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>

4 files changed, 8 insertions, 5 deletions

diff --git a/fs/binfmt_em86.c b/fs/binfmt_em86.c
index f9c88d0c8ced..32fb00b52cd0 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 = 1;      /* Well, the bang-shell is implicit... */
+        bprm->recursion_depth++; /* 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 8d7e88e02e0f..f2744ab4e5b3 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -117,7 +117,7 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
                 goto _ret;
 
         retval = -ENOEXEC;
-        if (bprm->misc_bang)
+        if (bprm->recursion_depth > BINPRM_MAX_RECURSION)
                 goto _ret;
 
         /* to keep locking time low, we copy the interpreter string */
@@ -197,7 +197,7 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
         if (retval < 0)
                 goto _error;
 
-        bprm->misc_bang = 1;
+        bprm->recursion_depth++;
 
         retval = search_binary_handler (bprm, regs);
         if (retval < 0)
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;
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 54980a3c7602..7394b5b349ff 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -39,6 +39,7 @@ struct linux_binprm{
 #ifdef __alpha__
         unsigned int taso:1;
 #endif
+        unsigned int recursion_depth;
         struct file * file;
         int e_uid, e_gid;
         kernel_cap_t cap_post_exec_permitted;
@@ -61,6 +62,7 @@ struct linux_binprm{
 #define BINPRM_FLAGS_EXECFD_BIT 1
 #define BINPRM_FLAGS_EXECFD (1 << BINPRM_FLAGS_EXECFD_BIT)
 
+#define BINPRM_MAX_RECURSION 4
 
 /*
  * This structure defines the functions that are used to load the binary formats that