exec: do not leave bprm->interp on stack (b66c5984) · Commits · 戴 / test

If a series of scripts are executed, each triggering module loading via unprintable bytes in the script header, kernel stack contents can leak into the command line. Normally execution of binfmt_script and binfmt_misc happens recursively. However, when modules are enabled, and unprintable bytes exist in the bprm->buf, execution will restart after attempting to load matching binfmt modules. Unfortunately, the logic in binfmt_script and binfmt_misc does not expect to get restarted. They leave bprm->interp pointing to their local stack. This means on restart bprm->interp is left pointing into unused stack memory which can then be copied into the userspace argv areas. After additional study, it seems that both recursion and restart remains the desirable way to handle exec with scripts, misc, and modules. As such, we need to protect the changes to interp. This changes the logic to require allocation for any changes to the bprm->interp. To avoid adding a new kmalloc to every exec, the default value is left as-is. Only when passing through binfmt_script or binfmt_misc does an allocation take place. For a proof of concept, see DoTest.sh from: http://www.halfdog.net/Security/2012/LinuxKernelBinfmtScriptStackDataDisclosure/ Signed-off-by:

Kees Cook <keescook@chromium.org> Cc: halfdog <me@halfdog.net> Cc: P J P <ppandit@redhat.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: <stable@vger.kernel.org> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_misc.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -172,7 +172,10 @@ static int load_misc_binary(struct linux_binprm *bprm)
		goto _error;
		bprm->argc ++;

		bprm->interp = iname; /* for binfmt_script */
		/* Update interp in case binfmt_script needs it. */
		retval = bprm_change_interp(iname, bprm);
		if (retval < 0)
		goto _error;

		interp_file = open_exec (iname);
		retval = PTR_ERR (interp_file);

fs/binfmt_script.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -80,7 +80,9 @@ static int load_script(struct linux_binprm *bprm)
		retval = copy_strings_kernel(1, &i_name, bprm);
		if (retval) return retval;
		bprm->argc++;
		bprm->interp = interp;
		retval = bprm_change_interp(interp, bprm);
		if (retval < 0)
		return retval;

		/*
		* OK, now restart the process with the interpreter's dentry.

fs/exec.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -1175,9 +1175,24 @@ void free_bprm(struct linux_binprm *bprm)
		mutex_unlock(&current->signal->cred_guard_mutex);
		abort_creds(bprm->cred);
		}
		/* If a binfmt changed the interp, free it. */
		if (bprm->interp != bprm->filename)
		kfree(bprm->interp);
		kfree(bprm);
		}

		int bprm_change_interp(char interp, struct linux_binprm bprm)
		{
		/* If a binfmt changed the interp, free it first. */
		if (bprm->interp != bprm->filename)
		kfree(bprm->interp);
		bprm->interp = kstrdup(interp, GFP_KERNEL);
		if (!bprm->interp)
		return -ENOMEM;
		return 0;
		}
		EXPORT_SYMBOL(bprm_change_interp);

		/*
		* install the new credentials for this executable
		*/

include/linux/binfmts.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -112,6 +112,7 @@ extern int setup_arg_pages(struct linux_binprm * bprm,
		unsigned long stack_top,
		int executable_stack);
		extern int bprm_mm_init(struct linux_binprm *bprm);
		extern int bprm_change_interp(char interp, struct linux_binprm bprm);
		extern int copy_strings_kernel(int argc, const char const argv,
		struct linux_binprm *bprm);
		extern int prepare_bprm_creds(struct linux_binprm *bprm);

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