Messages in this thread | | | Date | Mon, 19 Jun 2017 12:36:50 +0100 | From | Will Deacon <> | Subject | Re: [RFC PATCH] drivers/char: kmem: disable read/write if VMALLOC_START < PAGE_OFFSET |
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On Fri, Jun 09, 2017 at 04:32:28PM +0000, Ard Biesheuvel wrote: > 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 > 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, but the read/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 fail such calls with a warning if VMALLOC_START < PAGE_OFFSET. > (Note that kmem's mmap() interface is not affected by this) > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> > --- > > This is just an RFC. There may be better ways to deal with this, including > disabling /dev/kmem altogether on arm64.
FWIW: I'm fine with either approach.
Will
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