[PATCH] MIPS: Add support for ARCH_MMAP_RND_{COMPAT_}BITS

arch_mmap_rnd() uses hard-coded limits of 16MB for the randomisation
of mmap within 32bit processes and 256MB in 64bit processes. Since v4.4
other arches support tuning this value in /proc/sys/vm/mmap_rnd_bits.
Add support for this to MIPS.

Set the minimum(default) number of bits randomisation for 32bit to 8 -
which with 4k pagesize is unchanged from the current 16MB total
randomness. The minimum(default) for 64bit is 12bits, again with 4k
pagesize this is the same as the current 256MB.

This patch is necessary for MIPS32 to pass the Android CTS tests, with
the number of random bits set to 15.

Signed-off-by: Matt Redfearn <redacted>
---

 arch/mips/Kconfig   | 16 ++++++++++++++++
 arch/mips/mm/mmap.c | 10 +++++-----
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index b3c5bde43d34..d72cf6129b2c 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig

@@ -13,6 +13,8 @@ config MIPS
 	select HAVE_PERF_EVENTS
 	select PERF_USE_VMALLOC
 	select HAVE_ARCH_KGDB
+	select HAVE_ARCH_MMAP_RND_BITS if MMU
+	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_CBPF_JIT if !CPU_MICROMIPS

@@ -3073,6 +3075,20 @@ config MMU
 	bool
 	default y
 
+config ARCH_MMAP_RND_BITS_MIN
+	default 12 if 64BIT
+	default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+	default 18 if 64BIT
+	default 15
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+       default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+       default 15
+
 config I8253
 	bool
 	select CLKSRC_I8253

diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
index d08ea3ff0f53..d6d92c02308d 100644
--- a/arch/mips/mm/mmap.c
+++ b/arch/mips/mm/mmap.c

@@ -146,14 +146,14 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-	rnd = get_random_long();
-	rnd <<= PAGE_SHIFT;
+#ifdef CONFIG_COMPAT
 	if (TASK_IS_32BIT_ADDR)
-		rnd &= 0xfffffful;
+		rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
 	else
-		rnd &= 0xffffffful;
+#endif /* CONFIG_COMPAT */
+		rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
 
-	return rnd;
+	return rnd << PAGE_SHIFT;
 }
 
 void arch_pick_mmap_layout(struct mm_struct *mm)

-- 
2.7.4

On Thu, Nov 24, 2016 at 9:32 AM, Matt Redfearn [off-list ref] wrote:

Excellent!

Reviewed-by: Kees Cook <redacted>

Out of curiosity, how were the maxs of 15 and 18 chosen?

-Kees

-- 
Kees Cook
Nexus Security

Hi Kees


On 25/11/16 20:00, Kees Cook wrote:

On Thu, Nov 24, 2016 at 9:32 AM, Matt Redfearn [off-list ref] wrote:

quoted

arch_mmap_rnd() uses hard-coded limits of 16MB for the randomisation
of mmap within 32bit processes and 256MB in 64bit processes. Since v4.4
other arches support tuning this value in /proc/sys/vm/mmap_rnd_bits.
Add support for this to MIPS.

Set the minimum(default) number of bits randomisation for 32bit to 8 -
which with 4k pagesize is unchanged from the current 16MB total
randomness. The minimum(default) for 64bit is 12bits, again with 4k
pagesize this is the same as the current 256MB.

This patch is necessary for MIPS32 to pass the Android CTS tests, with
the number of random bits set to 15.

Signed-off-by: Matt Redfearn <redacted>
---

  arch/mips/Kconfig   | 16 ++++++++++++++++
  arch/mips/mm/mmap.c | 10 +++++-----
  2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index b3c5bde43d34..d72cf6129b2c 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig

@@ -13,6 +13,8 @@ config MIPS
         select HAVE_PERF_EVENTS
         select PERF_USE_VMALLOC
         select HAVE_ARCH_KGDB
+       select HAVE_ARCH_MMAP_RND_BITS if MMU
+       select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT
         select HAVE_ARCH_SECCOMP_FILTER
         select HAVE_ARCH_TRACEHOOK
         select HAVE_CBPF_JIT if !CPU_MICROMIPS

@@ -3073,6 +3075,20 @@ config MMU
         bool
         default y

+config ARCH_MMAP_RND_BITS_MIN
+       default 12 if 64BIT
+       default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+       default 18 if 64BIT
+       default 15
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+       default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+       default 15
+
  config I8253
         bool
         select CLKSRC_I8253

diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
index d08ea3ff0f53..d6d92c02308d 100644
--- a/arch/mips/mm/mmap.c
+++ b/arch/mips/mm/mmap.c

@@ -146,14 +146,14 @@ unsigned long arch_mmap_rnd(void)
  {
         unsigned long rnd;

-       rnd = get_random_long();
-       rnd <<= PAGE_SHIFT;
+#ifdef CONFIG_COMPAT
         if (TASK_IS_32BIT_ADDR)
-               rnd &= 0xfffffful;
+               rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
         else
-               rnd &= 0xffffffful;
+#endif /* CONFIG_COMPAT */
+               rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);

-       return rnd;
+       return rnd << PAGE_SHIFT;
  }

  void arch_pick_mmap_layout(struct mm_struct *mm)
--

2.7.4

Excellent!

