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SubjectRe: [PATCH v2] mm: Optional full ASLR for mmap() and mremap()
On Fri, Oct 02, 2020 at 02:09:17PM +0300, Topi Miettinen wrote:
> 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

Nit: this can be better illustrated with /proc/$pid/maps

> 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
>
>

--
Sincerely yours,
Mike.

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