From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755508AbaHVAf1 (ORCPT ); Thu, 21 Aug 2014 20:35:27 -0400 Received: from mail-pd0-f182.google.com ([209.85.192.182]:51190 "EHLO mail-pd0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754863AbaHVAf0 (ORCPT ); Thu, 21 Aug 2014 20:35:26 -0400 Message-ID: <53F69045.7010301@linaro.org> Date: Fri, 22 Aug 2014 09:35:17 +0900 From: AKASHI Takahiro User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Kees Cook CC: Catalin Marinas , Will Deacon , Deepak Saxena , arndb@arndb.de, "linux-arm-kernel@lists.infradead.org" , linaro-kernel@lists.linaro.org, LKML Subject: Re: [PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call References: <1408611405-8943-1-git-send-email-takahiro.akashi@linaro.org> <1408611405-8943-3-git-send-email-takahiro.akashi@linaro.org> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/22/2014 02:08 AM, Kees Cook wrote: > On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro > wrote: >> If tracer specifies -1 as a syscall number, this traced system call should >> be skipped with a value in x0 used as a return value. >> This patch enables this semantics, but there is a restriction here: >> >> when syscall(-1) is issued by user, tracer cannot skip this system call >> and modify a return value at syscall entry. >> >> In order to ease this flavor, we need to treat whatever value in x0 as >> a return value, but this might result in a bogus value being returned, >> especially when tracer doesn't do anything at this syscall. >> So we always return ENOSYS instead, while we have another chance to change >> a return value at syscall exit. >> >> 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(). >> >> * syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected >> in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS) >> is not used in this case, we may neglect the case. >> >> Signed-off-by: AKASHI Takahiro >> --- >> arch/arm64/include/asm/ptrace.h | 8 ++++++++ >> arch/arm64/kernel/entry.S | 4 ++++ >> arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++ >> 3 files changed, 32 insertions(+) >> >> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h >> index 501000f..a58cf62 100644 >> --- a/arch/arm64/include/asm/ptrace.h >> +++ b/arch/arm64/include/asm/ptrace.h >> @@ -65,6 +65,14 @@ >> #define COMPAT_PT_TEXT_ADDR 0x10000 >> #define COMPAT_PT_DATA_ADDR 0x10004 >> #define COMPAT_PT_TEXT_END_ADDR 0x10008 >> + >> +/* >> + * used to skip a system call when tracer changes its number to -1 >> + * with ptrace(PTRACE_SET_SYSCALL) >> + */ >> +#define RET_SKIP_SYSCALL -1 >> +#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1) >> + >> #ifndef __ASSEMBLY__ >> >> /* sizeof(struct user) for AArch32 */ >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S >> index f0b5e51..fdd6eae 100644 >> --- a/arch/arm64/kernel/entry.S >> +++ b/arch/arm64/kernel/entry.S >> @@ -25,6 +25,7 @@ >> #include >> #include >> #include >> +#include >> #include >> #include >> >> @@ -671,6 +672,8 @@ ENDPROC(el0_svc) >> __sys_trace: >> mov x0, sp >> bl syscall_trace_enter >> + cmp w0, #RET_SKIP_SYSCALL // skip 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 >> @@ -685,6 +688,7 @@ __sys_trace: >> >> __sys_trace_return: >> str x0, [sp] // save returned x0 >> +__sys_trace_return_skipped: // x0 already in regs[0] >> 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 8876049..c54dbcc 100644 >> --- a/arch/arm64/kernel/ptrace.c >> +++ b/arch/arm64/kernel/ptrace.c >> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, >> >> asmlinkage int syscall_trace_enter(struct pt_regs *regs) >> { >> + unsigned int saved_syscallno = regs->syscallno; >> + >> if (test_thread_flag(TIF_SYSCALL_TRACE)) >> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); >> >> + if (IS_SKIP_SYSCALL(regs->syscallno)) { >> + /* >> + * RESTRICTION: we can't modify a return value of user >> + * issued syscall(-1) here. In order to ease this flavor, >> + * we need to treat whatever value in x0 as a return value, >> + * but this might result in a bogus value being returned. >> + */ >> + /* >> + * NOTE: syscallno may also be set to -1 if fatal signal is >> + * detected in tracehook_report_syscall_entry(), but since >> + * a value set to x0 here is not used in this case, we may >> + * neglect the case. >> + */ >> + if (!test_thread_flag(TIF_SYSCALL_TRACE) || >> + (IS_SKIP_SYSCALL(saved_syscallno))) >> + regs->regs[0] = -ENOSYS; >> + } >> + > > I don't have a runtime environment yet for arm64, so I can't test this > directly myself, so I'm just trying to eyeball this. :) > > Once the seccomp logic is added here, I don't think using -2 as a > special value will work. Doesn't this mean the Oops is possible by the > user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and > the user passed -2 as the syscall, audit will be called only on entry, > and then skipped on exit? Oops, you're absolutely right. I didn't think of this case. syscall_trace_enter() should not return a syscallno directly, but always return -1 if syscallno < 0. (except when secure_computing() returns with -1) This also implies that tracehook_report_syscall() should