[PATCH] [RFC] Allow recursion in binfmt_script and binfmt_misc

["Kirill A. Shutemov" <kirill@shutemov.name>]

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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. 

I propose allow recursion, but limit its deep.

There is one more problem: on Alpha sh_bang used to remember if the 
application is TASO. I have no experience with Alpha. Is it possible to
use separate flag in struct linux_binprm for it?

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>

diff --git a/fs/binfmt_em86.c b/fs/binfmt_em86.c
index f9c88d0..f95ae97 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->sh_bang++;	/* 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 8d7e88e..50c665f 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -43,6 +43,8 @@ enum {Enabled, Magic};
 #define MISC_FMT_OPEN_BINARY (1<<30)
 #define MISC_FMT_CREDENTIALS (1<<29)
 
+#define MISC_FMT_MAX_RECURSION 4
+
 typedef struct {
 	struct list_head list;
 	unsigned long flags;		/* type, status, etc. */
@@ -117,7 +119,7 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 		goto _ret;
 
 	retval = -ENOEXEC;
-	if (bprm->misc_bang)
+	if (bprm->misc_bang >= MISC_FMT_MAX_RECURSION)
 		goto _ret;
 
 	/* to keep locking time low, we copy the interpreter string */
@@ -197,7 +199,7 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 	if (retval < 0)
 		goto _error;
 
-	bprm->misc_bang = 1;
+	bprm->misc_bang++;
 
 	retval = search_binary_handler (bprm, regs);
 	if (retval < 0)
diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index 9e3963f..172c954 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -15,6 +15,8 @@
 #include <linux/err.h>
 #include <linux/fs.h>
 
+#define SCRIPT_FMT_MAX_RECURSION 4
+
 static int load_script(struct linux_binprm *bprm,struct pt_regs *regs)
 {
 	char *cp, *i_name, *i_arg;
@@ -22,14 +24,15 @@ 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->sh_bang >= SCRIPT_FMT_MAX_RECURSION))
 		return -ENOEXEC;
 	/*
 	 * This section does the #! interpretation.
 	 * Sorta complicated, but hopefully it will work.  -TYT
 	 */
 
-	bprm->sh_bang = 1;
+	bprm->sh_bang++;
 	allow_write_access(bprm->file);
 	fput(bprm->file);
 	bprm->file = NULL;
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 826f623..1e21ff0 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -34,8 +34,8 @@ struct linux_binprm{
 #endif
 	struct mm_struct *mm;
 	unsigned long p; /* current top of mem */
-	unsigned int sh_bang:1,
-		     misc_bang:1;
+	unsigned char sh_bang;
+	unsigned char misc_bang;
 	struct file * file;
 	int e_uid, e_gid;
 	kernel_cap_t cap_post_exec_permitted;

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