[PATCH v2] drivers/kmem: disable on arm64

[Ard Biesheuvel]

On arm64, the /dev/kmem driver barely works, given that it assumes that
VMALLOC_START > PAGE_OFFSET, which is not the case on arm64. Due to the
dubious value of kmem on an architecture such as arm64, which is fairly
new and is not as forgiving as other architectures when it comes to
userland accesses to kernel memory using mismatched attributes*, let's
disable it altogether if CONFIG_ARM64=y.

* Mappings in the vmalloc region will be accessed using vread/vwrite when
  reading/writing to/from /dev/kmem, which will perform these accesses by
  kmap()'ing each page using writeback cacheable attributes. This will be
  correct in most cases, but this is not guaranteed.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
 drivers/char/Kconfig | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 31adbebf812e..8102ee7b3247 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -17,6 +17,8 @@ config DEVMEM
 
 config DEVKMEM
 	bool "/dev/kmem virtual device support"
+	# On arm64, VMALLOC_START < PAGE_OFFSET, which confuses kmem read/write
+	depends on !ARM64
 	help
 	  Say Y here if you want to support the /dev/kmem device. The
 	  /dev/kmem device is rarely used, but can be used for certain
-- 
2.7.4