Re: [PATCH] x86emul: fix SYSENTER/SYSCALL switching into 64-bit mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.02.2021 13:28, Andrew Cooper wrote:
> On 10/02/2021 09:57, Jan Beulich wrote:
>> When invoked by compat mode, mode_64bit() will be false at the start of
>> emulation. The logic after complete_insn, however, needs to consider the
>> mode switched into, in particular to avoid truncating RIP.
>>
>> Inspired by / paralleling and extending Linux commit 943dea8af21b ("KVM:
>> x86: Update emulator context mode if SYSENTER xfers to 64-bit mode").
>>
>> While there, tighten a related assertion in x86_emulate_wrapper() - we
>> want to be sure to not switch into an impossible mode when the code gets
>> built for 32-bit only (as is possible for the test harness).
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> In principle we could drop SYSENTER's ctxt->lma dependency when setting
>> _regs.r(ip). I wasn't certain whether leaving it as is serves as kind of
>> documentation ...
>>
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>> @@ -6127,6 +6127,10 @@ x86_emulate(
>>               (rc = ops->write_segment(x86_seg_ss, &sreg, ctxt)) )
>>              goto done;
>>  
>> +        if ( ctxt->lma )
>> +            /* In particular mode_64bit() needs to return true from here 
>> on. */
>> +            ctxt->addr_size = ctxt->sp_size = 64;
> 
> I think this is fine as presented, but don't we want the logical
> opposite for SYSRET/SYSEXIT ?
> 
> We truncate rip suitably already,

This is why I left them alone, i.e. ...

> but don't know what other checks may appear in the future.

... I thought we would deal with this if and when such checks
would appear. Just like considered in the post-description
remark, we could drop the conditional part from sysexit's
setting of _regs.r(ip), and _then_ we would indeed need a
respective change there, for the truncation to happen at
complete_insn:.

Jan