[PATCH v3] drivers/char: kmem: disable on arm64

From: Ard Biesheuvel
Date: Tue Jun 20 2017 - 02:59:10 EST

As it turns out, arm64 deviates from other architectures in the way it
maps the VMALLOC region: on most (all?) other architectures, it resides
strictly above the kernel's direct mapping of DRAM, but on arm64, this
is the other way around. For instance, for a 48-bit VA configuration,
we have

modules : 0xffff000000000000 - 0xffff000008000000 ( 128 MB)
vmalloc : 0xffff000008000000 - 0xffff7dffbfff0000 (129022 GB)
vmemmap : 0xffff7e0000000000 - 0xffff800000000000 ( 2048 GB maximum)
0xffff7e0000000000 - 0xffff7e0003ff0000 ( 63 MB actual)
memory : 0xffff800000000000 - 0xffff8000ffc00000 ( 4092 MB)

This has mostly gone unnoticed until now, but it does appear that it
breaks an assumption in the kcore read/write code, which does something

if (p < (unsigned long) high_memory) {
... use straight copy_[to|from]_user() using p as virtual address ...
if (count > 0) {
... use vread/vwrite for accesses past high_memory ...

The first condition will inadvertently hold for the VMALLOC region if
VMALLOC_START < PAGE_OFFSET [which is the case on arm64], but the read
or write will subsequently fail the virt_addr_valid() check, resulting
in a -ENXIO return value.

Given how kmem seems to be living in borrowed time anyway, and given
the fact that nobody noticed that the read/write interface is broken
on arm64 in the first place, let's not bother trying to fix it, but
simply disable the /dev/kmem interface entirely for arm64.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
v3: improve commit log
v2: disable /dev/kmem entirely rather than bandaiding it

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
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