Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB handler

[Alexandre Chartre]

On 5/23/24 19:53, Andrew Cooper wrote:
> On 23/05/2024 6:03 pm, Alexandre Chartre wrote:
> > On 5/23/24 17:36, Dave Hansen wrote:
> > > On 5/23/24 07:52, Alexandre Chartre wrote:
> > > > > Should we wrap up this gem and put it with the other entry selftests?
> > > >
> > > > It looks like tools/testing/selftests/x86/single_step_syscall.c tests
> > > > sysenter with TF set but it doesn't check if the kernel issues any
> > > > warning.
> > >
> > > Does it actually trip the warning though? I'm a bit surprised that
> > > nobody reported it if so.
> >
> > single_step_syscall does trigger the warning:
> >
> > $ ./single_step_syscall
> > [RUN]    Set TF and check nop
> > [OK]    Survived with TF set and 26 traps
> > [RUN]    Set TF and check syscall-less opportunistic sysret
> > [OK]    Survived with TF set and 30 traps
> > [RUN]    Set TF and check a fast syscall
> > [OK]    Survived with TF set and 40 traps
> > [RUN]    Fast syscall with TF cleared
> > [OK]    Nothing unexpected happened
> > [RUN]    Set TF and check SYSENTER
> >      Got SIGSEGV with RIP=ed7fe579, TF=256
> > [RUN]    Fast syscall with TF cleared
> > [OK]    Nothing unexpected happened
>
> :-/
>
> What about the exit code?
>
> I find the absence of a [FAIL] concerning...

$ ./single_step_syscall
[RUN]	Set TF and check nop
[OK]	Survived with TF set and 26 traps
[RUN]	Set TF and check syscall-less opportunistic sysret
[OK]	Survived with TF set and 30 traps
[RUN]	Set TF and check a fast syscall
[OK]	Survived with TF set and 40 traps
[RUN]	Fast syscall with TF cleared
[OK]	Nothing unexpected happened
[RUN]	Set TF and check SYSENTER
	Got SIGSEGV with RIP=bb44b579, TF=256
[RUN]	Fast syscall with TF cleared
[OK]	Nothing unexpected happened

$ echo $?
0

The program runs as expected (but it doesn't expect much than a SIGSEGV).
It triggers a warning from the kernel but it doesn't check if a warning
was produced.

alex.