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@chromium.org>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Qualys Security Advisory <qsa@qualys.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Grzegorz Andrejczuk <grzegorz.andrejczuk@intel.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pratyush Anand <panand@redhat.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index e8ab9a46bc6890f802cbdc5bef85f4f16b221183..1c18d83d3f094d1f5abfa09a08b67e9cb6b7376d 100644 (file)
--- a/arch/x86/include/asm/elf.h
+++ b/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 --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 5075fd5c62c86d93b30cb413f791d1810157b57c..7465c3ea5dd50f2b991a9d8db47e91654d1717a1 100644 (file)
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -927,17 +927,60 @@ static int load_elf_binary(struct linux_binprm *bprm)
                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) {