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Date: Tue, 18 Nov 2014 14:04:25 +0000 From: Will Deacon <will.deacon@....com> To: AKASHI Takahiro <takahiro.akashi@...aro.org> Cc: "keescook@...omium.org" <keescook@...omium.org>, Catalin Marinas <Catalin.Marinas@....com>, "dsaxena@...aro.org" <dsaxena@...aro.org>, "arndb@...db.de" <arndb@...db.de>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote: > If tracer specifies -1 as a syscall number, this traced system call should > be skipped with a return value specified in x0. > This patch implements this semantics, but there is one restriction here: > > syscall(-1) always return ENOSYS whatever value is stored in x0 > (a return value) at syscall entry. > > Normally, with ptrace off, syscall(-1) returns -ENOSYS. With ptrace on, > however, if a tracer didn't pay any attention to user-issued syscall(-1) > and just let it go, it would return a value in x0 as in other system call > cases. This means that this system call might succeed and yet see any bogus > return value. This should be definitely avoided. > > Please also note: > * syscall entry tracing and syscall exit tracing (ftrace tracepoint and > audit) are always executed, if enabled, even when skipping a system call > (that is, -1). > In this way, we can avoid a potential bug where audit_syscall_entry() > might be called without audit_syscall_exit() at the previous system call > being called, that would cause OOPs in audit_syscall_entry(). > > Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org> > --- > arch/arm64/kernel/entry.S | 3 +++ > arch/arm64/kernel/ptrace.c | 18 ++++++++++++++++++ > 2 files changed, 21 insertions(+) > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > index 726b910..01118b1 100644 > --- a/arch/arm64/kernel/entry.S > +++ b/arch/arm64/kernel/entry.S > @@ -670,6 +670,8 @@ ENDPROC(el0_svc) > __sys_trace: > mov x0, sp > bl syscall_trace_enter > + cmp w0, #-1 // skip the syscall? > + b.eq __sys_trace_return_skipped > adr lr, __sys_trace_return // return address > uxtw scno, w0 // syscall number (possibly new) > mov x1, sp // pointer to regs > @@ -684,6 +686,7 @@ __sys_trace: > > __sys_trace_return: > str x0, [sp] // save returned x0 > +__sys_trace_return_skipped: > mov x0, sp > bl syscall_trace_exit > b ret_to_user > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c > index 8b98781..34b1e85 100644 > --- a/arch/arm64/kernel/ptrace.c > +++ b/arch/arm64/kernel/ptrace.c > @@ -1149,6 +1149,8 @@ static void tracehook_report_syscall(struct pt_regs *regs, > > asmlinkage int syscall_trace_enter(struct pt_regs *regs) > { > + int orig_syscallno = regs->syscallno; > + > if (test_thread_flag(TIF_SYSCALL_TRACE)) > tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); > > @@ -1158,6 +1160,22 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs) > audit_syscall_entry(regs->syscallno, regs->orig_x0, regs->regs[1], > regs->regs[2], regs->regs[3]); > > + if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) { > + /* > + * user-issued syscall(-1): > + * RESTRICTION: We always return ENOSYS whatever value is > + * stored in x0 (a return value) at this point. > + * Normally, with ptrace off, syscall(-1) returns -ENOSYS. > + * With ptrace on, however, if a tracer didn't pay any > + * attention to user-issued syscall(-1) and just let it go > + * without a hack here, it would return a value in x0 as in > + * other system call cases. This means that this system call > + * might succeed and see any bogus return value. > + * This should be definitely avoided. > + */ > + regs->regs[0] = -ENOSYS; > + } I'm still really uncomfortable with this, and it doesn't seem to match what arch/arm/ does either. Doesn't it also prevent a tracer from skipping syscall(-1)? Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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