Re: [PATCH] arm64: mm: account for hotplug memory when randomizing the linear region

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jan 20, 2025 at 08:33:12AM -0800, Florian Fainelli wrote:
> 
> 
> On 1/20/2025 5:59 AM, Greg KH wrote:
> > On Mon, Jan 13, 2025 at 07:44:50AM -0800, Florian Fainelli wrote:
> > > 
> > > 
> > > On 1/12/2025 3:54 AM, Greg KH wrote:
> > > > On Thu, Jan 09, 2025 at 09:01:13AM -0800, Florian Fainelli wrote:
> > > > > On 1/9/25 08:54, Florian Fainelli wrote:
> > > > > > From: Ard Biesheuvel <ardb@kernel.org>
> > > > > > 
> > > > > > commit 97d6786e0669daa5c2f2d07a057f574e849dfd3e upstream
> > > > > > 
> > > > > > As a hardening measure, we currently randomize the placement of
> > > > > > physical memory inside the linear region when KASLR is in effect.
> > > > > > Since the random offset at which to place the available physical
> > > > > > memory inside the linear region is chosen early at boot, it is
> > > > > > based on the memblock description of memory, which does not cover
> > > > > > hotplug memory. The consequence of this is that the randomization
> > > > > > offset may be chosen such that any hotplugged memory located above
> > > > > > memblock_end_of_DRAM() that appears later is pushed off the end of
> > > > > > the linear region, where it cannot be accessed.
> > > > > > 
> > > > > > So let's limit this randomization of the linear region to ensure
> > > > > > that this can no longer happen, by using the CPU's addressable PA
> > > > > > range instead. As it is guaranteed that no hotpluggable memory will
> > > > > > appear that falls outside of that range, we can safely put this PA
> > > > > > range sized window anywhere in the linear region.
> > > > > > 
> > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > > > > Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> > > > > > Cc: Will Deacon <will@kernel.org>
> > > > > > Cc: Steven Price <steven.price@arm.com>
> > > > > > Cc: Robin Murphy <robin.murphy@arm.com>
> > > > > > Link: https://lore.kernel.org/r/20201014081857.3288-1-ardb@kernel.org
> > > > > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > > > > > Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
> > > > > 
> > > > > Forgot to update the patch subject, but this one is for 5.10.
> > > > 
> > > > You also forgot to tell us _why_ this is needed :(
> > > 
> > > This is explained in the second part of the first paragraph:
> > > 
> > > The consequence of this is that the randomization offset may be chosen such
> > > that any hotplugged memory located above memblock_end_of_DRAM() that appears
> > > later is pushed off the end of the linear region, where it cannot be
> > > accessed.
> > > 
> > > We use both memory hotplug and KASLR on our systems and that's how we
> > > eventually found out about the bug.
> > 
> > And you still have 5.10.y ARM64 systems that need this?  Why not move to
> > a newer kernel version already?
> 
> We still have ARM64 systems running 5.4 that need this, and the same bug
> applies to 5.10 that we used to support but dropped in favor of 5.15/6.1.
> Those are the kernel versions used by Android, and Android TV in particular,
> so it's kind of the way it goes for us.
> 
> > 
> > Anyway, I need an ack from the ARM64 maintainers that this is ok to
> > apply here before I can take it.
> 
> Just out of curiosity, the change is pretty innocuous and simple to review,
> why the extra scrutiny needed here?

Why shouldn't the maintainers review a proposed backport patch for core
kernel code that affects everyone who uses that arch?

thanks,

greg k-h