Re: [patch] remove MNT_NOEXEC check for PROT_EXEC mmaps

[Stas Sergeev]

Hi Guys.

Noone provided a reason for (or even tried to defend)
the fact that PROT_EXEC is rejected by "noexec" even
for MAP_PRIVATE, while, say, PROT_WRITE is *not* rejected
for "ro" filesystem for MAP_PRIVATE. What was argued is
only MAP_SHARED.
I take it as an indication that people basically agree
that "noexec" should not affect MAP_PRIVATE like that.
So how about the patch (like the attached one) which
changes only the behaveour of MAP_PRIVATE?

Reasons:
- MAP_PRIVATE should not behave like that, "ro" and PROT_WRITE
is a witness.
- This is not a security check - file-backed MAP_PRIVATE
can just be replaced with MAP_PRIVATE | MAP_ANONYMOUS
mmap and read().
- The programs (like AFAIK wine) use MAP_PRIVATE mmaps to
access the windows dlls, which are usually on a "noexec"
fat or ntfs partitions. Wine might be smart enough not to
break but fallback to read(), but this is slower and more
memory-consuming. Some other program may not be that smart
and break. So there is clearly a need for MAP_PRIVATE with
PROT_EXEC on the noexec partitions.

Please let me know why this patch should not be applied.

--- a/mm/mmap.c	2006-01-25 15:02:24.000000000 +0300
+++ b/mm/mmap.c	2006-09-21 13:19:15.000000000 +0400
@@ -900,7 +900,7 @@
 		if (!file->f_op || !file->f_op->mmap)
 			return -ENODEV;
 
-		if ((prot & PROT_EXEC) &&
+		if ((flags & MAP_SHARED) && (prot & PROT_EXEC) &&
 		    (file->f_vfsmnt->mnt_flags & MNT_NOEXEC))
 			return -EPERM;
 	}
@@ -911,7 +911,8 @@
 	 *  mounted, in which case we dont add PROT_EXEC.)
 	 */
 	if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
-		if (!(file && (file->f_vfsmnt->mnt_flags & MNT_NOEXEC)))
+		if (!(file && (flags & MAP_SHARED) &&
+				(file->f_vfsmnt->mnt_flags & MNT_NOEXEC)))
 			prot |= PROT_EXEC;
 
 	if (!len)
--- a/mm/nommu.c	2006-04-12 09:37:34.000000000 +0400
+++ b/mm/nommu.c	2006-09-21 13:21:32.000000000 +0400
@@ -495,7 +495,7 @@
 
 		/* handle executable mappings and implied executable
 		 * mappings */
-		if (file->f_vfsmnt->mnt_flags & MNT_NOEXEC) {
+		if ((flags & MAP_SHARED) && file->f_vfsmnt->mnt_flags & MNT_NOEXEC) {
 			if (prot & PROT_EXEC)
 				return -EPERM;
 		}