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From: Steven Price <steven.price@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-kernel@vger.kernel.org, Mike Rapoport <rppt@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Will Deacon <will.deacon@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: Bad virt_to_phys since commit 54c7a8916a887f35
Date: Thu, 16 May 2019 15:20:59 +0100	[thread overview]
Message-ID: <d265e5fe-c061-17a0-427d-0e6f31be17f3@arm.com> (raw)
In-Reply-To: <20190516141640.GC43059@lakrids.cambridge.arm.com>

On 16/05/2019 15:16, Mark Rutland wrote:
> On Thu, May 16, 2019 at 03:05:31PM +0100, Steven Price wrote:
>> On 16/05/2019 14:41, Mark Rutland wrote:
>>> On Thu, May 16, 2019 at 02:38:20PM +0100, Mark Rutland wrote:
>>>> Hi,
>>>>
>>>> Since commit:
>>>>
>>>>   54c7a8916a887f35 ("initramfs: free initrd memory if opening /initrd.image fails")
>>>
>>> Ugh, I dropped a paragarph here.
>>>
>>> Since that commit, I'm seeing a boot-time splat on arm64 when using
>>> CONFIG_DEBUG_VIRTUAL. I'm running an arm64 syzkaller instance, and this
>>> kills the VM, preventing further testing, which is unfortunate.
>>>
>>> Mark.
>>>
>>>> IIUC prior to that commit, we'd only attempt to free an intird if we had
>>>> one, whereas now we do so unconditionally. AFAICT, in this case
>>>> initrd_start has not been initialized (I'm not using an initrd or
>>>> initramfs on my system), so we end up trying virt_to_phys() on a bogus
>>>> VA in free_initrd_mem().
>>>>
>>>> Any ideas on the right way to fix this?
>>
>> Your analysis looks right to me. In my review I'd managed to spot the
>> change in behaviour when CONFIG_INITRAMFS_FORCE is set (the initrd is
>> freed), but I'd overlooked what happens if initrd_start == 0 (the
>> non-existent initrd is attempted to be freed).
>>
>> I suspect the following is sufficient to fix the problem:
>>
>> ----8<-----
>> diff --git a/init/initramfs.c b/init/initramfs.c
>> index 435a428c2af1..178130fd61c2 100644
>> --- a/init/initramfs.c
>> +++ b/init/initramfs.c
>> @@ -669,7 +669,7 @@ static int __init populate_rootfs(void)
>>  	 * If the initrd region is overlapped with crashkernel reserved region,
>>  	 * free only memory that is not part of crashkernel region.
>>  	 */
>> -	if (!do_retain_initrd && !kexec_free_initrd())
>> +	if (!do_retain_initrd && initrd_start && !kexec_free_initrd())
>>  		free_initrd_mem(initrd_start, initrd_end);
>>  	initrd_start = 0;
>>  	initrd_end = 0;
> 
> That works for me. If you spin this as a real patch:
> 
> Tested-by: Mark Rutland <mark.rutland@arm.com>
> 
> As I mentioned, initrd_start has not been initialized at all, so I
> suspect we should also update its declaration in init/do_mounts_initrd.c
> such that it is guaranteed to be initialized to zero. We get away with
> that today, but that won't necessarily hold with LTO and so on...

Well it's a global variable, so the C standard says it should be
initialised to 0...

I'll spin a real patch and add your Tested-by

Steve

  reply	other threads:[~2019-05-16 14:21 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16 13:38 Mark Rutland
2019-05-16 13:41 ` Mark Rutland
2019-05-16 14:05   ` Steven Price
2019-05-16 14:16     ` Mark Rutland
2019-05-16 14:20       ` Steven Price [this message]
2019-05-16 14:27         ` Mark Rutland
2019-05-23  9:31         ` Will Deacon
2019-05-23  9:54           ` Mike Rapoport
2019-05-23  9:58             ` Steven Price
2019-05-23 10:01               ` Will Deacon
2019-05-16 14:13   ` Mike Rapoport
2019-05-16 14:21     ` Mark Rutland
2019-05-16 14:23       ` Christoph Hellwig

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