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Message-ID: <505CBCD9.7060908@xenotime.net>
Date: Fri, 21 Sep 2012 12:15:37 -0700
From: Randy Dunlap <rdunlap@...otime.net>
To: halfdog <me@...fdog.net>
CC: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix kernel stack data disclosure in binfmt_script during
execve
On 09/20/2012 09:05 AM, halfdog wrote:
> halfdog wrote:
>
> Now this is the updated and also tested patch (vs. linux-3.5.4 kernel) to fix
> https://bugzilla.kernel.org/show_bug.cgi?id=46841 . See also
> http://www.halfdog.net/Security/2012/LinuxKernelBinfmtScriptStackDataDisclosure/
> This patch adresses the stack data disclosure but does not deal with the
> excessive recursion (to be handled in separate patch if needed).
>
> --- fs/binfmt_script.c 2012-09-14 22:28:08.000000000 +0000
> +++ fs/binfmt_script.c 2012-09-20 16:01:58.951942355 +0000
Incorrect diff/patch format for kernel patches.
It should be apply-able by using 'patch -p1'.
Oh, the patch is not signed off.
And Documentation/SubmittingPatches says signoffs are:
"using your real name (sorry, no pseudonyms or anonymous contributions.)"
There are also some kernel coding style issues that should be
fixed if this patch is to be merged.
> @@ -14,12 +14,24 @@
> #include <linux/err.h>
> #include <linux/fs.h>
>
> +/** Check if this handler is suitable to load the "binary" identified
/** means kernel-doc notation in the kernel, but this comment
block is not kernel-doc, so don't start it with /**
> + * by first BINPRM_BUF_SIZE bytes in bprm->buf.
> + * @returns -ENOEXEC if this handler is not suitable for that type
We don't use "@returns", just returns: <text>.
> + * of binary. In that case, the handler must not modify any of the
> + * data associated with bprm.
> + * Any error if the binary should have been handled by this loader
> + * but handling failed. In that case. FIXME: be defensive? also
> + * kill bprm->mm or bprm->file also to make it impossible, that
> + * upper search_binary_handler can continue handling?
> + * 0 (OK) otherwise, the new executable is ready in bprm->mm.
> + */
> static int load_script(struct linux_binprm *bprm,struct pt_regs *regs)
> {
> const char *i_arg, *i_name;
> char *cp;
> struct file *file;
> - char interp[BINPRM_BUF_SIZE];
> + char bprm_buf_copy[BINPRM_BUF_SIZE];
> + const char *bprm_old_interp_name;
> int retval;
>
> if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!') ||
> @@ -30,25 +42,29 @@ static int load_script(struct linux_binp
> * Sorta complicated, but hopefully it will work. -TYT
> */
>
> - bprm->recursion_depth++;
> - allow_write_access(bprm->file);
> - fput(bprm->file);
> - bprm->file = NULL;
> + /* Keep bprm unchanged until we known, that this is a script
> + * to be handled by this loader. Copy bprm->buf for sure,
> + * otherwise returning -ENOEXEC will make other handlers see
> + * modified data. (hd)
> + */
Kernel multi-line comment style is
/*
* line 1
* line 2
*/
> + memcpy(bprm_buf_copy, bprm->buf, BINPRM_BUF_SIZE);
>
> - bprm->buf[BINPRM_BUF_SIZE - 1] = '\0';
> - if ((cp = strchr(bprm->buf, '\n')) == NULL)
> - cp = bprm->buf+BINPRM_BUF_SIZE-1;
> + bprm_buf_copy[BINPRM_BUF_SIZE - 1]='\0';
> + if ((cp = strchr(bprm_buf_copy, '\n')) == NULL)
> + cp = bprm_buf_copy+BINPRM_BUF_SIZE-1;
> *cp = '\0';
> - while (cp > bprm->buf) {
> + while (cp > bprm_buf_copy) {
> cp--;
> if ((*cp == ' ') || (*cp == '\t'))
> *cp = '\0';
> else
> break;
> }
> - for (cp = bprm->buf+2; (*cp == ' ') || (*cp == '\t'); cp++);
> + for (cp = bprm_buf_copy+2; (*cp == ' ') || (*cp == '\t'); cp++);
> if (*cp == '\0')
> - return -ENOEXEC; /* No interpreter name found */
> + /* No interpreter name found. No problem to let other handlers
> + * retry, we did not change anything. */
multi-line comment style.
