diff options
| author | Ard Biesheuvel | 2017-06-20 08:59:00 +0200 |
|---|---|---|
| committer | Will Deacon | 2017-06-20 12:42:42 +0100 |
| commit | 06c35ef1fdf8d955684448683f7e48ac5f15ccfd (patch) | |
| tree | da44dc00c5df6e616218ef3f6e2cd6f73ed989fd /drivers/char | |
| parent | e27c7fa015d61c8be6a2c32b2144aad2ae6ec975 (diff) | |
drivers/char: kmem: disable on arm64
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 kmem read/write code, which does something
like
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.
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Diffstat (limited to 'drivers/char')
| -rw-r--r-- | drivers/char/Kconfig | 2 |
1 files changed, 2 insertions, 0 deletions
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 |