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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
>
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