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

Janosch Frank
2021-01-19T10:38:10Z

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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@linux.ibm.com>
>> Fixes: 084ea4d611a3d ("s390/mm: add (non)secure page access exceptions handlers")
>> Cc: stable@vger.kernel.org
>
> Reviewed-by: Christian Borntraeger <borntraeger@de.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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