Re: [PATCH] module/ksymtab: use 64-bit relative reference for target symbol

From: Ard Biesheuvel <ard.biesheuvel@arm.com>
To: linux-arm-kernel@lists.infradead.org
Cc: marc.zyngier@arm.com, james.morse@arm.com, will.deacon@arm.com,
	guillaume.gardet@arm.com, mark.rutland@arm.com, mingo@kernel.org,
	jeyu@kernel.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, arnd@arndb.de, x86@kernel.org
Subject: Re: [PATCH] module/ksymtab: use 64-bit relative reference for target symbol
Date: Thu, 23 May 2019 09:41:40 +0100	[thread overview]
Message-ID: <f2141ee5-d07a-6dd9-47c6-97e8fbdccf34@arm.com> (raw)
In-Reply-To: <293c9d0f-dc14-1413-e4b4-4299f0acfb9e@arm.com>

On 5/22/19 5:28 PM, Ard Biesheuvel wrote:
> 
> 
> On 5/22/19 4:02 PM, Ard Biesheuvel wrote:
>> The following commit
>>
>>    7290d5809571 ("module: use relative references for __ksymtab entries")
>>
>> updated the ksymtab handling of some KASLR capable architectures
>> so that ksymtab entries are emitted as pairs of 32-bit relative
>> references. This reduces the size of the entries, but more
>> importantly, it gets rid of statically assigned absolute
>> addresses, which require fixing up at boot time if the kernel
>> is self relocating (which takes a 24 byte RELA entry for each
>> member of the ksymtab struct).
>>
>> Since ksymtab entries are always part of the same module as the
>> symbol they export (or of the core kernel), it was assumed at the
>> time that a 32-bit relative reference is always sufficient to
>> capture the offset between a ksymtab entry and its target symbol.
>>
>> Unfortunately, this is not always true: in the case of per-CPU
>> variables, a per-CPU variable's base address (which usually differs
>> from the actual address of any of its per-CPU copies) could be at
>> an arbitrary offset from the ksymtab entry, and so it may be out
>> of range for a 32-bit relative reference.
>>

(Apologies for the 3-act monologue)

This turns out to be incorrect. The symbol address of per-CPU variables 
exported by modules is always in the vicinity of __per_cpu_start, and so 
it is simply a matter of making sure that the core kernel is in range 
for module ksymtab entries containing 32-bit relative references.

When running the arm64 with kaslr enabled, we currently randomize the 
module space based on the range of ADRP/ADD instruction pairs, which 
have a -/+ 4 GB range rather than the -/+ 2 GB range of 32-bit place 
relative data relocations. So we can fix this by simply reducing the 
randomization window to 2 GB.

So please disregard this patch (and the followup one against arch/x86/tools)

-- 
Ard.