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Date: Mon, 15 Sep 2014 17:17:47 -0700 From: AKASHI Takahiro <takahiro.akashi@...aro.org> To: Will Deacon <will.deacon@....com> CC: "linux@....linux.org.uk" <linux@....linux.org.uk>, "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>, "eparis@...hat.com" <eparis@...hat.com>, "rgb@...hat.com" <rgb@...hat.com>, "dsaxena@...aro.org" <dsaxena@...aro.org>, "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>, "linux-audit@...hat.com" <linux-audit@...hat.com> Subject: Re: [PATCH v2] arm: prevent BUG_ON in audit_syscall_entry() Will, Sorry for not responding quickly. On 09/11/2014 09:37 AM, Will Deacon wrote: > Hi Akashi, > > On Tue, Sep 09, 2014 at 05:49:59AM +0100, AKASHI Takahiro wrote: >> BUG_ON() in audit_syscall_entry() will be hit if user issues syscall(-1) >> while syscall auditing is enabled (that is, by starting auditd). > > [...] > >> diff --git a/arch/arm/include/asm/traps.h b/arch/arm/include/asm/traps.h >> index f555bb3..de01145 100644 >> --- a/arch/arm/include/asm/traps.h >> +++ b/arch/arm/include/asm/traps.h >> @@ -49,6 +49,7 @@ static inline int in_exception_text(unsigned long ptr) >> extern void __init early_trap_init(void *); >> extern void dump_backtrace_entry(unsigned long where, unsigned long from, unsigned long frame); >> extern void ptrace_break(struct task_struct *tsk, struct pt_regs *regs); >> +extern int arm_syscall(int no, struct pt_regs *regs); >> >> extern void *vectors_page; >> >> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S >> index e52fe5a..28d3931 100644 >> --- a/arch/arm/kernel/entry-common.S >> +++ b/arch/arm/kernel/entry-common.S >> @@ -426,7 +426,6 @@ ENTRY(vector_swi) >> local_restart: >> ldr r10, [tsk, #TI_FLAGS] @ check for syscall tracing >> stmdb sp!, {r4, r5} @ push fifth and sixth args >> - > > You don't need this cosmetic change. Typo. I will fix it. >> tst r10, #_TIF_SYSCALL_WORK @ are we tracing syscalls? >> bne __sys_trace >> >> @@ -476,10 +475,11 @@ __sys_trace: >> cmp scno, #-1 @ skip the syscall? >> bne 2b >> add sp, sp, #S_OFF @ restore stack >> - b ret_slow_syscall >> + b __sys_trace_return_skipped > > Can't you just remove the add as well, them fall-through here? I'm afraid that we can't remove this branch because we don't want to override a value of r0 in regs which a tracer may have already changed while skipping a syscall. >> >> __sys_trace_return: >> str r0, [sp, #S_R0 + S_OFF]! @ save returned r0 >> +__sys_trace_return_skipped: >> mov r0, sp >> bl syscall_trace_exit >> b ret_slow_syscall >> diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c >> index 0c27ed6..68b42cd 100644 >> --- a/arch/arm/kernel/ptrace.c >> +++ b/arch/arm/kernel/ptrace.c >> @@ -930,7 +930,9 @@ static void tracehook_report_syscall(struct pt_regs *regs, >> >> asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno) >> { >> - current_thread_info()->syscall = scno; >> + int orig_scno; >> + >> + current_thread_info()->syscall = orig_scno = scno; >> >> /* Do the secure computing check first; failures should be fast. */ >> if (secure_computing(scno) == -1) >> @@ -941,31 +943,40 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno) >> >> scno = current_thread_info()->syscall; >> >> - if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) >> - trace_sys_enter(regs, scno); >> + if (scno >= 0 && scno < NR_syscalls) { > > Is this supposed to work for OABI? If so, better use __NR_SYSCALL_BASE. Good point. I'm not quite sure how it works for OABI, but looking into entry-comon.S, there is some code: > #if defined(CONFIG_OABI_COMPAT) > bics r10, r10, #0xff000000 > eorne scno, r10, #__NR_OABI_SYSCALL_BASE > ldrne tbl, =sys_oabi_call_table > #elif !defined(CONFIG_AEABI) > bic scno, scno, #0xff000000 @ mask off SWI op-code > eor scno, scno, #__NR_SYSCALL_BASE @ check OS number > #endif > > local_restart: It seems to me that scno, actually r7 in regs, is already offset. >> + if (test_thread_flag(TIF_SYSCALL_TRACEPOINT)) >> + trace_sys_enter(regs, scno); >> + >> + audit_syscall_entry(AUDIT_ARCH_ARM, scno, >> + regs->ARM_r0, regs->ARM_r1, >> + regs->ARM_r2, regs->ARM_r3); >> + } >> >> - audit_syscall_entry(AUDIT_ARCH_ARM, scno, regs->ARM_r0, regs->ARM_r1, >> - regs->ARM_r2, regs->ARM_r3); >> + /* user-issued syscall of -1 */ >> + if (scno == -1 && orig_scno == -1) > > Make this an else if, for clarity? Sure. I will fix it. >> + arm_syscall(scno, regs); > > Doesn't this always result in bad_syscall being called, which sends a SIGILL > to the task? Shouldn't we simply return -ENOSYS instead? You could do that > in the assembly code. I meant so (that is, resulting in bad_syscall). As I mentioned earlier, a task calling syscall(-1, or whatever native value) is always signaled on arm. Meanwhile, whether it is intended or not, this behavior is not simulated in the current arm64 compat syscalls. >> return scno; >> } >> >> asmlinkage void syscall_trace_exit(struct pt_regs *regs) >> { >> - /* >> - * Audit the syscall before anything else, as a debugger may >> - * come in and change the current registers. >> - */ >> - audit_syscall_exit(regs); >> + if (current_thread_info()->syscall < NR_syscalls) { > > Again, not going to work for OABI. The same comment above. Thanks, -Takahiro AKASHI > 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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