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Date: Wed, 16 Jul 2014 13:21:35 -0700
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: LKML <linux-kernel@...r.kernel.org>,
Will Drewry <wad@...omium.org>,
James Morris <james.l.morris@...cle.com>,
Oleg Nesterov <oleg@...hat.com>,
"x86@...nel.org" <x86@...nel.org>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>, linux-mips@...ux-mips.org,
linux-arch <linux-arch@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>,
Alexei Starovoitov <ast@...mgrid.com>
Subject: Re: [PATCH 5/7] x86: Split syscall_trace_enter into two phases
On Tue, Jul 15, 2014 at 12:32 PM, Andy Lutomirski <luto@...capital.net> wrote:
> This splits syscall_trace_enter into syscall_trace_enter_phase1 and
> syscall_trace_enter_phase2. Only phase 2 has full pt_regs, and only
> phase 2 is permitted to modify any of pt_regs except for orig_ax.
>
> The intent is that phase 1 can be called from the syscall fast path.
>
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
> arch/x86/include/asm/ptrace.h | 5 ++
> arch/x86/kernel/ptrace.c | 139 ++++++++++++++++++++++++++++++++++++------
> 2 files changed, 125 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index 14fd6fd..dcbfb49 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -75,6 +75,11 @@ convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs);
> extern void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
> int error_code, int si_code);
>
> +
> +extern unsigned long syscall_trace_enter_phase1(struct pt_regs *, u32 arch);
> +extern long syscall_trace_enter_phase2(struct pt_regs *, u32 arch,
> + unsigned long phase1_result);
> +
> extern long syscall_trace_enter(struct pt_regs *);
> extern void syscall_trace_leave(struct pt_regs *);
>
> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> index 39296d2..8e05418 100644
> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -1441,13 +1441,111 @@ void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
> force_sig_info(SIGTRAP, &info, tsk);
> }
>
> +static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
> +{
> + if (arch == AUDIT_ARCH_X86_64) {
> + audit_syscall_entry(arch, regs->orig_ax, regs->di,
> + regs->si, regs->dx, regs->r10);
> + } else {
> + audit_syscall_entry(arch, regs->orig_ax, regs->bx,
> + regs->cx, regs->dx, regs->si);
> + }
> +}
> +
> /*
> - * We must return the syscall number to actually look up in the table.
> - * This can be -1L to skip running any syscall at all.
> + * We can return 0 to resume the syscall or anything else to go to phase
> + * 2. If we resume the syscall, we need to put something appropriate in
> + * regs->orig_ax.
> + *
> + * NB: We don't have full pt_regs here, but regs->orig_ax and regs->ax
> + * are fully functional.
> + *
> + * For phase 2's benefit, our return value is:
> + * 0: resume the syscall
> + * 1: go to phase 2; no seccomp phase 2 needed
> + * 2: go to phase 2; pass return value to seccomp
> */
> -long syscall_trace_enter(struct pt_regs *regs)
> +unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
> +{
> + unsigned long ret = 0;
> + u32 work;
> +
> + BUG_ON(regs != task_pt_regs(current));
> +
> + work = ACCESS_ONCE(current_thread_info()->flags) &
> + _TIF_WORK_SYSCALL_ENTRY;
> +
> +#ifdef CONFIG_SECCOMP
> + /*
> + * Do seccomp first -- it should minimize exposure of other
> + * code, and keeping seccomp fast is probably more valuable
> + * than the rest of this.
> + */
> + if (work & _TIF_SECCOMP) {
> + struct seccomp_data sd;
> +
> + sd.arch = arch;
> + sd.nr = regs->orig_ax;
> + sd.instruction_pointer = regs->ip;
> + if (arch == AUDIT_ARCH_X86_64) {
> + sd.args[0] = regs->di;
> + sd.args[1] = regs->si;
> + sd.args[2] = regs->dx;
> + sd.args[3] = regs->r10;
> + sd.args[4] = regs->r8;
> + sd.args[5] = regs->r9;
> + } else {
> + sd.args[0] = regs->bx;
> + sd.args[1] = regs->cx;
> + sd.args[2] = regs->dx;
> + sd.args[3] = regs->si;
> + sd.args[4] = regs->di;
> + sd.args[5] = regs->bp;
> + }
> +
> + BUILD_BUG_ON(SECCOMP_PHASE1_OK != 0);
> + BUILD_BUG_ON(SECCOMP_PHASE1_SKIP != 1);
> +
> + ret = seccomp_phase1(&sd);
> + if (ret == SECCOMP_PHASE1_SKIP) {
> + regs->orig_ax = -ENOSYS;
Before, seccomp didn't touch orig_ax on a skip. I don't see any
problem with this, and it's probably more clear this way, but are you
sure there aren't unexpected side-effects from this?
