Re: binfmts.h MAX_ARG_STRINGS excessive value allows heap spraying

[Leonard den Ottolander <leonard-lists-2Avth2y2NeLyQNdsBcn8aGZHpeb/A1Y/@public.gmane.org>]

--- a/include/uapi/linux/binfmts.h	2016-11-23 21:02:31.000000000 +0100
+++ b/include/uapi/linux/binfmts.h	2017-03-09 01:52:14.716319950 +0100
@@ -9,10 +9,15 @@ struct pt_regs;
  * These are the maximum length and maximum number of strings passed to the
  * execve() system call.  MAX_ARG_STRLEN is essentially random but serves to
  * prevent the kernel from being unduly impacted by misaddressed pointers.
- * MAX_ARG_STRINGS is chosen to fit in a signed 32-bit integer.
+ * MAX_ARG_STRINGS * MAX_ARG_STRLEN should be smaller than the 4GiB
+ *  address space on 32 bit to avoid heap spraying.
+ * MAX_ARG_STRSLEN to the rescue - the MAX_ARG_PAGES concept was there
+ *  for a reason.
+ * We can now safely increase STRINGS * STRLEN beyond 4GiB if need be.
  */
-#define MAX_ARG_STRLEN (PAGE_SIZE * 32)
-#define MAX_ARG_STRINGS 0x7FFFFFFF
+#define MAX_ARG_STRSLEN 262144
+#define MAX_ARG_STRLEN 65536
+#define MAX_ARG_STRINGS 4096
 
 /* sizeof(linux_binprm->buf) */
 #define BINPRM_BUF_SIZE 128
--- a/fs/exec.c	2017-02-20 08:04:43.000000000 +0100
+++ b/fs/exec.c	2017-03-09 01:54:25.931476370 +0100
@@ -459,6 +459,7 @@ static int copy_strings(int argc, struct
 	char *kaddr = NULL;
 	unsigned long kpos = 0;
 	int ret;
+	int total_bytes = 0;
 
 	while (argc-- > 0) {
 		const char __user *str;
@@ -478,6 +479,11 @@ static int copy_strings(int argc, struct
 		if (!valid_arg_len(bprm, len))
 			goto out;
 
+		/* -E2BIG is fine for now */
+		total_bytes += len;
+		if (total_bytes > MAX_ARG_STRSLEN)
+			goto out;
+
 		/* We're going to work our way backwords. */
 		pos = bprm->p;
 		str += len;

I've successfully built a kernel on a system with a kernel using above
values.

As we have now introduced a safeguard (MAX_ARG_STRSLEN capping the total
amount of memory reserved) the values of MAX_ARG_STRLEN and
MAX_ARG_STRINGS can be increased beyond where their multiplication
reaches 2^32.

So if we really want to support lets say users having directories of
128k files we can now safely set MAX_ARG_STRINGS to 131072 and assuming
an average file name length of 32 set MAX_ARG_STRSLEN to 4194304.

Seems a little excessive to me for a default, but it is now safe.

Regards,
Leonard.