s390/uaccess: test if current->mm is set before walking page tables

If get_fs() == USER_DS we better test if current->mm is not zero before
walking page tables.
The page table walk code would try to lock mm->page_table_lock, however
if mm is zero this might crash.

Now it is arguably incorrect trying to access userspace if current->mm
is zero, however we have seen that and s390 would be the only architecture
which would crash in such a case.
So we better make the page table walk code a bit more robust and report
always a fault instead.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/s390/lib/uaccess_pt.c b/arch/s390/lib/uaccess_pt.c
index 0632dc50da78..5b77c341a1e3 100644
--- a/arch/s390/lib/uaccess_pt.c
+++ b/arch/s390/lib/uaccess_pt.c
@@ -153,6 +153,8 @@ static __always_inline size_t __user_copy_pt(unsigned long uaddr, void *kptr,
         unsigned long offset, done, size, kaddr;
         void *from, *to;
 
+        if (!mm)
+                return n;
         done = 0;
 retry:
         spin_lock(&mm->page_table_lock);
@@ -262,6 +264,8 @@ static size_t strnlen_user_pt(size_t count, const char __user *src)
                 return 0;
         if (segment_eq(get_fs(), KERNEL_DS))
                 return strnlen_kernel(count, src);
+        if (!mm)
+                return 0;
         done = 0;
 retry:
         spin_lock(&mm->page_table_lock);
@@ -323,6 +327,8 @@ static size_t copy_in_user_pt(size_t n, void __user *to,
 
         if (segment_eq(get_fs(), KERNEL_DS))
                 return copy_in_kernel(n, to, from);
+        if (!mm)
+                return n;
         done = 0;
 retry:
         spin_lock(&mm->page_table_lock);
@@ -411,6 +417,8 @@ int futex_atomic_op_pt(int op, u32 __user *uaddr, int oparg, int *old)
 
         if (segment_eq(get_fs(), KERNEL_DS))
                 return __futex_atomic_op_pt(op, uaddr, oparg, old);
+        if (unlikely(!current->mm))
+                return -EFAULT;
         spin_lock(&current->mm->page_table_lock);
         uaddr = (u32 __force __user *)
                 __dat_user_addr((__force unsigned long) uaddr, 1);
@@ -448,6 +456,8 @@ int futex_atomic_cmpxchg_pt(u32 *uval, u32 __user *uaddr,
 
         if (segment_eq(get_fs(), KERNEL_DS))
                 return __futex_atomic_cmpxchg_pt(uval, uaddr, oldval, newval);
+        if (unlikely(!current->mm))
+                return -EFAULT;
         spin_lock(&current->mm->page_table_lock);
         uaddr = (u32 __force __user *)
                 __dat_user_addr((__force unsigned long) uaddr, 1);