From 28278b332bc73f45189939a6c1797cba0eb5879f Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 20 Dec 2012 15:05:16 -0800 Subject: [PATCH] exec: do not leave bprm->interp on stack commit b66c5984017533316fd1951770302649baf1aa33 upstream. 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 Cc: halfdog Cc: P J P Cc: Alexander Viro Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- fs/binfmt_misc.c | 5 ++++- fs/binfmt_script.c | 4 +++- fs/exec.c | 15 +++++++++++++++ include/linux/binfmts.h | 1 + 4 files changed, 23 insertions(+), 2 deletions(-) diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c index 1befe2ec8186..54639527dc43 100644 --- a/fs/binfmt_misc.c +++ b/fs/binfmt_misc.c @@ -176,7 +176,10 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs) 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); diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c index 396a9884591f..e39c18afe48d 100644 --- a/fs/binfmt_script.c +++ b/fs/binfmt_script.c @@ -82,7 +82,9 @@ static int load_script(struct linux_binprm *bprm,struct pt_regs *regs) 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. diff --git a/fs/exec.c b/fs/exec.c index 044c13ffdc42..08f3e4e4b614 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1192,9 +1192,24 @@ void free_bprm(struct linux_binprm *bprm) mutex_unlock(¤t->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 */ diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h index 8845613fd7e3..384e37f6834f 100644 --- a/include/linux/binfmts.h +++ b/include/linux/binfmts.h @@ -126,6 +126,7 @@ 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); -- 2.34.1