Avoid potential kernel stack overflow in binfmt_misc.c

PATCH New Pavel Emelyanov 1 participant 1 message Mar 13, 2008

Subsystems (2)

VFS exec and binfmt

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Pavel Emelyanov

author

2008-03-13 16:37 UTC #0 lore raw

PATCH 1/1 Avoid potential kernel stack overflow in binfmt_misc.c

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 <[email protected]>

---

fs/binfmt_em86.c +1 -1

index f95ae97..f9c88d0 100644
@@ -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;

fs/binfmt_misc.c +6 -0

index b53c7e5..d7d1b00 100644
@@ -116,6 +116,12 @@ 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);

fs/binfmt_script.c +1 -1

index ab33939..9e3963f 100644
@@ -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;

include/linux/binfmts.h +2 -1

index 1dd7567..b512e48 100644
@@ -34,7 +34,8 @@ struct linux_binprm{
 #endif
 	struct mm_struct *mm;
 	unsigned long p; /* current top of mem */
-	int sh_bang;
+	unsigned int sh_bang:1,
+		     misc_bang:1;
 	struct file * file;
 	int e_uid, e_gid;
 	kernel_cap_t cap_inheritable, cap_permitted;