also have a return value. Will, is this fine with you? -Takahiro AKASHI > -Kees > >> if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) >> trace_sys_enter(regs, regs->syscallno); >> >> -- >> 1.7.9.5 >> > > > From mboxrd@z Thu Jan 1 00:00:00 1970 From: takahiro.akashi@linaro.org (AKASHI Takahiro) Date: Fri, 22 Aug 2014 09:35:17 +0900 Subject: [PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call In-Reply-To: References: <1408611405-8943-1-git-send-email-takahiro.akashi@linaro.org> <1408611405-8943-3-git-send-email-takahiro.akashi@linaro.org> Message-ID: <53F69045.7010301@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 08/22/2014 02:08 AM, Kees Cook wrote: > On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro > wrote: >> If tracer specifies -1 as a syscall number, this traced system call should >> be skipped with a value in x0 used as a return value. >> This patch enables this semantics, but there is a restriction here: >> >> when syscall(-1) is issued by user, tracer cannot skip this system call >> and modify a return value at syscall entry. >> >> In order to ease this flavor, we need to treat whatever value in x0 as >> a return value, but this might result in a bogus value being returned, >> especially when tracer doesn't do anything at this syscall. >> So we always return ENOSYS instead, while we have another chance to change >> a return value at syscall exit. >> >> 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(). >> >> * syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected >> in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS) >> is not used in this case, we may neglect the case. >> >> Signed-off-by: AKASHI Takahiro >> --- >> arch/arm64/include/asm/ptrace.h | 8 ++++++++ >> arch/arm64/kernel/entry.S | 4 ++++ >> arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++ >> 3 files changed, 32 insertions(+) >> >> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h >> index 501000f..a58cf62 100644 >> --- a/arch/arm64/include/asm/ptrace.h >> +++ b/arch/arm64/include/asm/ptrace.h >> @@ -65,6 +65,14 @@ >> #define COMPAT_PT_TEXT_ADDR 0x10000 >> #define COMPAT_PT_DATA_ADDR 0x10004 >> #define COMPAT_PT_TEXT_END_ADDR 0x10008 >> + >> +/* >> + * used to skip a system call when tracer changes its number to -1 >> + * with ptrace(PTRACE_SET_SYSCALL) >> + */ >> +#define RET_SKIP_SYSCALL -1 >> +#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1) >> + >> #ifndef __ASSEMBLY__ >> >> /* sizeof(struct user) for AArch32 */ >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S >> index f0b5e51..fdd6eae 100644 >> --- a/arch/arm64/kernel/entry.S >> +++ b/arch/arm64/kernel/entry.S >> @@ -25,6 +25,7 @@ >> #include >> #include >> #include >> +#include >> #include >> #include >> >> @@ -671,6 +672,8 @@ ENDPROC(el0_svc) >> __sys_trace: >> mov x0, sp >> bl syscall_trace_enter >> + cmp w0, #RET_SKIP_SYSCALL // skip 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 >> @@ -685,6 +688,7 @@ __sys_trace: >> >> __sys_trace_return: >> str x0, [sp] // save returned x0 >> +__sys_trace_return_skipped: // x0 already in regs[0] >> 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 8876049..c54dbcc 100644 >> --- a/arch/arm64/kernel/ptrace.c >> +++ b/arch/arm64/kernel/ptrace.c >> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs, >> >> asmlinkage int syscall_trace_enter(struct pt_regs *regs) >> { >> + unsigned int saved_syscallno = regs->syscallno; >> + >> if (test_thread_flag(TIF_SYSCALL_TRACE)) >> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER); >> >> + if (IS_SKIP_SYSCALL(regs->syscallno)) { >> + /* >> + * RESTRICTION: we can't modify a return value of user >> + * issued syscall(-1) here. In order to ease this flavor, >> + * we need to treat whatever value in x0 as a return value, >> + * but this might result in a bogus value being returned. >> + */ >> + /* >> + * NOTE: syscallno may also be set to -1 if fatal signal is >> + * detected in tracehook_report_syscall_entry(), but since >> + * a value set to x0 here is not used in this case, we may >> + * neglect the case. >> + */ >> + if (!test_thread_flag(TIF_SYSCALL_TRACE) || >> + (IS_SKIP_SYSCALL(saved_syscallno))) >> + regs->regs[0] = -ENOSYS; >> + } >> + > > I don't have a runtime environment yet for arm64, so I can't test this > directly myself, so I'm just trying to eyeball this. :) > > Once the seccomp logic is added here, I don't think using -2 as a > special value will work. Doesn't this mean the Oops is possible by the > user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and > the user passed -2 as the syscall, audit will be called only on entry, > and then skipped on exit? Oops, you're absolutely right. I didn't think of this case. syscall_trace_enter() should not return a syscallno directly, but always return -1 if syscallno < 0. (except when secure_computing() returns with -1) This also implies that tracehook_report_syscall() should also have a return value. Will, is this fine with you? -Takahiro AKASHI > -Kees > >> if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) >> trace_sys_enter(regs, regs->syscallno); >> >> -- >> 1.7.9.5 >> > > >