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SubjectRe: [PATCH] exec: do not leave bprm->interp on stack

Hello Kees, all,

Please have a look at a *NEW* patch at the end of this mail. It seems to fix
both the issues, stack disclosure + undue recursions.

It uses modprobe "--first-time" option which returns an error code when trying
to load a module which is already present or unload one which is not present.

Checking this return value at - request_module() - and breaking the loop in
case of an error seems to be doing the trick.

+-- On Mon, 19 Nov 2012, Kees Cook wrote --+
| I think to avoid the explosion of request_module calls in the abusive case,
| we could simply return ELOOP instead of ENOEXEC on max recursion.

-> http://www.spinics.net/lists/mm-commits/msg92433.html

1. returning -ELOOP has a side effect of not reaching to request_module()
ever, for:
==
#ifdef CONFIG_MODULES
1415 if (retval != -ENOEXEC || bprm->mm == NULL) {
1416 break;
1417 } else {
...
==

2. above patch does not seem to fix the 2^6(64) recursions issue, for:
==
+ bprm->recursion_depth = depth + 1;
retval = fn(bprm);
bprm->recursion_depth = depth;
==

setting - recursion_dept = depth - again and the outer for(try=0;try<2...)
loop seems to be causing the 2^6 recursions.


Please have a look at this *NEW* patch to fix both the issues, stack
disclosure + undue recursions.

===
diff --git a/fs/exec.c b/fs/exec.c
index 0039055..dec467f 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1423,7 +1423,14 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
break; /* -ENOEXEC */
if (try)
break; /* -ENOEXEC */
- request_module("binfmt-%04x", *(unsigned short *)(&bprm->buf[2]));
+ if (request_module("binfmt-%04x",
+ *(unsigned short *)(&bprm->buf[2])))
+ {
+ printk(KERN_WARNING
+ "request_module: failed to load: binfmt-%04x",
+ *(unsigned short *)(&bprm->buf[2]));
+ break;
+ }
}
#else
break;
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 1c317e3..7ec0e3e 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -83,7 +83,7 @@ static int call_modprobe(char *module_name, int wait)
NULL
};

- char **argv = kmalloc(sizeof(char *[5]), GFP_KERNEL);
+ char **argv = kmalloc(sizeof(char *[6]), GFP_KERNEL);
if (!argv)
goto out;

@@ -93,9 +93,10 @@ static int call_modprobe(char *module_name, int wait)

argv[0] = modprobe_path;
argv[1] = "-q";
- argv[2] = "--";
- argv[3] = module_name; /* check free_modprobe_argv() */
- argv[4] = NULL;
+ argv[2] = "--first-time";
+ argv[3] = "--";
+ argv[4] = module_name; /* check free_modprobe_argv() */
+ argv[5] = NULL;

return call_usermodehelper_fns(modprobe_path, argv, envp,
wait | UMH_KILLABLE, NULL, free_modprobe_argv, NULL);
===


Thank you.
--
Prasad J Pandit / Red Hat Security Response Team
DB7A 84C5 D3F9 7CD1 B5EB C939 D048 7860 3655 602B


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