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From b66c5984017533316fd1951770302649baf1aa33 Mon Sep 17 00:00:00 2001 |
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From: Kees Cook <keescook@chromium.org> |
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Date: Thu, 20 Dec 2012 15:05:16 -0800 |
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Subject: exec: do not leave bprm->interp on stack |
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|
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From: Kees Cook <keescook@chromium.org> |
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|
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commit b66c5984017533316fd1951770302649baf1aa33 upstream. |
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|
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If a series of scripts are executed, each triggering module loading via |
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unprintable bytes in the script header, kernel stack contents can leak |
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into the command line. |
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|
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Normally execution of binfmt_script and binfmt_misc happens recursively. |
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However, when modules are enabled, and unprintable bytes exist in the |
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bprm->buf, execution will restart after attempting to load matching |
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binfmt modules. Unfortunately, the logic in binfmt_script and |
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binfmt_misc does not expect to get restarted. They leave bprm->interp |
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pointing to their local stack. This means on restart bprm->interp is |
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left pointing into unused stack memory which can then be copied into the |
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userspace argv areas. |
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|
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After additional study, it seems that both recursion and restart remains |
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the desirable way to handle exec with scripts, misc, and modules. As |
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such, we need to protect the changes to interp. |
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|
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This changes the logic to require allocation for any changes to the |
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bprm->interp. To avoid adding a new kmalloc to every exec, the default |
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value is left as-is. Only when passing through binfmt_script or |
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binfmt_misc does an allocation take place. |
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|
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For a proof of concept, see DoTest.sh from: |
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|
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http://www.halfdog.net/Security/2012/LinuxKernelBinfmtScriptStackDataDisclosure/ |
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|
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Signed-off-by: Kees Cook <keescook@chromium.org> |
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Cc: halfdog <me@halfdog.net> |
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Cc: P J P <ppandit@redhat.com> |
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Cc: Alexander Viro <viro@zeniv.linux.org.uk> |
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Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
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|
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--- |
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fs/binfmt_misc.c | 5 ++++- |
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fs/binfmt_script.c | 4 +++- |
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fs/exec.c | 15 +++++++++++++++ |
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include/linux/binfmts.h | 1 + |
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4 files changed, 23 insertions(+), 2 deletions(-) |
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|
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--- a/fs/binfmt_misc.c |
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+++ b/fs/binfmt_misc.c |
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@@ -176,7 +176,10 @@ static int load_misc_binary(struct linux |
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goto _error; |
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bprm->argc ++; |
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|
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- bprm->interp = iname; /* for binfmt_script */ |
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+ /* Update interp in case binfmt_script needs it. */ |
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+ retval = bprm_change_interp(iname, bprm); |
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+ if (retval < 0) |
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+ goto _error; |
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|
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interp_file = open_exec (iname); |
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retval = PTR_ERR (interp_file); |
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--- a/fs/binfmt_script.c |
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+++ b/fs/binfmt_script.c |
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@@ -82,7 +82,9 @@ static int load_script(struct linux_binp |
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retval = copy_strings_kernel(1, &i_name, bprm); |
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if (retval) return retval; |
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bprm->argc++; |
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- bprm->interp = interp; |
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+ retval = bprm_change_interp(interp, bprm); |
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+ if (retval < 0) |
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+ return retval; |
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|
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/* |
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* OK, now restart the process with the interpreter's dentry. |
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--- a/fs/exec.c |
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+++ b/fs/exec.c |
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@@ -1206,9 +1206,24 @@ void free_bprm(struct linux_binprm *bprm |
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mutex_unlock(¤t->signal->cred_guard_mutex); |
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abort_creds(bprm->cred); |
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} |
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+ /* If a binfmt changed the interp, free it. */ |
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+ if (bprm->interp != bprm->filename) |
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+ kfree(bprm->interp); |
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kfree(bprm); |
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} |
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|
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+int bprm_change_interp(char *interp, struct linux_binprm *bprm) |
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+{ |
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+ /* If a binfmt changed the interp, free it first. */ |
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+ if (bprm->interp != bprm->filename) |
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+ kfree(bprm->interp); |
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+ bprm->interp = kstrdup(interp, GFP_KERNEL); |
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+ if (!bprm->interp) |
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+ return -ENOMEM; |
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+ return 0; |
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+} |
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+EXPORT_SYMBOL(bprm_change_interp); |
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+ |
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/* |
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* install the new credentials for this executable |
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*/ |
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--- a/include/linux/binfmts.h |
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+++ b/include/linux/binfmts.h |
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@@ -128,6 +128,7 @@ extern int setup_arg_pages(struct linux_ |
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unsigned long stack_top, |
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int executable_stack); |
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extern int bprm_mm_init(struct linux_binprm *bprm); |
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+extern int bprm_change_interp(char *interp, struct linux_binprm *bprm); |
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extern int copy_strings_kernel(int argc, const char *const *argv, |
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struct linux_binprm *bprm); |
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extern int prepare_bprm_creds(struct linux_binprm *bprm); |
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