> + return -ENOEXEC;
> i_name = cp;
> i_arg = NULL;
> for ( ; *cp && (*cp != ' ') && (*cp != '\t'); cp++)
> @@ -57,45 +73,84 @@ static int load_script(struct linux_binp
> *cp++ = '\0';
> if (*cp)
> i_arg = cp;
> - strcpy (interp, i_name);
> +
> + /* So this is our point-of-no-return: modification of bprm
> + * will be irreversible, so if we fail to setup execution
> + * using the new interpreter name (i_name), we have to make
> + * sure, that no other handler tries again. (hd)
> + */
ditto.
> +
> /*
> * OK, we've parsed out the interpreter name and
> * (optional) argument.
> * Splice in (1) the interpreter's name for argv[0]
> - * (2) (optional) argument to interpreter
> - * (3) filename of shell script (replace argv[0])
> + * (2) (optional) argument to interpreter
> + * (3) filename of shell script (replace argv[0])
> *
> * This is done in reverse order, because of how the
> * user environment and arguments are stored.
> */
> +
> + /* Ugly: we store pointer to local stack frame in bprm,
> + * so make sure to clear this up before returning.
> + */
ditto.
> + bprm_old_interp_name = bprm->interp;
> + bprm->interp = i_name;
> +
> retval = remove_arg_zero(bprm);
> - if (retval)
> - return retval;
> - retval = copy_strings_kernel(1, &bprm->interp, bprm);
> - if (retval < 0) return retval;
> + if (retval) goto out;
Really should be
if (retval)
goto out;
> + /* copy_strings_kernel is ok here, even when racy: since no
> + * user can be attached to new mm, there is nobody to race
> + * with and call is safe for now. The return code of
> + * copy_strings_kernel cannot be -ENOEXEC in any case,
> + * so no special checks needed. (hd)
> + */
style.
> + retval = copy_strings_kernel(1, &bprm_old_interp_name, bprm);
> + if (retval < 0) goto out;
> bprm->argc++;
> if (i_arg) {
> retval = copy_strings_kernel(1, &i_arg, bprm);
> - if (retval < 0) return retval;
> + if (retval < 0) goto out;
style.
> bprm->argc++;
> }
> - retval = copy_strings_kernel(1, &i_name, bprm);
> - if (retval) return retval;
> + retval = copy_strings_kernel(1, &bprm->interp, bprm);
> + if (retval) goto out;
style. (but Al can override these if he wants to)
> bprm->argc++;
> - bprm->interp = interp;
>
> /*
> * OK, now restart the process with the interpreter's dentry.
> + * Release old file first
indentation mucked up.
> */
> - file = open_exec(interp);
> - if (IS_ERR(file))
> - return PTR_ERR(file);
> -
> + allow_write_access(bprm->file);
> + fput(bprm->file);
> + bprm->file = NULL;
> + file = open_exec(bprm->interp);
> + if (IS_ERR(file)) {
> + retval=PTR_ERR(file);
> + goto out;
> + }
> bprm->file = file;
> + /* Caveat: This also updates the credentials of the next exec. */
> retval = prepare_binprm(bprm);
> if (retval < 0)
> - return retval;
> - return search_binary_handler(bprm,regs);
> + goto out;
> + bprm->recursion_depth++;
> + retval=search_binary_handler(bprm,regs);
> +
> +out: /* Make sure, we do not return local stack frame data. If
> + * it would be needed after returning, we would have needed
> + * to allocate memory or use copy from new bprm->mm anyway. (hd)
> + */
Comment block probably should come before the label.
and the indentation is mucked up.
> + bprm->interp = bprm_old_interp_name;
> + if(!retval) {
> + /* The handlers for starting of interpreter failed.
> + * bprm is already modified, hence we are dead here.
> + * Make sure, that we do not return -ENOEXEC, that would
> + * allow searching for handlers to continue. (hd).
> + */
style
> + if(retval==-ENOEXEC) retval=-EINVAL;
missing space before '('.
etc.
> + }
> + return(retval);
> }
>
> static struct linux_binfmt script_format = {
>
--
~Randy
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