From fee5e57d00c55a1a7b3415b6875cb2a0dd00a906 Mon Sep 17 00:00:00 2001 From: Josh Boyer Date: Mon, 26 Nov 2012 09:03:26 -0500 Subject: CVE-2012-4530: stack disclosure binfmt_script load_script (rhbz 868285 880147) --- exec-do-not-leave-bprm-interp-on-stack.patch | 118 +++++++++++++++++++++++++++ 1 file changed, 118 insertions(+) create mode 100644 exec-do-not-leave-bprm-interp-on-stack.patch (limited to 'exec-do-not-leave-bprm-interp-on-stack.patch') diff --git a/exec-do-not-leave-bprm-interp-on-stack.patch b/exec-do-not-leave-bprm-interp-on-stack.patch new file mode 100644 index 00000000..5198824e --- /dev/null +++ b/exec-do-not-leave-bprm-interp-on-stack.patch @@ -0,0 +1,118 @@ +From 6752ab4cb863fc63ed85f1ca78a42235c09fad83 Mon Sep 17 00:00:00 2001 +From: Kees Cook +Date: Mon, 26 Nov 2012 09:07:50 -0500 +Subject: [PATCH 1/2] exec: do not leave bprm->interp on stack + +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 +Cc: +Signed-off-by: Andrew Morton +--- + 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 790b3cd..772428d 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 d3b8c1f..df49d48 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 0039055..c6e6de4 100644 +--- a/fs/exec.c ++++ b/fs/exec.c +@@ -1175,9 +1175,24 @@ 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 cfcc6bf..de0628e 100644 +--- a/include/linux/binfmts.h ++++ b/include/linux/binfmts.h +@@ -114,6 +114,7 @@ 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); +-- +1.8.0 + -- cgit