Reviewed-by: Kees Cook <redacted>

Out of curiosity, how were the maxs of 15 and 18 chosen?

The maximum of 15 bits for 32 bit processes was the minimum required to 
pass the Android CTS tests with Nougat using 4k pages, but with 64k 
pages would be on the limit of the virtual address space. For 64 bit 
processes, I just allowed an 3 extra bits of randomness within the 
larger virtual address space, similar to the ARM64 limit.

Thanks,
Matt

-Kees

On Mon, Nov 28, 2016 at 8:10 AM, Matt Redfearn [off-list ref] wrote:

Hi Kees



On 25/11/16 20:00, Kees Cook wrote:

quoted

On Thu, Nov 24, 2016 at 9:32 AM, Matt Redfearn [off-list ref]
wrote:

quoted

arch_mmap_rnd() uses hard-coded limits of 16MB for the randomisation
of mmap within 32bit processes and 256MB in 64bit processes. Since v4.4
other arches support tuning this value in /proc/sys/vm/mmap_rnd_bits.
Add support for this to MIPS.

Set the minimum(default) number of bits randomisation for 32bit to 8 -
which with 4k pagesize is unchanged from the current 16MB total
randomness. The minimum(default) for 64bit is 12bits, again with 4k
pagesize this is the same as the current 256MB.

This patch is necessary for MIPS32 to pass the Android CTS tests, with
the number of random bits set to 15.

Signed-off-by: Matt Redfearn <redacted>
---

  arch/mips/Kconfig   | 16 ++++++++++++++++
  arch/mips/mm/mmap.c | 10 +++++-----
  2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index b3c5bde43d34..d72cf6129b2c 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig

@@ -13,6 +13,8 @@ config MIPS
         select HAVE_PERF_EVENTS
         select PERF_USE_VMALLOC
         select HAVE_ARCH_KGDB
+       select HAVE_ARCH_MMAP_RND_BITS if MMU
+       select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT
         select HAVE_ARCH_SECCOMP_FILTER
         select HAVE_ARCH_TRACEHOOK
         select HAVE_CBPF_JIT if !CPU_MICROMIPS

@@ -3073,6 +3075,20 @@ config MMU
         bool
         default y

+config ARCH_MMAP_RND_BITS_MIN
+       default 12 if 64BIT
+       default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+       default 18 if 64BIT
+       default 15
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+       default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+       default 15
+
  config I8253
         bool
         select CLKSRC_I8253

diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
index d08ea3ff0f53..d6d92c02308d 100644
--- a/arch/mips/mm/mmap.c
+++ b/arch/mips/mm/mmap.c

@@ -146,14 +146,14 @@ unsigned long arch_mmap_rnd(void)
  {
         unsigned long rnd;

-       rnd = get_random_long();
-       rnd <<= PAGE_SHIFT;
+#ifdef CONFIG_COMPAT
         if (TASK_IS_32BIT_ADDR)
-               rnd &= 0xfffffful;
+               rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits)
- 1);
         else
-               rnd &= 0xffffffful;
+#endif /* CONFIG_COMPAT */
+               rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);

-       return rnd;
+       return rnd << PAGE_SHIFT;
  }

  void arch_pick_mmap_layout(struct mm_struct *mm)
--

2.7.4

Excellent!

Reviewed-by: Kees Cook <redacted>

Out of curiosity, how were the maxs of 15 and 18 chosen?

The maximum of 15 bits for 32 bit processes was the minimum required to pass
the Android CTS tests with Nougat using 4k pages, but with 64k pages would
be on the limit of the virtual address space. For 64 bit processes, I just
allowed an 3 extra bits of randomness within the larger virtual address
space, similar to the ARM64 limit.

arm64's limit depends on the VA size. What does MIPS's 64-bit VA space
look like? e.g. by default on arm64 with 4k pages, the VA space is 39
bits, which gives 24 bits of entropy (rather than 18 here, which seems
low to me for a 64-bit userspace).

