arm64: ptrace: Consistently use pseudo-singlestep exceptions

Although the arm64 single-step state machine can be fast-forwarded in
cases where we wish to generate a SIGTRAP without actually executing an
instruction, this has two major limitations outside of simply skipping
an instruction due to emulation.

1. Stepping out of a ptrace signal stop into a signal handler where
   SIGTRAP is blocked. Fast-forwarding the stepping state machine in
   this case will result in a forced SIGTRAP, with the handler reset to
   SIG_DFL.

2. The hardware implicitly fast-forwards the state machine when executing
   an SVC instruction for issuing a system call. This can interact badly
   with subsequent ptrace stops signalled during the execution of the
   system call (e.g. SYSCALL_EXIT or seccomp traps), as they may corrupt
   the stepping state by updating the PSTATE for the tracee.

Resolve both of these issues by injecting a pseudo-singlestep exception
on entry to a signal handler and also on return to userspace following a
system call.

Cc: <stable@vger.kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Tested-by: Luis Machado <luis.machado@linaro.org>
Reported-by: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 5f5b868292f5..7c14466a12af 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -139,7 +139,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
 		local_daif_mask();
 		flags = current_thread_info()->flags;
-		if (!has_syscall_work(flags)) {
+		if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP)) {
 			/*
 			 * We're off to userspace, where interrupts are
 			 * always enabled after we restore the flags from