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Subject[PATCH v2] mm: Optional full ASLR for mmap() and mremap()
Date
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.

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

In this example, with value of 2, ld.so.cache, libc, an anonymous mmap
and locale-archive are located close to each other:
$ strace /bin/sync
...
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=189096, ...}) = 0
mmap(NULL, 189096, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7d9c1e7f2000
...
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0n\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1839792, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7d9c1e7f0000
mmap(NULL, 1852680, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7d9c1e62b000
...
openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=5642592, ...}) = 0
mmap(NULL, 5642592, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7d9c1e0c9000

With 3, they are located in unrelated addresses:
$ echo 3 > /proc/sys/kernel/randomize_va_space
$ /bin/sync
...
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=189096, ...}) = 0
mmap(NULL, 189096, PROT_READ, MAP_PRIVATE, 3, 0) = 0xeda4fbea000
...
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0n\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1839792, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb8fb9c1d000
mmap(NULL, 1852680, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xaabd8598000
...
openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=5642592, ...}) = 0
mmap(NULL, 5642592, PROT_READ, MAP_PRIVATE, 3, 0) = 0xbe351ab8000

Signed-off-by: Topi Miettinen <toiwoton@gmail.com>
---
v2: also randomize mremap(..., MREMAP_MAYMOVE)
---
Documentation/admin-guide/hw-vuln/spectre.rst | 6 +++---
Documentation/admin-guide/sysctl/kernel.rst | 11 +++++++++++
init/Kconfig | 2 +-
mm/mmap.c | 7 ++++++-
mm/mremap.c | 15 +++++++++++++++
5 files changed, 36 insertions(+), 5 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..acd0612155d9 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1060,6 +1060,17 @@ 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.
+
== ===========================================================================


diff --git a/init/Kconfig b/init/Kconfig
index d6a0b31b13dc..c5ea2e694f6a 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1859,7 +1859,7 @@ 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/mmap.c b/mm/mmap.c
index 40248d84ad5f..489368f43af1 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>
@@ -206,7 +207,7 @@ 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)
@@ -1407,6 +1408,10 @@ 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)
+ addr = arch_mmap_rnd();
+
/* Obtain the address to map to. we verify (or select) it and ensure
* that it represents a valid section of the address space.
*/
diff --git a/mm/mremap.c b/mm/mremap.c
index 138abbae4f75..c7fd1ab5fb5f 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -24,6 +24,7 @@
#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>
@@ -720,6 +721,20 @@ 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!
+ */
+ new_addr = arch_mmap_rnd();
+
+ ret = mremap_to(addr, old_len, new_addr, new_len,
+ &locked, flags, &uf, &uf_unmap_early,
+ &uf_unmap);
+ goto out;
+ }
+
+
/*
* Always allow a shrinking remap: that just unmaps
* the unnecessary pages..
--
2.28.0
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