exec: binfmt_misc: fix race between load_misc_binary() and kill_node()

load_misc_binary() makes a local copy of fmt->interpreter under
entries_lock to avoid the race with kill_node() but this is not enough;
the whole Node can be freed after we drop entries_lock, not only the
->interpreter string.

Add dget/dput(fmt->dentry) to ensure bm_evict_inode() can't destroy/free
this Node.

Link: http://lkml.kernel.org/r/20170922143650.GA17227@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ben Woodard <woodard@redhat.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jim Foraker <foraker1@llnl.gov>
Cc: Travis Gummels <tgummels@redhat.com>
Cc: <tdhooge@llnl.gov>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index babfe7b..f5f8c25 100644 (file)
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -138,20 +138,23 @@ static int load_misc_binary(struct linux_binprm *bprm)
 
        retval = -ENOEXEC;
        if (!enabled)
-               goto ret;
+               return retval;
 
        /* to keep locking time low, we copy the interpreter string */
        read_lock(&entries_lock);
        fmt = check_file(bprm);
-       if (fmt)
+       if (fmt) {
+               dget(fmt->dentry);
                strlcpy(iname, fmt->interpreter, BINPRM_BUF_SIZE);
+       }
        read_unlock(&entries_lock);
        if (!fmt)
-               goto ret;
+               return retval;
 
        /* Need to be able to load the file after exec */
+       retval = -ENOENT;
        if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
-               return -ENOENT;
+               goto ret;
 
        if (!(fmt->flags & MISC_FMT_PRESERVE_ARGV0)) {
                retval = remove_arg_zero(bprm);
@@ -238,6 +241,7 @@ static int load_misc_binary(struct linux_binprm *bprm)
                goto error;
 
 ret:
+       dput(fmt->dentry);
        return retval;
 error:
        if (fd_binary > 0)