Re: [PATCH] binfmt_misc.c: avoid potential kernel stack overflow

[Pavel Emelyanov <xemul@openvz.org>]

(Put lkml in Cc. The original message is beyond)

Oops! My fault. The problem is that in case of modularized binfmt,
the appropriate binary handler gets registered _before_ the script
one and sets the misc_bang flag even too early.

Thus when we launch a script the load_misc_binary sets this bang,
then returns error, since the binary is actually a script, then the
load_script_binary successfully loads the script, then it loads the
misc binary again, which exits with the -ENOEXEC error due to bang 
set.

This patch helped my box, what about yours?

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 7562053..8d7e88e 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -120,8 +120,6 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 	if (bprm->misc_bang)
 		goto _ret;
 
-	bprm->misc_bang = 1;
-
 	/* to keep locking time low, we copy the interpreter string */
 	read_lock(&entries_lock);
 	fmt = check_file(bprm);
@@ -199,6 +197,8 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 	if (retval < 0)
 		goto _error;
 
+	bprm->misc_bang = 1;
+
 	retval = search_binary_handler (bprm, regs);
 	if (retval < 0)
 		goto _error;

> On Tue, Apr 29, 2008 at 12:59:24AM -0700, Pavel Emelyanov wrote:
>> This can be triggered with root help only, but...
>>
>> Register the ":text:E::txt::/root/cat.txt:' rule in binfmt_misc (by root) and
>> try launching the cat.txt file (by anyone) :) The result is - the endless
>> recursion in the load_misc_binary -> open_exec -> load_misc_binary chain and
>> stack overflow.
>>
>> There's a similar problem with binfmt_script, and there's a sh_bang memner on
>> linux_binprm structure to handle this, but simply raising this in binfmt_misc
>> may break some setups when the interpreter of some misc binaries is a script.
>>
>> So the proposal is to turn sh_bang into a bit, add a new one (the misc_bang)
>> and raise it in load_misc_binary.  After this, even if we set up the misc ->
>> script -> misc loop for binfmts one of them will step on its own bang and
>> exit.
> 
> This patch causes problem in some cases, if the kernel compiled with 
> CONFIG_BINFMT_MISC=m:
> 
> $ pwd
> /tmp/chroot
> $ cat test0.c 
> #include <stdio.h>
> 
> int main(void)
> {
>         printf("test\n");
>         return 0;
> }
> $ gcc test0.c -o test0 -static
> $ sudo sh -c 'echo ":test:M::123::/test0:" > /proc/sys/fs/binfmt_misc/register'
> $ cat test1
> 123
> $ cat test2
> #!/test1
> $ sudo chroot /tmp/chroot /test2
> chroot: cannot run command `/test2': No such file or directory
> $ sudo strace chroot /tmp/chroot /test2
> ...
> execve("/test2", ["/test2"], [/* 54 vars */]) = -1 ENOEXEC (Exec format error)
> ...
> 
> This test works fine if I revert the patch or compile with CONFIG_BINFMT_MISC=y.
>