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

[AKASHI Takahiro]

BUG_ON() in audit_syscall_entry() will be hit if user issues syscall(-1)
while syscall auditing is enabled (that is, by starting auditd).

------------[ cut here ]------------
kernel BUG at /home/akashi/arm/armv7/linux/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 ]---

In fact, syscall(-1) just fails (not signaled despite the expectation,
this is another minor bug), but the succeeding syscall hits BUG_ON.

When auditing syscall(-1), audit_syscall_entry() is called anyway, but
audit_syscall_exit() is not called and then 'in_syscall' flag in thread's
audit context is kept on. In this way, audit_syscall_entry() against
the succeeding syscall will see BUG_ON(in_syscall).

This patch fixes this bug by
1) preventing syscall tracing, ftrace tracepoint and audit, from being
   executed in case of invalid or pseudo system calls,
2) handling user-issued syscall(-1) with arm_syscall().

Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
---
 arch/arm/include/asm/traps.h   |    1 +
 arch/arm/kernel/entry-common.S |    4 ++--
 arch/arm/kernel/ptrace.c       |   47 +++++++++++++++++++++++++---------------
 3 files changed, 32 insertions(+), 20 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..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
-
 	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
 
 __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) {
+		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)
+		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

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