Re: Search for patch for kernel stack data disclosure inbinfmt_script during execve

[Kirill A. Shutemov]

On Wed, Aug 22, 2012 at 09:49:35PM +0000, halfdog wrote:
> Got a hint via IRC, that I should not send patch idea for review to
> "generic" list, but to maintainers and last (or relevant) comitters of code.
> 
> http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=commitdiff;h=bf2a9a39639b8b51377905397a5005f444e9a892
> 
> CC to generic just for the records
> 
> halfdog wrote:
> > halfdog wrote:
> >> I'm searching for a patch for linux kernel stack disclosure in 
> >> binfmt_script with crafted interpreter names when CONFIG_MODULES
> >> is active (see [1]).
> > 
> > Please disregard my previous proposal [2], since it did not address
> > the problem directly (referencing local stack frame data from bprm
> > structure) but worked around it. I suspect, that this could increase
> > probability to reintroduce similar bugs.
> > 
> > Opinions on (untested sketch for) second solution: Could someone look
> > on the source code comments and changes in patch to judge, if this is
> > going in the right direction?
> > 
> > 
> > Explanation of patch: Since load_script will start to irreversibly
> > change bprm structures at some point (using stack local data was one
> > of those changes), try to delay this point. Run checks if load_script
> > could be the right handler, if not give other binfmt handlers the
> > chance to do so.
> > 
> > If binfmt_script is the right one, try to load the interpreter
> > (causing bprm modification), if failing make sure that no other binfmt
> > handler has the chance to continue on the now modified bprm data.
> > 
> > CAVEAT: This assumes, that if binfmt_script could handle the load,
> > that it would be the one and only binfmt with that ability, so no
> > other one, e.g. binfmt_misc should have the chance to do so. If this
> > assumption is wrong, leaving binfmt_script would have to rollback all
> > bprm changes (e.g. restore old credentials).
> > 
> > hd
> > 
> > [1]
> > http://www.halfdog.net/Security/2012/LinuxKernelBinfmtScriptStackDataDisclosure/
> > [2] http://lkml.org/lkml/2012/8/18/75

What about (untested):

diff --git a/fs/exec.c b/fs/exec.c
index 574cf4d..ef13850 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1438,7 +1438,8 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
 		}
 		read_unlock(&binfmt_lock);
 #ifdef CONFIG_MODULES
-		if (retval != -ENOEXEC || bprm->mm == NULL) {
+		if (retval != -ENOEXEC || bprm->mm == NULL ||
+				bprm->recursion_depth > BINPRM_MAX_RECURSION) {
 			break;
 		} else {
 #define printable(c) (((c)=='\t') || ((c)=='\n') || (0x20<=(c) && (c)<=0x7e))
-- 
 Kirill A. Shutemov

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