ARM: remove user cmpxchg syscall

Mark Brand reports that a NEEDS_SYSCALL_FOR_CMPXCHG enabled kernel would
open a security hole in the ghost syscall used to implement cmpxchg, as
it fails to validate the user pointer.

However, in order for this option to be enabled, you'd need to be
building a pre-ARMv6 kernel with SMP support.  There is only one system
known which fits that, which is an early ARM SMP FPGA implementation
based on the ARM926T.

In any case, the Kconfig does not allow SMP to be enabled for pre-ARMv6
systems.

Moreover, even if NEEDS_SYSCALL_FOR_CMPXCHG were to be enabled, the
kernel would not build as __ARM_NR_cmpxchg64 is not defined.

The simple answer is to remove the buggy code.
Reported-by: Mark Brand <markbrand@google.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/unistd.h

@@ -21,13 +21,6 @@
  */
 #define __NR_syscalls  (388)
 
-/*
- * *NOTE*: This is a ghost syscall private to the kernel.  Only the
- * __kuser_cmpxchg code in entry-armv.S should be aware of its
- * existence.  Don't ever use this from user code.
- */
-#define __ARM_NR_cmpxchg		(__ARM_NR_BASE+0x00fff0)
-
 #define __ARCH_WANT_STAT64
 #define __ARCH_WANT_SYS_GETHOSTNAME
 #define __ARCH_WANT_SYS_PAUSE

arch/arm/kernel/entry-armv.S

@@ -427,8 +427,7 @@ ENDPROC(__fiq_abt)
 	.endm
 
 	.macro	kuser_cmpxchg_check
-#if !defined(CONFIG_CPU_32v6K) && defined(CONFIG_KUSER_HELPERS) && \
-    !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
+#if !defined(CONFIG_CPU_32v6K) && defined(CONFIG_KUSER_HELPERS)
 #ifndef CONFIG_MMU
 #warning "NPTL on non MMU needs fixing"
 #else
@@ -859,20 +858,7 @@ __kuser_helper_start:
 
 __kuser_cmpxchg64:				@ 0xffff0f60
 
-#if defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
-
-	/*
-	 * Poor you.  No fast solution possible...
-	 * The kernel itself must perform the operation.
-	 * A special ghost syscall is used for that (see traps.c).
-	 */
-	stmfd	sp!, {r7, lr}
-	ldr	r7, 1f			@ it's 20 bits
-	swi	__ARM_NR_cmpxchg64
-	ldmfd	sp!, {r7, pc}
-1:	.word	__ARM_NR_cmpxchg64
-
-#elif defined(CONFIG_CPU_32v6K)
+#if defined(CONFIG_CPU_32v6K)
 
 	stmfd	sp!, {r4, r5, r6, r7}
 	ldrd	r4, r5, [r0]			@ load old val
@@ -948,20 +934,7 @@ __kuser_memory_barrier:				@ 0xffff0fa0
 
 __kuser_cmpxchg:				@ 0xffff0fc0
 
-#if defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
-
-	/*
-	 * Poor you.  No fast solution possible...
-	 * The kernel itself must perform the operation.
-	 * A special ghost syscall is used for that (see traps.c).
-	 */
-	stmfd	sp!, {r7, lr}
-	ldr	r7, 1f			@ it's 20 bits
-	swi	__ARM_NR_cmpxchg
-	ldmfd	sp!, {r7, pc}
-1:	.word	__ARM_NR_cmpxchg
-
-#elif __LINUX_ARM_ARCH__ < 6
+#if __LINUX_ARM_ARCH__ < 6
 
 #ifdef CONFIG_MMU
 

arch/arm/kernel/traps.c

@@ -625,58 +625,6 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 		set_tls(regs->ARM_r0);
 		return 0;
 
