[PATCH v4] mm: Optional full ASLR for mmap() and mremap()

From: Topi Miettinen
Date: Mon Oct 26 2020 - 12:06:08 EST

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Writing a new value of 3 to /proc/sys/kernel/randomize_va_space
enables full randomization of memory mappings created with mmap(NULL,
...). With 2, the base of the VMA used for such mappings is random,
but the mappings are created in predictable places within the VMA and
in sequential order. With 3, new VMAs are created to fully randomize
the mappings. Also mremap(..., MREMAP_MAYMOVE) will move the mappings
even if not necessary.

The method is to randomize the new address without considering
VMAs. If the address fails checks because of overlap with the stack
area (or in case of mremap(), overlap with the old mapping), the
operation is retried a few times before falling back to old method.

On 32 bit systems this may cause problems due to increased VM
fragmentation if the address space gets crowded.

On all systems, it will reduce performance and increase memory
usage due to less efficient use of page tables and inability to
merge adjacent VMAs with compatible attributes.

In this example with value of 2, dynamic loader, libc, anonymous
memory reserved with mmap() and locale-archive are located close to
each other:

$ cat /proc/self/maps (only first line for each object shown for brevity)
58c1175b1000-58c1175b3000 r--p 00000000 fe:0c 1868624 /usr/bin/cat
79752ec17000-79752f179000 r--p 00000000 fe:0c 2473999 /usr/lib/locale/locale-archive
79752f179000-79752f279000 rw-p 00000000 00:00 0
79752f279000-79752f29e000 r--p 00000000 fe:0c 2402415 /usr/lib/x86_64-linux-gnu/libc-2.31.so
79752f43a000-79752f440000 rw-p 00000000 00:00 0
79752f46f000-79752f470000 r--p 00000000 fe:0c 2400484 /usr/lib/x86_64-linux-gnu/ld-2.31.so
79752f49b000-79752f49c000 rw-p 00000000 00:00 0
7ffdcad9e000-7ffdcadbf000 rw-p 00000000 00:00 0 [stack]
7ffdcadd2000-7ffdcadd6000 r--p 00000000 00:00 0 [vvar]
7ffdcadd6000-7ffdcadd8000 r-xp 00000000 00:00 0 [vdso]

With 3, they are located at unrelated addresses:
$ echo 3 > /proc/sys/kernel/randomize_va_space
$ cat /proc/self/maps (only first line for each object shown for brevity)
1206a8fa000-1206a8fb000 r--p 00000000 fe:0c 2400484 /usr/lib/x86_64-linux-gnu/ld-2.31.so
1206a926000-1206a927000 rw-p 00000000 00:00 0
19174173000-19174175000 rw-p 00000000 00:00 0
ac82f419000-ac82f519000 rw-p 00000000 00:00 0
afa66a42000-afa66fa4000 r--p 00000000 fe:0c 2473999 /usr/lib/locale/locale-archive
d8656ba9000-d8656bce000 r--p 00000000 fe:0c 2402415 /usr/lib/x86_64-linux-gnu/libc-2.31.so
d8656d6a000-d8656d6e000 rw-p 00000000 00:00 0
5df90b712000-5df90b714000 r--p 00000000 fe:0c 1868624 /usr/bin/cat
7ffe1be4c000-7ffe1be6d000 rw-p 00000000 00:00 0 [stack]
7ffe1bf07000-7ffe1bf0b000 r--p 00000000 00:00 0 [vvar]
7ffe1bf0b000-7ffe1bf0d000 r-xp 00000000 00:00 0 [vdso]

CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Jann Horn <jannh@xxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: Matthew Wilcox <willy@xxxxxxxxxxxxx>
CC: Mike Rapoport <rppt@xxxxxxxxxx>
Signed-off-by: Topi Miettinen <toiwoton@xxxxxxxxx>
---
v2: also randomize mremap(..., MREMAP_MAYMOVE)
v3: avoid stack area and retry in case of bad random address (Jann
Horn), improve description in kernel.rst (Matthew Wilcox)
v4: use /proc/$pid/maps in the example (Mike Rapaport), CCs (Andrew
Morton), only check randomize_va_space == 3
---
Documentation/admin-guide/hw-vuln/spectre.rst | 6 ++--
Documentation/admin-guide/sysctl/kernel.rst | 15 ++++++++++
init/Kconfig | 2 +-
mm/internal.h | 8 +++++
mm/mmap.c | 30 +++++++++++++------
mm/mremap.c | 27 +++++++++++++++++
6 files changed, 75 insertions(+), 13 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index e05e581af5cf..9ea250522077 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -254,7 +254,7 @@ Spectre variant 2
left by the previous process will also be cleared.

User programs should use address space randomization to make attacks
- more difficult (Set /proc/sys/kernel/randomize_va_space = 1 or 2).
+ more difficult (Set /proc/sys/kernel/randomize_va_space = 1, 2 or 3).

3. A virtualized guest attacking the host
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -499,8 +499,8 @@ Spectre variant 2
more overhead and run slower.

User programs should use address space randomization
- (/proc/sys/kernel/randomize_va_space = 1 or 2) to make attacks more
- difficult.
+ (/proc/sys/kernel/randomize_va_space = 1, 2 or 3) to make attacks
+ more difficult.

