Re: [PATCH 2/2] arm64: Allow changing of attributes outside of modules

From: Laura Abbott
Date: Thu Nov 05 2015 - 20:35:17 EST

On 11/04/2015 11:44 PM, Ard Biesheuvel wrote:
On 3 November 2015 at 22:48, Laura Abbott <labbott@xxxxxxxxxxxxxxxxx> wrote:

Currently, the set_memory_* functions that are implemented for arm64
are restricted to module addresses only. This was mostly done
because arm64 maps normal zone memory with larger page sizes to
improve TLB performance. This has the side effect though of making it
difficult to adjust attributes at the PAGE_SIZE granularity. There are
an increasing number of use cases related to security where it is
necessary to change the attributes of kernel memory. Add functionality
to the page attribute changing code under a Kconfig to let systems
designers decide if they want to make the trade off of security for TLB

Signed-off-by: Laura Abbott <labbott@xxxxxxxxxxxxxxxxx>
arch/arm64/Kconfig.debug | 11 +++++++
arch/arm64/mm/mm.h | 3 ++
arch/arm64/mm/mmu.c | 2 +-
arch/arm64/mm/pageattr.c | 74 ++++++++++++++++++++++++++++++++++++++++++++----
4 files changed, 84 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
index d6285ef..abc6922 100644
--- a/arch/arm64/Kconfig.debug
+++ b/arch/arm64/Kconfig.debug
@@ -89,6 +89,17 @@ config DEBUG_ALIGN_RODATA

If in doubt, say N


I don't think this belongs in Kconfig.debug, and I don't think this
should default to off.

We know we currently have no users of set_memory_xx() in arch/arm64
that operate on linear mapping addresses, so we will not introduce any
performance regressions by adding this functionality now. By putting
this feature behind a debug option, every newly introduced call
set_memory_xx() that operates on linear or vmalloc() addresses needs
to deal with -EINVAL (or depend on DEBUG_CHANGE_PAGEATTR), or it may
error out at runtime if the feature is not enabled.

I stuck it in Kconfig.debug to have it match with the rest of the
module and DEBUG_RODATA options. I'll pull it out.
+ bool "Allow all kernel memory to have attributes changed"
+ help
+ If this option is selected, APIs that change page attributes
+ (RW <-> RO, X <-> NX) will be valid for all memory mapped in
+ the kernel space. The trade off is that there may be increased
+ TLB pressure from finer grained page mapping. Turn on this option
+ if performance is more important than security

This is backwards

+ If in doubt, say N
source "drivers/hwtracing/coresight/Kconfig"



diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index e47ed1c..48a4ce9 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c


@@ -45,17 +108,18 @@ static int change_memory_common(unsigned long addr, int numpages,
int ret;
struct page_change_data data;

+ if (addr < PAGE_OFFSET && !is_vmalloc_addr((void *)addr))
+ return -EINVAL;

Doesn't this exclude the module area?

if (!IS_ALIGNED(addr, PAGE_SIZE)) {
start &= PAGE_MASK;
end = start + size;

- if (start < MODULES_VADDR || start >= MODULES_END)
- return -EINVAL;
- if (end < MODULES_VADDR || end >= MODULES_END)
- return -EINVAL;
+ ret = check_address(addr);
+ if (ret)
+ return ret;

data.set_mask = set_mask;
data.clear_mask = clear_mask;

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