exec: use -ELOOP for max recursion depth
Kees Cook [Tue, 18 Dec 2012 00:03:20 +0000 (16:03 -0800)]
commit d740269867021faf4ce38a449353d2b986c34a67 upstream.

To avoid an explosion of request_module calls on a chain of abusive
scripts, fail maximum recursion with -ELOOP instead of -ENOEXEC. As soon
as maximum recursion depth is hit, the error will fail all the way back
up the chain, aborting immediately.

This also has the side-effect of stopping the user's shell from attempting
to reexecute the top-level file as a shell script. As seen in the
dash source:

        if (cmd != path_bshell && errno == ENOEXEC) {
                *argv-- = cmd;
                *argv = cmd = path_bshell;
                goto repeat;
        }

The above logic was designed for running scripts automatically that lacked
the "#!" header, not to re-try failed recursion. On a legitimate -ENOEXEC,
things continue to behave as the shell expects.

Additionally, when tracking recursion, the binfmt handlers should not be
involved. The recursion being tracked is the depth of calls through
search_binary_handler(), so that function should be exclusively responsible
for tracking the depth.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/binfmt_em86.c
fs/binfmt_misc.c
fs/binfmt_script.c
fs/exec.c
include/linux/binfmts.h

index 2790c7e..575796a 100644 (file)
@@ -42,7 +42,6 @@ static int load_em86(struct linux_binprm *bprm,struct pt_regs *regs)
                        return -ENOEXEC;
        }
 
-       bprm->recursion_depth++; /* Well, the bang-shell is implicit... */
        allow_write_access(bprm->file);
        fput(bprm->file);
        bprm->file = NULL;
index e172439..87fa90d 100644 (file)
@@ -117,10 +117,6 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
        if (!enabled)
                goto _ret;
 
-       retval = -ENOEXEC;
-       if (bprm->recursion_depth > BINPRM_MAX_RECURSION)
-               goto _ret;
-
        /* to keep locking time low, we copy the interpreter string */
        read_lock(&entries_lock);
        fmt = check_file(bprm);
@@ -200,8 +196,6 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
        if (retval < 0)
                goto _error;
 
-       bprm->recursion_depth++;
-
        retval = search_binary_handler (bprm, regs);
        if (retval < 0)
                goto _error;
index df49d48..8ae4be1 100644 (file)
@@ -22,15 +22,13 @@ static int load_script(struct linux_binprm *bprm,struct pt_regs *regs)
        char interp[BINPRM_BUF_SIZE];
        int retval;
 
-       if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!') ||
-           (bprm->recursion_depth > BINPRM_MAX_RECURSION))
+       if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!'))
                return -ENOEXEC;
        /*
         * This section does the #! interpretation.
         * Sorta complicated, but hopefully it will work.  -TYT
         */
 
-       bprm->recursion_depth++;
        allow_write_access(bprm->file);
        fput(bprm->file);
        bprm->file = NULL;
index 51d8629..e3a7e36 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1389,6 +1389,10 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
        struct linux_binfmt *fmt;
        pid_t old_pid, old_vpid;
 
+       /* This allows 4 levels of binfmt rewrites before failing hard. */
+       if (depth > 5)
+               return -ELOOP;
+
        retval = security_bprm_check(bprm);
        if (retval)
                return retval;
@@ -1413,12 +1417,8 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
                        if (!try_module_get(fmt->module))
                                continue;
                        read_unlock(&binfmt_lock);
+                       bprm->recursion_depth = depth + 1;
                        retval = fn(bprm, regs);
-                       /*
-                        * Restore the depth counter to its starting value
-                        * in this call, so we don't have to rely on every
-                        * load_binary function to restore it on return.
-                        */
                        bprm->recursion_depth = depth;
                        if (retval >= 0) {
                                if (depth == 0) {
index eb53e15..5bab59b 100644 (file)
@@ -68,8 +68,6 @@ struct linux_binprm {
 #define BINPRM_FLAGS_EXECFD_BIT 1
 #define BINPRM_FLAGS_EXECFD (1 << BINPRM_FLAGS_EXECFD_BIT)
 
-#define BINPRM_MAX_RECURSION 4
-
 /* Function parameter for binfmt->coredump */
 struct coredump_params {
        long signr;