[PATCH AUTOSEL 5.6 149/606] arm64: Fix PTRACE_SYSEMU semantics

Sasha Levin
2020-06-09T00:18:00Z

From: Keno Fischer <keno@juliacomputing.com>

commit 1cf6022bd9161081215028203919c33fcfa6debb upstream.

Quoth the man page:
```
If the tracee was restarted by PTRACE_SYSCALL or PTRACE_SYSEMU, the
tracee enters syscall-enter-stop just prior to entering any system
call (which will not be executed if the restart was using
PTRACE_SYSEMU, regardless of any change made to registers at this
point or how the tracee is restarted after this stop).
```

The parenthetical comment is currently true on x86 and powerpc,
but not currently true on arm64. arm64 re-checks the _TIF_SYSCALL_EMU
flag after the syscall entry ptrace stop. However, at this point,
it reflects which method was used to re-start the syscall
at the entry stop, rather than the method that was used to reach it.
Fix that by recording the original flag before performing the ptrace
stop, bringing the behavior in line with documentation and x86/powerpc.

Fixes: f086f67485c5 ("arm64: ptrace: add support for syscall emulation")
Cc: <stable@vger.kernel.org> # 5.3.x-
Signed-off-by: Keno Fischer <keno@juliacomputing.com>
Acked-by: Will Deacon <will@kernel.org>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Bin Lu <Bin.Lu@arm.com>
[catalin.marinas@arm.com: moved 'flags' bit masking]
[catalin.marinas@arm.com: changed 'flags' type to unsigned long]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/kernel/ptrace.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index cd6e5fa48b9c..c30f77bd875f 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1829,10 +1829,11 @@ static void tracehook_report_syscall(struct pt_regs *regs,

int syscall_trace_enter(struct pt_regs *regs)
{
- if (test_thread_flag(TIF_SYSCALL_TRACE) ||
- test_thread_flag(TIF_SYSCALL_EMU)) {
+ unsigned long flags = READ_ONCE(current_thread_info()->flags);
+
+ if (flags & (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE)) {
tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
- if (!in_syscall(regs) || test_thread_flag(TIF_SYSCALL_EMU))
+ if (!in_syscall(regs) || (flags & _TIF_SYSCALL_EMU))
return -1;
}

--
2.25.1