From 6e5543434e9620e53fe7a7959a870f1e38cc23d0 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 29 Nov 2012 14:18:55 +1100 Subject: [PATCH] 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 b0b70fbea06c..b0c1755b8baa 100644 --- a/fs/binfmt_misc.c +++ b/fs/binfmt_misc.c @@ -176,7 +176,10 @@ 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); diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c index 8c954997e7f7..4834f2ceeab1 100644 --- a/fs/binfmt_script.c +++ b/fs/binfmt_script.c @@ -82,7 +82,9 @@ 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. diff --git a/fs/exec.c b/fs/exec.c index b71b08ce7120..bf5097388148 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 2630c9b41a86..7f0e297b13f9 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); -- 2.39.5