Re: [PATCH 2/3] arm64: signal: sigreturn() and rt_sigreturn() sometime returns the wrong signals

[Will Deacon <will@kernel.org>]

[+Eric as he actually understands how this is supposed to work]

On Tue, Apr 20, 2021 at 04:50:13PM +0000, Liam Howlett wrote:
> arm64_notify_segfault() was used to force a SIGSEGV in all error cases
> in sigreturn() and rt_sigreturn() to avoid writing a new sig handler.
> There is now a better sig handler to use which does not search the VMA
> address space and return a slightly incorrect error code.  Restore the
> older and correct si_code of SI_KERNEL by using arm64_notify_die().  In
> the case of !access_ok(), simply return SIGSEGV with si_code
> SEGV_ACCERR.
> 
> This change requires exporting arm64_notfiy_die() to the arm64 traps.h
> 
> Fixes: f71016a8a8c5 (arm64: signal: Call arm64_notify_segfault when
> failing to deliver signal)
> Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> ---
>  arch/arm64/include/asm/traps.h |  2 ++
>  arch/arm64/kernel/signal.c     |  8 ++++++--
>  arch/arm64/kernel/signal32.c   | 18 ++++++++++++++----
>  3 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/traps.h b/arch/arm64/include/asm/traps.h
> index 54f32a0675df..9b76144fcba6 100644
> --- a/arch/arm64/include/asm/traps.h
> +++ b/arch/arm64/include/asm/traps.h
> @@ -29,6 +29,8 @@ void arm64_notify_segfault(unsigned long addr);
>  void arm64_force_sig_fault(int signo, int code, unsigned long far, const char *str);
>  void arm64_force_sig_mceerr(int code, unsigned long far, short lsb, const char *str);
>  void arm64_force_sig_ptrace_errno_trap(int errno, unsigned long far, const char *str);
> +void arm64_notify_die(const char *str, struct pt_regs *regs, int signo,
> +		      int sicode, unsigned long far, int err);
>  
>  /*
>   * Move regs->pc to next instruction and do necessary setup before it
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index 6237486ff6bb..9fde6dc760c3 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -544,7 +544,7 @@ SYSCALL_DEFINE0(rt_sigreturn)
>  	frame = (struct rt_sigframe __user *)regs->sp;
>  
>  	if (!access_ok(frame, sizeof (*frame)))
> -		goto badframe;
> +		goto e_access;
>  
>  	if (restore_sigframe(regs, frame))
>  		goto badframe;
> @@ -555,7 +555,11 @@ SYSCALL_DEFINE0(rt_sigreturn)
>  	return regs->regs[0];
>  
>  badframe:
> -	arm64_notify_segfault(regs->sp);
> +	arm64_notify_die("Bad frame", regs, SIGSEGV, SI_KERNEL, regs->sp, 0);
> +	return 0;
> +
> +e_access:
> +	force_signal_inject(SIGSEGV, SEGV_ACCERR, regs->sp, 0);
>  	return 0;

This seems really error-prone to me, but maybe I'm just missing some
context. What's the rule for reporting an si_code of SI_KERNEL vs
SEGV_ACCERR, and is the former actually valid for SIGSEGV?

With this change, pointing the (signal) stack to a kernel address will
result in SEGV_ACCERR but pointing it to something like a PROT_NONE user
address will give SI_KERNEL (well, assuming that we manage to deliver
the SEGV somehow). I'm having a hard time seeing why that's a useful
distinction to make..

If it's important to get this a particular way around, please can you
add some selftests?

Will