ARM: vdso: Mark vDSO code as read-only

Message ID 20160216213659.GA47194@davidb.org
State New
Headers show

Commit Message

David Brown Feb. 16, 2016, 9:36 p.m.
Although the arm vDSO is cleanly separated by code/data with the code
being read-only in userspace mappings, the code page is still writable
from the kernel.  There have been exploits (such as
http://itszn.com/blog/?p=21) that take advantage of this on x86 to go
from a bad kernel write to full root.

Prevent this specific exploit on arm by putting the vDSO code page in
post-init read-only memory as well.

Before:
vdso: 1 text pages at base 80927000
root@Vexpress:/ cat /sys/kernel/debug/kernel_page_tables
---[ Modules ]---
---[ Kernel Mapping ]---
0x80000000-0x80100000           1M     RW NX SHD
0x80100000-0x80600000           5M     ro x  SHD
0x80600000-0x80800000           2M     ro NX SHD
0x80800000-0xbe000000         984M     RW NX SHD

After:
vdso: 1 text pages at base 8072b000
root@Vexpress:/ cat /sys/kernel/debug/kernel_page_tables
---[ Modules ]---
---[ Kernel Mapping ]---
0x80000000-0x80100000           1M     RW NX SHD
0x80100000-0x80600000           5M     ro x  SHD
0x80600000-0x80800000           2M     ro NX SHD
0x80800000-0xbe000000         984M     RW NX SHD

Inspired by https://lkml.org/lkml/2016/1/19/494 based on work by the
PaX Team, Brad Spengler, and Kees Cook.

Signed-off-by: David Brown <david.brown@linaro.org>

---
This patch depends on Kees Cook's series
https://lkml.org/lkml/2016/1/19/497 which adds the ro_after_init
section.

 arch/arm/vdso/vdso.S | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

-- 
2.7.1

Comments

David Brown Feb. 17, 2016, 5:20 a.m. | #1
On Tue, Feb 16, 2016 at 01:52:33PM -0800, Kees Cook wrote:
>On Tue, Feb 16, 2016 at 1:36 PM, David Brown <david.brown@linaro.org> wrote:

>> Although the arm vDSO is cleanly separated by code/data with the code

>> being read-only in userspace mappings, the code page is still writable

>> from the kernel.  There have been exploits (such as

>> http://itszn.com/blog/?p=21) that take advantage of this on x86 to go

>> from a bad kernel write to full root.

>>

>> Prevent this specific exploit on arm by putting the vDSO code page in

>> post-init read-only memory as well.

>

>Is the vdso dynamically built at init time like on x86, or can this

>just use .rodata directly?


On ARM, it is patched during init.  Arm64's is just plain read-only.

David
David Brown Feb. 17, 2016, 11:43 p.m. | #2
On Wed, Feb 17, 2016 at 03:00:52PM -0800, Kees Cook wrote:
>On Tue, Feb 16, 2016 at 9:20 PM, David Brown <david.brown@linaro.org> wrote:

>> On Tue, Feb 16, 2016 at 01:52:33PM -0800, Kees Cook wrote:

>>>

>>> On Tue, Feb 16, 2016 at 1:36 PM, David Brown <david.brown@linaro.org>

>>> wrote:

>>>>

>>>> Although the arm vDSO is cleanly separated by code/data with the code

>>>> being read-only in userspace mappings, the code page is still writable

>>>> from the kernel.  There have been exploits (such as

>>>> http://itszn.com/blog/?p=21) that take advantage of this on x86 to go

>>>> from a bad kernel write to full root.

>>>>

>>>> Prevent this specific exploit on arm by putting the vDSO code page in

>>>> post-init read-only memory as well.

>>>

>>>

>>> Is the vdso dynamically built at init time like on x86, or can this

>>> just use .rodata directly?

>>

>>

>> On ARM, it is patched during init.  Arm64's is just plain read-only.

>

>Okay, great. I've added this to my postinit-readonly series (which I

>just refreshed and sent out again...)


However, this distinction between .rodata and .data..ro_after_init is
kind of fuzzy, anyway, since they both get made actually read-only at
the same time (post init).  The patch actually does work fine with the
vDSO page in .rodata, since the patching happens during init.

Is there a possible future consideration to perhaps make .rodata read
only much earlier?

David

Patch

diff --git a/arch/arm/vdso/vdso.S b/arch/arm/vdso/vdso.S
index b2b97e3..a62a7b6 100644
--- a/arch/arm/vdso/vdso.S
+++ b/arch/arm/vdso/vdso.S
@@ -23,9 +23,8 @@ 
 #include <linux/const.h>
 #include <asm/page.h>
 
-	__PAGE_ALIGNED_DATA
-
 	.globl vdso_start, vdso_end
+	.section .data..ro_after_init
 	.balign PAGE_SIZE
 vdso_start:
 	.incbin "arch/arm/vdso/vdso.so"