sysctl: refactor sysctl string writing logic

Consolidate buffer length checking with new-line/end-of-line checking.
Additionally, instead of reading user memory twice, just do the
assignment during the loop.

This change doesn't affect the potential races here.  It was already
possible to read a sysctl that was in the middle of a write.  In both
cases, the string will always be NULL terminated.  The pre-existing race
remains a problem to be solved.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 7 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 3e214beabbe9..ac6847feaa83 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1717,21 +1717,18 @@ static int _proc_do_string(char *data, int maxlen, int write,
         }
 
         if (write) {
+                /* Start writing from beginning of buffer. */
                 len = 0;
+                *ppos += *lenp;
                 p = buffer;
-                while (len < *lenp) {
+                while ((p - buffer) < *lenp && len < maxlen - 1) {
                         if (get_user(c, p++))
                                 return -EFAULT;
                         if (c == 0 || c == '\n')
                                 break;
-                        len++;
+                        data[len++] = c;
                 }
-                if (len >= maxlen)
-                        len = maxlen-1;
-                if(copy_from_user(data, buffer, len))
-                        return -EFAULT;
                 data[len] = 0;
-                *ppos += *lenp;
         } else {
                 len = strlen(data);
                 if (len > maxlen)