-Kees

-- 
Kees Cook
Nexus Security

As can be seen from the following comment in arch/arm64/Kconfig:
""
# max bits determined by the following formula:
#  VA_BITS - PAGE_SHIFT - 3
config ARCH_MMAP_RND_BITS_MAX
"""

The max value was determined based on the available address space 
(VA_BITS - PAGE_SHIFT) and an account of how far mmap_base could be 
moved (the "- 3" part).  MIPS uses the same calculation for the MAX_GAP 
in arch/mips/mm/mmap.c, namely 5/6 the TASK_SIZE, so I'd expect a 
similar formula for MIPS from a cursory first-glance.

These values likely will stick, so it'd be best to get them to be as 
large as possible from the get-go.  I imagine something similar to ARM64 
would be set up based on the following in 
arch/mips/include/asm/processor.h, but of course defer to the mips 
maintainer:

"""
#ifdef CONFIG_MIPS_VA_BITS_48
#define TASK_SIZE64     (0x1UL << 
((cpu_data[0].vmbits>48)?48:cpu_data[0].vmbits))
#else
#define TASK_SIZE64     0x10000000000UL
#endif
"""

Thanks,
Dan

On 11/28/16 10:17 AM, Kees Cook wrote:

On Mon, Nov 28, 2016 at 8:10 AM, Matt Redfearn [off-list ref] wrote:

quoted

Hi Kees



On 25/11/16 20:00, Kees Cook wrote:

quoted

On Thu, Nov 24, 2016 at 9:32 AM, Matt Redfearn [off-list ref]
wrote:

quoted

arch_mmap_rnd() uses hard-coded limits of 16MB for the randomisation
of mmap within 32bit processes and 256MB in 64bit processes. Since v4.4
other arches support tuning this value in /proc/sys/vm/mmap_rnd_bits.
Add support for this to MIPS.

Set the minimum(default) number of bits randomisation for 32bit to 8 -
which with 4k pagesize is unchanged from the current 16MB total
randomness. The minimum(default) for 64bit is 12bits, again with 4k
pagesize this is the same as the current 256MB.

This patch is necessary for MIPS32 to pass the Android CTS tests, with
the number of random bits set to 15.

Signed-off-by: Matt Redfearn <redacted>
---

  arch/mips/Kconfig   | 16 ++++++++++++++++
  arch/mips/mm/mmap.c | 10 +++++-----
  2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index b3c5bde43d34..d72cf6129b2c 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig

@@ -13,6 +13,8 @@ config MIPS
         select HAVE_PERF_EVENTS
         select PERF_USE_VMALLOC
         select HAVE_ARCH_KGDB
+       select HAVE_ARCH_MMAP_RND_BITS if MMU
+       select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT
         select HAVE_ARCH_SECCOMP_FILTER
         select HAVE_ARCH_TRACEHOOK
         select HAVE_CBPF_JIT if !CPU_MICROMIPS

@@ -3073,6 +3075,20 @@ config MMU
         bool
         default y

+config ARCH_MMAP_RND_BITS_MIN
+       default 12 if 64BIT
+       default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+       default 18 if 64BIT
+       default 15
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+       default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+       default 15
+
  config I8253
         bool
         select CLKSRC_I8253

diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
index d08ea3ff0f53..d6d92c02308d 100644
--- a/arch/mips/mm/mmap.c
+++ b/arch/mips/mm/mmap.c

@@ -146,14 +146,14 @@ unsigned long arch_mmap_rnd(void)
  {
         unsigned long rnd;

-       rnd = get_random_long();
-       rnd <<= PAGE_SHIFT;
+#ifdef CONFIG_COMPAT
         if (TASK_IS_32BIT_ADDR)
-               rnd &= 0xfffffful;
+               rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits)
- 1);
         else
-               rnd &= 0xffffffful;
+#endif /* CONFIG_COMPAT */
+               rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);

-       return rnd;
+       return rnd << PAGE_SHIFT;
  }

  void arch_pick_mmap_layout(struct mm_struct *mm)
--

2.7.4

Excellent!

Reviewed-by: Kees Cook <redacted>

Out of curiosity, how were the maxs of 15 and 18 chosen?

The maximum of 15 bits for 32 bit processes was the minimum required to pass
the Android CTS tests with Nougat using 4k pages, but with 64k pages would
be on the limit of the virtual address space. For 64 bit processes, I just
allowed an 3 extra bits of randomness within the larger virtual address
space, similar to the ARM64 limit.

arm64's limit depends on the VA size. What does MIPS's 64-bit VA space
look like? e.g. by default on arm64 with 4k pages, the VA space is 39
bits, which gives 24 bits of entropy (rather than 18 here, which seems
low to me for a 64-bit userspace).

-Kees