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authorKees Cook <keescook@chromium.org>2012-12-20 15:05:16 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-12-20 17:40:19 -0800
commitb66c5984017533316fd1951770302649baf1aa33 (patch)
tree78d1e5fc82a057c62699734602c8e5f7ca86b7a2
parent9f9c9cbb60576a1518d0bf93fb8e499cffccf377 (diff)
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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 <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>
-rw-r--r--fs/binfmt_misc.c5
-rw-r--r--fs/binfmt_script.c4
-rw-r--r--fs/exec.c15
-rw-r--r--include/linux/binfmts.h1
4 files changed, 23 insertions, 2 deletions
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 9be335fb8a7..0c8869fdd14 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -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);
diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index 1610a91637e..5027a3e1492 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -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.
diff --git a/fs/exec.c b/fs/exec.c
index d8e1191cb11..237d5342786 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -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
*/
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index a4c2b565c83..bdf3965f0a2 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -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);