linux-kernel - Re: [PATCH 2/2] s390: mm: Fix secure storage access exception handling

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Date:   Tue, 19 Jan 2021 11:38:10 +0100
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, thuth@...hat.com, david@...hat.com,
        imbrenda@...ux.ibm.com, cohuck@...hat.com,
        linux-s390@...r.kernel.org, gor@...ux.ibm.com,
        mihajlov@...ux.ibm.com
Subject: Re: [PATCH 2/2] s390: mm: Fix secure storage access exception
 handling

On 1/19/21 11:25 AM, Christian Borntraeger wrote:
> 
> 
> On 19.01.21 11:04, Janosch Frank wrote:
>> Turns out that the bit 61 in the TEID is not always 1 and if that's
>> the case the address space ID and the address are
>> unpredictable. Without an address and it's address space ID we can't
>> export memory and hence we can only send a SIGSEGV to the process or
>> panic the kernel depending on who caused the exception.
>>
>> Signed-off-by: Janosch Frank <frankja@...ux.ibm.com>
>> Fixes: 084ea4d611a3d ("s390/mm: add (non)secure page access exceptions handlers")
>> Cc: stable@...r.kernel.org
> 
> Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>

Thanks!

> 
> some small things to consider (or to reject)
> 
>> ---
>>  arch/s390/mm/fault.c | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
>> index e30c7c781172..5442937e5b4b 100644
>> --- a/arch/s390/mm/fault.c
>> +++ b/arch/s390/mm/fault.c
>> @@ -791,6 +791,20 @@ void do_secure_storage_access(struct pt_regs *regs)
>>  	struct page *page;
>>  	int rc;
>>  
>> +	/* There are cases where we don't have a TEID. */
>> +	if (!(regs->int_parm_long & 0x4)) {
>> +		/*
>> +		 * Userspace could for example try to execute secure
>> +		 * storage and trigger this. We should tell it that it
>> +		 * shouldn't do that.
> 
> Maybe something like
> 		/*
> 		 * when this happens, userspace did something that it
> 		 * was not supposed to do, e.g. branching into secure
> 		 * secure memory. Trigger a segmentation fault.
>> +		 */

Sounds good

>> +		if (user_mode(regs)) {
>> +			send_sig(SIGSEGV, current, 0);
>> +			return;
>> +		} else
>> +			panic("Unexpected PGM 0x3d with TEID bit 61=0");
> 
> use BUG instead of panic? That would kill this process, but it allows
> people to maybe save unaffected data.

That would make sense, will do

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