@xxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Pratyush Anand <panand@xxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/elf.h | 13 ++++---
fs/binfmt_elf.c | 59 ++++++++++++++++++++++++++++++-----
2 files changed, 58 insertions(+), 14 deletions(-)
diff -puN arch/x86/include/asm/elf.h~binfmt_elf-use-elf_et_dyn_base-only-for-pie arch/x86/include/asm/elf.h
--- a/arch/x86/include/asm/elf.h~binfmt_elf-use-elf_et_dyn_base-only-for-pie
+++ a/arch/x86/include/asm/elf.h
@@ -245,12 +245,13 @@ extern int force_personality32;
#define CORE_DUMP_USE_REGSET
#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 "./ld.so 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 : \
+ 0x100000000UL)
/* 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,
diff -puN fs/binfmt_elf.c~binfmt_elf-use-elf_et_dyn_base-only-for-pie fs/binfmt_elf.c
--- a/fs/binfmt_elf.c~binfmt_elf-use-elf_et_dyn_base-only-for-pie
+++ a/fs/binfmt_elf.c
@@ -927,17 +927,60 @@ static int load_elf_binary(struct linux_
elf_flags = MAP_PRIVATE | MAP_DENYWRITE | MAP_EXECUTABLE;
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
+ * "./ld.so 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,
loc->elf_ex.e_phnum);
if (!total_size) {
_
--
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