Allow recursion in binfmt_script and binfmt_misc

binfmt_script and binfmt_misc disallow recursion to avoid stack overflow
using sh_bang and misc_bang.  It causes problem in some cases:

$ echo '#!/bin/ls' > /tmp/t0
$ echo '#!/tmp/t0' > /tmp/t1
$ echo '#!/tmp/t1' > /tmp/t2
$ chmod +x /tmp/t*
$ /tmp/t2
zsh: exec format error: /tmp/t2

Similar problem with binfmt_misc.

This patch introduces field 'recursion_depth' into struct linux_binprm to
track recursion level in binfmt_misc and binfmt_script.  If recursion
level more then BINPRM_MAX_RECURSION it generates -ENOEXEC.

[akpm@linux-foundation.org: make linux_binprm.recursion_depth a uint]
Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Pavel Emelyanov <xemul@openvz.org>
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>

1 files changed, 3 insertions, 2 deletions

diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index 9e3963f7ebf1..08343505e184 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -22,14 +22,15 @@ 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->sh_bang)) 
+        if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!') ||
+            (bprm->recursion_depth > BINPRM_MAX_RECURSION))
                 return -ENOEXEC;
         /*
          * This section does the #! interpretation.
          * Sorta complicated, but hopefully it will work.  -TYT
          */
 
-        bprm->sh_bang = 1;
+        bprm->recursion_depth++;
         allow_write_access(bprm->file);
         fput(bprm->file);
         bprm->file = NULL;