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

[Cornelia Huck <cohuck@redhat.com>]

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On Tue, 19 Jan 2021 11:38:10 +0100
Janosch Frank <frankja@linux.ibm.com> wrote:

> 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

s/when/When/ :)

> > 		 * 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

With BUG():

Reviewed-by: Cornelia Huck <cohuck@redhat.com>

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