-Kees
> + ret = 0;
> + } else if (ret != SECCOMP_PHASE1_OK) {
> + return ret; /* Go directly to phase 2 */
> + }
> +
> + work &= ~_TIF_SECCOMP;
> + }
> +#endif
> +
> + /* Do our best to finish without phase 2. */
> + if (work == 0)
> + return ret; /* seccomp only (ret == 0 here) */
> +
> +#ifdef CONFIG_AUDITSYSCALL
> + if (work == _TIF_SYSCALL_AUDIT) {
> + /*
> + * If there is no more work to be done except auditing,
> + * then audit in phase 1. Phase 2 always audits, so, if
> + * we audit here, then we can't go on to phase 2.
> + */
> + do_audit_syscall_entry(regs, arch);
> + return 0;
> + }
> +#endif
> +
> + return 1; /* Something is enabled that we can't handle in phase 1 */
> +}
> +
> +/* Returns the syscall nr to run (which should match regs->orig_ax). */
> +long syscall_trace_enter_phase2(struct pt_regs *regs, u32 arch,
> + unsigned long phase1_result)
> {
> long ret = 0;
> + u32 work = ACCESS_ONCE(current_thread_info()->flags) &
> + _TIF_WORK_SYSCALL_ENTRY;
> +
> + BUG_ON(regs != task_pt_regs(current));
>
> user_exit();
>
> @@ -1458,17 +1556,20 @@ long syscall_trace_enter(struct pt_regs *regs)
> * do_debug() and we need to set it again to restore the user
> * state. If we entered on the slow path, TF was already set.
> */
> - if (test_thread_flag(TIF_SINGLESTEP))
> + if (work & _TIF_SINGLESTEP)
> regs->flags |= X86_EFLAGS_TF;
>
> - /* do the secure computing check first */
> - if (secure_computing()) {
> + /*
> + * Call seccomp_phase2 before running the other hooks so that
> + * they can see any changes made by a seccomp tracer.
> + */
> + if (phase1_result > 1 && seccomp_phase2(phase1_result)) {
> /* seccomp failures shouldn't expose any additional code. */
> ret = -1L;
> goto out;
> }
>
> - if (unlikely(test_thread_flag(TIF_SYSCALL_EMU)))
> + if (unlikely(work & _TIF_SYSCALL_EMU))
> ret = -1L;
>
> if ((ret || test_thread_flag(TIF_SYSCALL_TRACE)) &&
> @@ -1478,23 +1579,23 @@ long syscall_trace_enter(struct pt_regs *regs)
> if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
> trace_sys_enter(regs, regs->orig_ax);
>
> - if (is_ia32_task())
> - audit_syscall_entry(AUDIT_ARCH_I386,
> - regs->orig_ax,
> - regs->bx, regs->cx,
> - regs->dx, regs->si);
> -#ifdef CONFIG_X86_64
> - else
> - audit_syscall_entry(AUDIT_ARCH_X86_64,
> - regs->orig_ax,
> - regs->di, regs->si,
> - regs->dx, regs->r10);
> -#endif
> + do_audit_syscall_entry(regs, arch);
>
> out:
> return ret ?: regs->orig_ax;
> }
>
> +long syscall_trace_enter(struct pt_regs *regs)
> +{
> + u32 arch = is_ia32_task() ? AUDIT_ARCH_I386 : AUDIT_ARCH_X86_64;
> + unsigned long phase1_result = syscall_trace_enter_phase1(regs, arch);
> +
> + if (phase1_result == 0)
> + return regs->orig_ax;
> + else
> + return syscall_trace_enter_phase2(regs, arch, phase1_result);
> +}
> +
> void syscall_trace_leave(struct pt_regs *regs)
> {
> bool step;
> --
> 1.9.3
>
--
Kees Cook
Chrome OS Security
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