3. VM mitigation
^^^^^^^^^^^^^^^^
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index d4b32cc32bb7..bc3bb74d544d 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1060,6 +1060,21 @@ that support this feature.
Systems with ancient and/or broken binaries should be configured
with ``CONFIG_COMPAT_BRK`` enabled, which excludes the heap from process
address space randomization.
+
+3 Additionally enable full randomization of memory mappings created
+ with mmap(NULL, ...). With 2, the base of the VMA used for such
+ mappings is random, but the mappings are created in predictable
+ places within the VMA and in sequential order. With 3, new VMAs
+ are created to fully randomize the mappings. Also mremap(...,
+ MREMAP_MAYMOVE) will move the mappings even if not necessary.
+
+ On 32 bit systems this may cause problems due to increased VM
+ fragmentation if the address space gets crowded.
+
+ On all systems, it will reduce performance and increase memory
+ usage due to less efficient use of page tables and inability to
+ merge adjacent VMAs with compatible attributes.
+
== ===========================================================================

diff --git a/init/Kconfig b/init/Kconfig
index c9446911cf41..6146e2cd3b77 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1863,7 +1863,7 @@ config COMPAT_BRK
also breaks ancient binaries (including anything libc5 based).
This option changes the bootup default to heap randomization
disabled, and can be overridden at runtime by setting
- /proc/sys/kernel/randomize_va_space to 2.
+ /proc/sys/kernel/randomize_va_space to 2 or 3.

On non-ancient distros (post-2000 ones) N is usually a safe choice.

diff --git a/mm/internal.h b/mm/internal.h
index c43ccdddb0f6..b964c8dbb242 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -618,4 +618,12 @@ struct migration_target_control {
gfp_t gfp_mask;
};

+#ifndef arch_get_mmap_end
+#define arch_get_mmap_end(addr) (TASK_SIZE)
+#endif
+
+#ifndef arch_get_mmap_base
+#define arch_get_mmap_base(addr, base) (base)
+#endif
+
#endif /* __MM_INTERNAL_H */
diff --git a/mm/mmap.c b/mm/mmap.c
index d91ecb00d38c..3677491e999b 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -47,6 +47,7 @@
#include <linux/pkeys.h>
#include <linux/oom.h>
#include <linux/sched/mm.h>
+#include <linux/elf-randomize.h>

#include <linux/uaccess.h>
#include <asm/cacheflush.h>
@@ -73,6 +74,8 @@ const int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
int mmap_rnd_compat_bits __read_mostly = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
#endif

+#define MAX_RANDOM_MMAP_RETRIES 5
+
static bool ignore_rlimit_data;
core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);

@@ -206,7 +209,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
#ifdef CONFIG_COMPAT_BRK
/*
* CONFIG_COMPAT_BRK can still be overridden by setting
- * randomize_va_space to 2, which will still cause mm->start_brk
+ * randomize_va_space to >= 2, which will still cause mm->start_brk
* to be arbitrarily shifted
*/
if (current->brk_randomized)
@@ -1445,6 +1448,23 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
if (mm->map_count > sysctl_max_map_count)
return -ENOMEM;

+ /* Pick a random address even outside current VMAs? */
+ if (!addr && randomize_va_space == 3) {
+ int i = MAX_RANDOM_MMAP_RETRIES;
+ unsigned long max_addr = arch_get_mmap_base(addr, mm->mmap_base);
+
+ do {
+ /* Try a few times to find a free area */
+ addr = arch_mmap_rnd();
+ if (addr >= max_addr)
+ continue;
+ addr = get_unmapped_area(file, addr, len, pgoff, flags);
+ } while (--i >= 0 && !IS_ERR_VALUE(addr));
+
+ if (IS_ERR_VALUE(addr))
+ addr = 0;
+ }
+
/* Obtain the address to map to. we verify (or select) it and ensure
* that it represents a valid section of the address space.
*/
@@ -2142,14 +2162,6 @@ unsigned long vm_unmapped_area(struct vm_unmapped_area_info *info)
return addr;
}

-#ifndef arch_get_mmap_end
-#define arch_get_mmap_end(addr) (TASK_SIZE)
-#endif
-
-#ifndef arch_get_mmap_base
-#define arch_get_mmap_base(addr, base) (base)
-#endif
-
/* Get an address range which is currently unmapped.
* For shmat() with addr=0.
*
diff --git a/mm/mremap.c b/mm/mremap.c
index 138abbae4f75..c5b2ed2bfd2d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -24,12 +24,15 @@
#include <linux/uaccess.h>
#include <linux/mm-arch-hooks.h>
#include <linux/userfaultfd_k.h>
+#include <linux/elf-randomize.h>

#include <asm/cacheflush.h>
#include <asm/tlbflush.h>

#include "internal.h"

+#define MAX_RANDOM_MREMAP_RETRIES 5
+
static pmd_t *get_old_pmd(struct mm_struct *mm, unsigned long addr)
{
pgd_t *pgd;
@@ -720,6 +723,30 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
goto out;
}

+ if ((flags & MREMAP_MAYMOVE) && randomize_va_space == 3) {
+ /*
+ * Caller is happy with a different address, so let's
+ * move even if not necessary!
+ */
+ int i = MAX_RANDOM_MREMAP_RETRIES;
+ unsigned long max_addr = arch_get_mmap_base(addr, mm->mmap_base);
+
+ do {
+ /* Try a few times to find a free area */
+ new_addr = arch_mmap_rnd();
+ if (new_addr >= max_addr)
+ continue;
+ ret = mremap_to(addr, old_len, new_addr, new_len,
+ &locked, flags, &uf, &uf_unmap_early,
+ &uf_unmap);
+ if (!IS_ERR_VALUE(ret))
+ goto out;
+ } while (--i >= 0);
+
+ /* Give up and try the old address */
+ new_addr = addr;
+ }
+
/*
* Always allow a shrinking remap: that just unmaps
* the unnecessary pages..

base-commit: 3650b228f83adda7e5ee532e2b90429c03f7b9ec
--
2.28.0

Next message: Arnd Bergmann: "[PATCH 1/2] libfc: move scsi/fc_encode.h to libfc"
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