linux-kernel - Re: [PATCH v2] selftests/sgx: Use rip relative addressing for encl

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Date:   Thu, 31 Mar 2022 03:59:07 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        Shuah Khan <shuah@...nel.org>
Cc:     linux-sgx@...r.kernel.org, nathaniel@...fian.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] selftests/sgx: Use rip relative addressing for
 encl_stack

On Wed, 2022-03-30 at 17:10 -0700, Reinette Chatre wrote:
> Hi Jarkko,
> 
> On 3/30/2022 3:28 PM, Jarkko Sakkinen wrote:
> > Simplify the test_encl_bootstrap.S flow by using RIP-relative addressing.
> 
> It is not clear to me how this is simpler. At this point there is no
> functional change (except for what appears to be an unintended bug - more below).
> At this time the change seems more code utilizing subtle compiler features
> to accomplish the same.
> 
> Could you please share more about your plans following this change? I need
> to understand this better since it is also an area changed by the SGX2 testing
> code.
> 
> > The compiler automatically puts relative addresses for RIP index addresses.
> 
> I was not aware of this. A comment would be helpful to understand the implementation.
> 
> > 
> > In order to get a clean and tweakless solution, define separate entry point
> > for each TCS.
> > 
> > Cc: Reinette Chatre <reinette.chatre@...el.com>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> > ---
> > v2:
> > * Based on Reinette's example, make proper structuring with separate
> >   entry points for each TCS.
> > ---
> >  .../selftests/sgx/test_encl_bootstrap.S       | 30 +++++++++++--------
> >  1 file changed, 18 insertions(+), 12 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/sgx/test_encl_bootstrap.S b/tools/testing/selftests/sgx/test_encl_bootstrap.S
> > index 82fb0dfcbd23..cc2353f38bcc 100644
> > --- a/tools/testing/selftests/sgx/test_encl_bootstrap.S
> > +++ b/tools/testing/selftests/sgx/test_encl_bootstrap.S
> > @@ -10,12 +10,13 @@
> >         .section ".tcs", "aw"
> >         .balign 4096
> >  
> > +encl_tcs1:
> >         .fill   1, 8, 0                 # STATE (set by CPU)
> >         .fill   1, 8, 0                 # FLAGS
> >         .quad   encl_ssa_tcs1           # OSSA
> >         .fill   1, 4, 0                 # CSSA (set by CPU)
> >         .fill   1, 4, 1                 # NSSA
> > -       .quad   encl_entry              # OENTRY
> > +       .quad   encl_entry1             # OENTRY
> >         .fill   1, 8, 0                 # AEP (set by EENTER and ERESUME)
> >         .fill   1, 8, 0                 # OFSBASE
> >         .fill   1, 8, 0                 # OGSBASE
> > @@ -23,13 +24,13 @@
> >         .fill   1, 4, 0xFFFFFFFF        # GSLIMIT
> >         .fill   4024, 1, 0              # Reserved
> >  
> > -       # TCS2
> > +encl_tcs2:
> >         .fill   1, 8, 0                 # STATE (set by CPU)
> >         .fill   1, 8, 0                 # FLAGS
> >         .quad   encl_ssa_tcs2           # OSSA
> >         .fill   1, 4, 0                 # CSSA (set by CPU)
> >         .fill   1, 4, 1                 # NSSA
> > -       .quad   encl_entry              # OENTRY
> > +       .quad   encl_entry2             # OENTRY
> >         .fill   1, 8, 0                 # AEP (set by EENTER and ERESUME)
> >         .fill   1, 8, 0                 # OFSBASE
> >         .fill   1, 8, 0                 # OGSBASE
> > @@ -39,15 +40,19 @@
> >  
> >         .text
> >  
> > -encl_entry:
> > -       # RBX contains the base address for TCS, which is the first address
> > -       # inside the enclave for TCS #1 and one page into the enclave for
> > -       # TCS #2. By adding the value of encl_stack to it, we get
> > -       # the absolute address for the stack.
> > -       lea     (encl_stack)(%rbx), %rax
> > +encl_entry1:
> > +       lea     (encl_stack1)(%rip), %rax
> >         xchg    %rsp, %rax
> >         push    %rax
> > +       jmp     encl_continue
> >  
> > +encl_entry2:
> > +       lea     (encl_stack2)(%rip), %rax
> > +       xchg    %rsp, %rax
> > +       push    %rax
> > +       jmp     encl_continue
> > +
> 
> The code duplication (xchg and push) is not needed.
> 
> > +encl_continue:
> >         push    %rcx # push the address after EENTER
> >         push    %rbx # push the enclave base address
> >  
> > @@ -84,13 +89,14 @@ encl_entry:
> >  
> >  encl_ssa_tcs1:
> >         .space 4096
> > +
> >  encl_ssa_tcs2:
> >         .space 4096
> >  
> > +encl_stack1:
> 
> Stack grows the other way so by placing the entry here the stack of
> TCS #1 will clobber the SSA of TCS #2.
> 
> >         .balign 4096
> > -       # Stack of TCS #1
> >         .space 4096
> > -encl_stack:
> > +
> > +encl_stack2:
> 
> Here the stack of TCS #2 will actually use the stack of TCS #1.
> 
> >         .balign 4096
> > -       # Stack of TCS #2
> >         .space 4096
> 
> Last page will be unused.
> 
> Reinette

Thanks for the remarks. I'll fix the issues.

BR, Jarkko