binfmt_misc.c: avoid potential kernel stack overflow

This can be triggered with root help only, but...

Register the ":text:E::txt::/root/cat.txt:' rule in binfmt_misc (by root) and
try launching the cat.txt file (by anyone) :) The result is - the endless
recursion in the load_misc_binary -> open_exec -> load_misc_binary chain and
stack overflow.

There's a similar problem with binfmt_script, and there's a sh_bang memner on
linux_binprm structure to handle this, but simply raising this in binfmt_misc
may break some setups when the interpreter of some misc binaries is a script.

So the proposal is to turn sh_bang into a bit, add a new one (the misc_bang)
and raise it in load_misc_binary.  After this, even if we set up the misc ->
script -> misc loop for binfmts one of them will step on its own bang and
exit.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 8 insertions, 2 deletions

diff --git a/fs/binfmt_em86.c b/fs/binfmt_em86.c
index f95ae9789c91..f9c88d0c8ced 100644
--- a/fs/binfmt_em86.c
+++ b/fs/binfmt_em86.c
@@ -43,7 +43,7 @@ static int load_em86(struct linux_binprm *bprm,struct pt_regs *regs)
                         return -ENOEXEC;
         }
 
-        bprm->sh_bang++;        /* Well, the bang-shell is implicit... */
+        bprm->sh_bang = 1;      /* Well, the bang-shell is implicit... */
         allow_write_access(bprm->file);
         fput(bprm->file);
         bprm->file = NULL;
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index dbf0ac0523de..7191306367c5 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -115,6 +115,12 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
         if (!enabled)
                 goto _ret;
 
+        retval = -ENOEXEC;
+        if (bprm->misc_bang)
+                goto _ret;
+
+        bprm->misc_bang = 1;
+
         /* to keep locking time low, we copy the interpreter string */
         read_lock(&entries_lock);
         fmt = check_file(bprm);
diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index ab33939b12a7..9e3963f7ebf1 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -29,7 +29,7 @@ static int load_script(struct linux_binprm *bprm,struct pt_regs *regs)
          * Sorta complicated, but hopefully it will work.  -TYT
          */
 
-        bprm->sh_bang++;
+        bprm->sh_bang = 1;
         allow_write_access(bprm->file);
         fput(bprm->file);
         bprm->file = NULL;