-#ifdef CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG
-	/*
-	 * Atomically store r1 in *r2 if *r2 is equal to r0 for user space.
-	 * Return zero in r0 if *MEM was changed or non-zero if no exchange
-	 * happened.  Also set the user C flag accordingly.
-	 * If access permissions have to be fixed up then non-zero is
-	 * returned and the operation has to be re-attempted.
-	 *
-	 * *NOTE*: This is a ghost syscall private to the kernel.  Only the
-	 * __kuser_cmpxchg code in entry-armv.S should be aware of its
-	 * existence.  Don't ever use this from user code.
-	 */
-	case NR(cmpxchg):
-	for (;;) {
-		extern void do_DataAbort(unsigned long addr, unsigned int fsr,
-					 struct pt_regs *regs);
-		unsigned long val;
-		unsigned long addr = regs->ARM_r2;
-		struct mm_struct *mm = current->mm;
-		pgd_t *pgd; pmd_t *pmd; pte_t *pte;
-		spinlock_t *ptl;
-
-		regs->ARM_cpsr &= ~PSR_C_BIT;
-		down_read(&mm->mmap_sem);
-		pgd = pgd_offset(mm, addr);
-		if (!pgd_present(*pgd))
-			goto bad_access;
-		pmd = pmd_offset(pgd, addr);
-		if (!pmd_present(*pmd))
-			goto bad_access;
-		pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
-		if (!pte_present(*pte) || !pte_write(*pte) || !pte_dirty(*pte)) {
-			pte_unmap_unlock(pte, ptl);
-			goto bad_access;
-		}
-		val = *(unsigned long *)addr;
-		val -= regs->ARM_r0;
-		if (val == 0) {
-			*(unsigned long *)addr = regs->ARM_r1;
-			regs->ARM_cpsr |= PSR_C_BIT;
-		}
-		pte_unmap_unlock(pte, ptl);
-		up_read(&mm->mmap_sem);
-		return val;
-
-		bad_access:
-		up_read(&mm->mmap_sem);
-		/* simulate a write access fault */
-		do_DataAbort(addr, 15 + (1 << 11), regs);
-	}
-#endif
-
 	default:
 		/* Calls 9f00xx..9f07ff are defined to return -ENOSYS
 		   if not implemented, rather than raising SIGILL.  This

arch/arm/mm/Kconfig

@@ -419,28 +419,24 @@ config CPU_THUMBONLY
 config CPU_32v3
 	bool
 	select CPU_USE_DOMAINS if MMU
-	select NEEDS_SYSCALL_FOR_CMPXCHG if SMP
 	select NEED_KUSER_HELPERS
 	select TLS_REG_EMUL if SMP || !MMU
 
 config CPU_32v4
 	bool
 	select CPU_USE_DOMAINS if MMU
-	select NEEDS_SYSCALL_FOR_CMPXCHG if SMP
 	select NEED_KUSER_HELPERS
 	select TLS_REG_EMUL if SMP || !MMU
 
 config CPU_32v4T
 	bool
 	select CPU_USE_DOMAINS if MMU
-	select NEEDS_SYSCALL_FOR_CMPXCHG if SMP
 	select NEED_KUSER_HELPERS
 	select TLS_REG_EMUL if SMP || !MMU
 
 config CPU_32v5
 	bool
 	select CPU_USE_DOMAINS if MMU
-	select NEEDS_SYSCALL_FOR_CMPXCHG if SMP
 	select NEED_KUSER_HELPERS
 	select TLS_REG_EMUL if SMP || !MMU
 
@@ -805,14 +801,6 @@ config TLS_REG_EMUL
 	  a few prototypes like that in existence) and therefore access to
 	  that required register must be emulated.
 
-config NEEDS_SYSCALL_FOR_CMPXCHG
-	bool
-	select NEED_KUSER_HELPERS
-	help
-	  SMP on a pre-ARMv6 processor?  Well OK then.
-	  Forget about fast user space cmpxchg support.
-	  It is just not possible.
-
 config NEED_KUSER_HELPERS
 	bool