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Date:   Tue, 10 May 2022 15:03:33 +0100
From:   Will Deacon <will@kernel.org>
To:     Francis Laniel <flaniel@linux.microsoft.com>
Cc:     linux-arm-kernel@lists.infradead.org,
        linux-trace-devel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Peter Collingbourne <pcc@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Daniel Kiss <daniel.kiss@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v1 1/1] arm64: Forget syscall if different from
 execve*()
Message-ID: <20220510140333.GA28104@willie-the-truck>
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On Tue, May 10, 2022 at 03:00:11PM +0100, Francis Laniel wrote:
> Le mardi 10 mai 2022, 11:59:48 BST Will Deacon a ?crit :
> > On Mon, May 09, 2022 at 04:19:57PM +0100, Francis Laniel wrote:
> > > diff --git a/arch/arm64/include/asm/processor.h
> > > b/arch/arm64/include/asm/processor.h index 73e38d9a540c..e12ceb363d6a
> > > 100644
> > > --- a/arch/arm64/include/asm/processor.h
> > > +++ b/arch/arm64/include/asm/processor.h
> > > @@ -34,6 +34,8 @@
> > > 
> > >  #include <vdso/processor.h>
> > > 
> > > +#include <asm-generic/unistd.h>
> > > +
> > > 
> > >  #include <asm/alternative.h>
> > >  #include <asm/cpufeature.h>
> > >  #include <asm/hw_breakpoint.h>
> > > 
> > > @@ -250,8 +252,12 @@ void tls_preserve_current_state(void);
> > > 
> > >  static inline void start_thread_common(struct pt_regs *regs, unsigned
> > >  long pc) {
> > > 
> > > +	s32 previous_syscall = regs->syscallno;
> > > 
> > >  	memset(regs, 0, sizeof(*regs));
> > > 
> > > -	forget_syscall(regs);
> > > +	if (previous_syscall == __NR_execve || previous_syscall ==
> > > __NR_execveat)
> > > +		regs->syscallno = previous_syscall;
> > > +	else
> > > +		forget_syscall(regs);
> > 
> > Hmm, this really looks like a bodge and it doesn't handle the compat case
> > either.
> > 
> > How do other architectures handle this?
> 
> My understanding of others architectures is quite limited, but here are my 
> findings and understanding of some of them:
> * arm (32 bits) EABI: start_thread() sets r7 to previous r7 for ELF FDPIC  and 
> to 0 for other binfmts [1].
> * arm (32 bits) OABI: syscall number is set to -1 if 
> ptrace_report_syscall_entry() failed [2].
> * mips: start_thread() does not modify current_thread_info->syscall which is 
> taken directly from v0 [3, 4].
> * riscv: start_thread() does not modify a7 [5].
> * x86_64: start_thread_common() does not touch orig_ax which seems to contain 
> the syscall number [6].

Hmm, so the million dollar question is why on Earth we have that
forget_syscall() call to start with. Amusingly I've, err, forgotten;
forget_forget_syscall() perhaps?

Catalin? It's been there since day one afaict.

Will