Re: [patch] remove MNT_NOEXEC check for PROT_EXEC mmaps

[Stas Sergeev]

Hi.

Jakub Jelinek wrote:
> Even assuming ld.so would be hacked up so that it parses /proc/mounts
> to see if you are trying to run an executable via ld.so from
> noexec mount (which isn't going to happen),
No, the solution I wanted to evaluate, is "chmod 'go-x' ld.so".
For that to work, something like this is needed:
http://uwsg.ucs.indiana.edu/hypermail/linux/kernel/0609.3/2322.html
Then you can't invoke ld.so directly and if you happen to
have "noexec" on all the writeable mounts, then you can't
also use your own ld.so.

> if mmap with PROT_EXEC
> is allowed on noexec mounts, you can always put there a shared
> library instead of a binary and put some interesting stuff in its
> constructors and then just LD_PRELOAD=/dev/shm/libmyhack.so /bin/true
Of course if ld.so would check /proc/mounts, then it will
do so also for the shared libs, so LD_PRELOAD won't trick it.
I understand that parsing /proc/mounts is silly (I have
admitted that earlier in that thread already), but why not to
at least check the access(X_OK)? ld.so _must_ check access(X_OK)
before executing - why not yet?
Oh wait, access(X_OK) doesn't seem to work...
The attached patch is needed to get it working.
Does the patch look good? I think it was just a bug.

> Really, if noexec is supposed to make any sense at all, it needs
> to prevent PROT_EXEC mapping/mprotect, otherwise it is completely
> useless.
Why not having an exec perm on a file doesn't prevent PROT_EXEC then?

In any case, guys, can the attached patch be applied?
Arjan, it enforces "noexec", just as you wanted to see. :)
Please say "no" now, not when I mail it to Andrew, if possible.

--- a/fs/namei.c	2006-08-29 14:15:47.000000000 +0400
+++ b/fs/namei.c	2006-10-04 11:28:52.000000000 +0400
@@ -249,9 +249,11 @@
 
 	/*
 	 * MAY_EXEC on regular files requires special handling: We override
-	 * filesystem execute permissions if the mode bits aren't set.
+	 * filesystem execute permissions if the mode bits aren't set or
+	 * the fs is mounted with the "noexec" flag.
 	 */
-	if ((mask & MAY_EXEC) && S_ISREG(mode) && !(mode & S_IXUGO))
+	if ((mask & MAY_EXEC) && S_ISREG(mode) && (!(mode & S_IXUGO) ||
+			(nd && nd->mnt && (nd->mnt->mnt_flags & MNT_NOEXEC))))
 		return -EACCES;
 
 	/* Ordinary permission routines do not understand MAY_APPEND. */