From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux@arm.linux.org.uk (Russell King - ARM Linux) Date: Thu, 14 May 2015 20:35:53 +0100 Subject: arm syscall fast path can miss a ptrace syscall-exit In-Reply-To: <5554F3E4.8020307@redhat.com> References: <5554F3E4.8020307@redhat.com> Message-ID: <20150514193553.GD2067@n2100.arm.linux.org.uk> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, May 14, 2015 at 12:13:40PM -0700, Josh Stone wrote: > I've discovered a case where both arm and arm64 will miss a ptrace > syscall-exit that they should report. If the syscall is entered without > TIF_SYSCALL_TRACE set, then it goes on the fast path. It's then > possible to have TIF_SYSCALL_TRACE added in the middle of the syscall, > but ret_fast_syscall doesn't check this flag again. Yes, we assume that if TIF_SYSCALL_TRACE was not set before the call, it isn't set after. That appears to be an invalid assumption. Here's a patch for ARM - untested atm. There's still a possible hole - if we exit the syscall, then do "work" before returning (such as reschedling to another process), and _then_ have syscall tracing enabled, we won't trace the exit. I think that's acceptable as I see no difference between that and having restored state for userspace, and then immediately processing an interrupt and scheduling on the IRQ exit path. arch/arm/kernel/entry-common.S | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S index f8ccc21fa032..4e7f40c577e6 100644 --- a/arch/arm/kernel/entry-common.S +++ b/arch/arm/kernel/entry-common.S @@ -33,7 +33,9 @@ ret_fast_syscall: UNWIND(.fnstart ) UNWIND(.cantunwind ) disable_irq @ disable interrupts - ldr r1, [tsk, #TI_FLAGS] + ldr r1, [tsk, #TI_FLAGS] @ re-check for syscall tracing + tst r1, #_TIF_SYSCALL_WORK + bne __sys_trace_return tst r1, #_TIF_WORK_MASK bne fast_work_pending asm_trace_hardirqs_on -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net.