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

[Arnd Bergmann]

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.

> - 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?

      Arnd