Re: [PATCH v2] arm: prevent BUG_ON in audit_syscall_entry()

On 14/10/01, AKASHI Takahiro wrote:

BUG_ON() in audit_syscall_entry() is hit under the following conditions:

1. syscall auditing is enabled (that is, by running auditd).
2. user process issues syscall(-1)

In fact, syscall(-1) just fails (not raising SIGILL, that is an expected behavior if not audited), and the succeeding syscall will hit this BUG_ON.

Ok, this looks related to: https://lkml.org/lkml/2014/6/16/682

Andy has another plan for upstream: https://lkml.org/lkml/2014/6/16/730

Which finally got posted here: https://lkml.org/lkml/2014/9/5/736 v5 0/5 x86: two-phase syscall tracing and seccomp fastpath

This is all x86 stuff, but sounds like it may be a problem across other architectures as well.

Thanks for testing this!

------------[ cut here ]------------ kernel BUG at (...)/kernel/auditsc.c:1534! Internal error: Oops - BUG: 0 [#1] SMP ARM Modules linked in: CPU: 0 PID: 61 Comm: syscall_arm Not tainted 3.17.0-rc4 #60 task: ea0c4380 ti: ea48e000 task.ti: ea48e000 PC is at __audit_syscall_entry+0xe4/0x110 LR is at 0xea0c4380 pc : [<c02b2ae8>] lr : [<ea0c4380>] psr: 20000013 sp : ea48ff68 ip : 00000001 fp : 0000869c r10: 00000200 r9 : ea48e000 r8 : c020f4e4 r7 : 000000c5 r6 : ea48e000 r5 : ea48ffb0 r4 : ea490800 r3 : bef1e718 r2 : 00000001 r1 : 000000c5 r0 : 40000028 Flags: nzCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user Control: 10c5387d Table: 8a4ac059 DAC: 00000015 Process syscall_arm (pid: 61, stack limit = 0xea48e250) Stack: (0xea48ff68 to 0xea490000) ff60: 540e7a69 3ab5e840 00000200 000000c5 ea48ffb0 ea48e000 ff80: 000000c5 c02114bc bef1e718 00000001 b6efdb58 ffffffff 08400000 000000c5 ffa0: c020f4e4 c020f49c b6efdb58 ffffffff 00000001 bef1e718 bef1e718 00000001 ffc0: b6efdb58 ffffffff 08400000 000000c5 00000000 b6f22850 00000008 0000869c ffe0: 000000c5 bef1e704 b6ea315f b6e318e6 20000030 00000001 00000000 00000000 [<c02b2ae8>] (__audit_syscall_entry) from [<c02114bc>] (syscall_trace_enter+0xf0/0x120) [<c02114bc>] (syscall_trace_enter) from [<c020f49c>] (__sys_trace+0xc/0x38) Code: e584500c e5842004 e28dd00c e8bd80f0 (e7f001f2) ---[ end trace 0c4441660aba5692 ]---

Under the current implementation, audit_syscall_entry() is called even against syscall(-1), but audit_syscall_exit() is not. Because 'in_syscall' flag in a current thread's audit context is kept on, next called audit_syscall_entry() will see BUG_ON(in_syscall).

This patch fixes this bug by

1. preventing syscall enter/exit tracing, ftrace tracepoint and audit, from being executed if an invalid (including -1) or pseudo syscall number (starting from __NR_ARM_BASE + 1). Please note that tracehook_report_syscall(PTRACE_SYSCALL_EXIT) is still executed as it should be paired with tracehook_report_syscall(PTRACE_SYSCALL_ENTER).

2. handling user-issued syscall(-1) with arm_syscall() for compatibility if tracing is on. Since UL(-1) > __NR_ARM_BASE - __NR_SYSCALL_BASE, arm_syscall() is called at local_restart in entry-common.S if tracing is off.

Signed-off-by: AKASHI Takahiro takahiro.akashi@linaro.org

arch/arm/include/asm/traps.h | 1 + arch/arm/kernel/entry-common.S | 3 ++- arch/arm/kernel/ptrace.c | 49 +++++++++++++++++++++++++--------------- 3 files changed, 34 insertions(+), 19 deletions(-)

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..4c62324 100644 --- a/arch/arm/kernel/entry-common.S +++ b/arch/arm/kernel/entry-common.S @@ -476,10 +476,11 @@ __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

__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..d458367 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 +943,42 @@ 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) {

• 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);
  

• } else if (scno == -1 && orig_scno == -1) {

• /*
  

•  * For compatibility, we handles user-issued syscall(-1)
  

•  * here with arm_syscall().
  

•  */
  

• arm_syscall(scno, regs);
  

• }

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) {

• /*
  

•  * Audit the syscall before anything else, as a debugger may
  

•  * come in and change the current registers.
  

•  */
  

• audit_syscall_exit(regs);
  

• /*

• * Note that we haven't updated the ->syscall field for the
  

• * current thread. This isn't a problem because it will have
  

• * been set on syscall entry and there hasn't been an opportunity
  

• * for a PTRACE_SET_SYSCALL since then.
  

• */
  

• if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))

• trace_sys_exit(regs, regs_return_value(regs));
  

• /*
  

•  * Note that we haven't updated the ->syscall field for the
  

•  * current thread. This isn't a problem because it will have
  

•  * been set on syscall entry and there hasn't been
  

•  * an opportunity for a PTRACE_SET_SYSCALL since then.
  

•  */
  

• if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
  

• 	trace_sys_exit(regs, regs_return_value(regs));
  

• }

if (test_thread_flag(TIF_SYSCALL_TRACE)) tracehook_report_syscall(regs, PTRACE_SYSCALL_EXIT); -- 1.7.9.5

- RGB

-- Richard Guy Briggs rbriggs@redhat.com Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat Remote, Ottawa, Canada Voice: +1.647.777.2635, Internal: (81) 32635, Alt: +1.613.693.0684x3545