s390/uaccess: fix strncpy_from_user/strnlen_user zero maxlen case

If the maximum length specified for the to be accessed string for
strncpy_from_user() and strnlen_user() is zero the following incorrect
values would be returned or incorrect memory accesses would happen:

strnlen_user_std() and strnlen_user_pt() incorrectly return "1"
strncpy_from_user_pt() would incorrectly access "dst[maxlen - 1]"
strncpy_from_user_mvcos() would incorrectly return "-EFAULT"

Fix all these oddities by adding early checks.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

3 files changed, 8 insertions, 0 deletions

diff --git a/arch/s390/lib/uaccess_mvcos.c b/arch/s390/lib/uaccess_mvcos.c
index 7f3eb3d56024..1829742bf479 100644
--- a/arch/s390/lib/uaccess_mvcos.c
+++ b/arch/s390/lib/uaccess_mvcos.c
@@ -184,6 +184,8 @@ static size_t strncpy_from_user_mvcos(size_t count, const char __user *src,
 {
         size_t done, len, offset, len_str;
 
+        if (unlikely(!count))
+                return 0;
         done = 0;
         do {
                 offset = (size_t)src & ~PAGE_MASK;
diff --git a/arch/s390/lib/uaccess_pt.c b/arch/s390/lib/uaccess_pt.c
index a70ee84c0241..c1aaf22c326b 100644
--- a/arch/s390/lib/uaccess_pt.c
+++ b/arch/s390/lib/uaccess_pt.c
@@ -172,6 +172,8 @@ static size_t strnlen_user_pt(size_t count, const char __user *src)
         unsigned long offset, done, len, kaddr;
         size_t len_str;
 
+        if (unlikely(!count))
+                return 0;
         if (segment_eq(get_fs(), KERNEL_DS))
                 return strnlen((const char __kernel __force *) src, count) + 1;
         done = 0;
@@ -202,6 +204,8 @@ static size_t strncpy_from_user_pt(size_t count, const char __user *src,
 {
         size_t n = strnlen_user_pt(count, src);
 
+        if (unlikely(!count))
+                return 0;
         if (!n)
                 return -EFAULT;
         if (n > count)
diff --git a/arch/s390/lib/uaccess_std.c b/arch/s390/lib/uaccess_std.c
index 6fbd06338270..79c6c7d76e08 100644
--- a/arch/s390/lib/uaccess_std.c
+++ b/arch/s390/lib/uaccess_std.c
@@ -188,6 +188,8 @@ size_t strnlen_user_std(size_t size, const char __user *src)
         register unsigned long reg0 asm("0") = 0UL;
         unsigned long tmp1, tmp2;
 
+        if (unlikely(!size))
+                return 0;
         asm volatile(
                 "   la    %2,0(%1)\n"
                 "   la    %3,0(%0,%1)\n"