Re: [PATCH] ARM: ptrace: Restore syscall skipping and restart while tracing

[Kees Cook]

On Wed, Aug 09, 2023 at 09:47:24PM +0200, Arnd Bergmann wrote:
> On Fri, Aug 4, 2023, at 09:10, Kees Cook wrote:
> > Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
> > thread_info->abi_syscall"), the seccomp selftests "syscall_errno",
> > "syscall_faked", and "syscall_restart" have been broken. This was
> > related to two issues:
> 
> While it looks like my patch introduced both problems, it might
> be better to split your fix into two bits.

Okay, sounds good.

> > - seccomp and PTRACE depend on using the special value of "-1" for
> >   skipping syscalls. This value wasn't working because it was getting
> >   masked by __NR_SYSCALL_MASK in both PTRACE_SET_SYSCALL and
> >   get_syscall_nr().
> 
> > Explicitly test for -1 in PTRACE_SET_SYSCALL and get_syscall_nr(),
> > leaving it exposed when present, allowing tracers to skip syscalls
> > again.
> 
> This part looks good to me, at least it seems to be one of multiple
> ways of doing this, depending on how we want to encode the
> syscall skipping in the variable.
> 
> > - the syscall entry label "local_restart" is used for resuming syscalls
> >   interrupted by signals, but the updated syscall number (in scno) was
> >   not being stored in current_thread_info()->abi_syscall, causing traced
> >   syscall restarting to fail.
> >
> > Move the AEABI-only assignment of current_thread_info()->abi_syscall
> > after the "local_restart" label to allow tracers to survive syscall
> > restarting.
> 
> I'm not following exactly what you are doing here yet, but I suspect
> this part is wrong:
> 
> > diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> > index bcc4c9ec3aa4..08bd624e4c6f 100644
> > --- a/arch/arm/kernel/entry-common.S
> > +++ b/arch/arm/kernel/entry-common.S
> > @@ -246,8 +246,6 @@ ENTRY(vector_swi)
> >  	bic	scno, scno, #0xff000000		@ mask off SWI op-code
> >  	str	scno, [tsk, #TI_ABI_SYSCALL]
> >  	eor	scno, scno, #__NR_SYSCALL_BASE	@ check OS number
> > -#else
> > -	str	scno, [tsk, #TI_ABI_SYSCALL]
> >  #endif
> >  	/*
> >  	 * Reload the registers that may have been corrupted on entry to
> > @@ -256,6 +254,9 @@ ENTRY(vector_swi)
> >   TRACE(	ldmia	sp, {r0 - r3}		)
> > 
> >  local_restart:
> > +#if defined(CONFIG_AEABI) && !defined(CONFIG_OABI_COMPAT)
> > +	str	scno, [tsk, #TI_ABI_SYSCALL]	@ store scno for syscall restart
> > +#endif
> >  	ldr	r10, [tsk, #TI_FLAGS]		@ check for syscall tracing
> >  	stmdb	sp!, {r4, r5}			@ push fifth and sixth args
> > 
> 
> If the local_restart code has to store the syscall number
> for an EABI-only kernel, wouldn't it have to also do this
> for a kernel with OABI-only or OABI_COMPAT support?

This is the part I wasn't sure about. Initially I was thinking it didn't
matter because it's only a problem for a seccomp tracer, but I realize
it might be exposed to a PTRACE tracer too. I was only able to test with
EABI since seccomp is disabled for OABI_COMPAT.

Anyway, syscall restart is done this way:

        movlt   scno, #(__NR_restart_syscall - __NR_SYSCALL_BASE)

Can a EABI call restart an OABI syscall? I think so?

So maybe we just need to add:

	str     scno, [tsk, #TI_ABI_SYSCALL]    @ store scno for syscall restart

after that instead of moving it like I did originally?

Let me test that...

-- 
Kees Cook