Pass correct length to strnlen_user in fs/exec.c

Replace valid_arg_len function in fs/exec.c with max_arg_len function and pass the correct length to strnlen_user.

fs/exec.c +5 -6

@@ -296,9 +296,9 @@ err:
 	return err;
 }
 
-static bool valid_arg_len(struct linux_binprm *bprm, long len)
+static long max_arg_len(struct linux_binprm *bprm)
 {
-	return len <= MAX_ARG_STRLEN;
+	return MAX_ARG_STRLEN;
 }
 
 #else

@@ -354,9 +354,9 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	return 0;
 }
 
-static bool valid_arg_len(struct linux_binprm *bprm, long len)
+static long max_arg_len(struct linux_binprm *bprm)
 {
-	return len <= bprm->p;
+	return bprm->p;
 }
 
 #endif /* CONFIG_MMU */

@@ -474,18 +474,19 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
 		const char __user *str;
 		int len;
 		unsigned long pos;
+		long max_len = max_arg_len(bprm);
 
 		ret = -EFAULT;
 		str = get_user_arg_ptr(argv, argc);
 		if (IS_ERR(str))
 			goto out;
 
-		len = strnlen_user(str, MAX_ARG_STRLEN);
-		if (!len || len > MAX_ARG_STRLEN)

Quoted 46 lines · On Thu, 08 Sep 2011 10:39:24 +1000 Ryan Mallon <[email protected]> wrote:

Replace valid_arg_len function in fs/exec.c with max_arg_len function and pass the correct length to strnlen_user.

fs/exec.c +7 -6

@@ -296,9 +296,9 @@ err:
 	return err;
 }
 
-static bool valid_arg_len(struct linux_binprm *bprm, long len)
+static long max_arg_len(struct linux_binprm *bprm)
 {
-	return len <= MAX_ARG_STRLEN;
+	return MAX_ARG_STRLEN;
 }
 
 #else

@@ -354,9 +354,9 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	return 0;
 }
 
-static bool valid_arg_len(struct linux_binprm *bprm, long len)
+static long max_arg_len(struct linux_binprm *bprm)
 {
-	return len <= bprm->p;
+	return bprm->p;
 }
 
 #endif /* CONFIG_MMU */

@@ -474,18 +474,19 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
 		const char __user *str;
 		int len;
 		unsigned long pos;
+		long max_len = max_arg_len(bprm);
 
 		ret = -EFAULT;
 		str = get_user_arg_ptr(argv, argc);
 		if (IS_ERR(str))
 			goto out;
 
-		len = strnlen_user(str, MAX_ARG_STRLEN);
-		if (!len || len > MAX_ARG_STRLEN)
+		len = strnlen_user(str, max_len);
+		if (!len)
 			goto out;
 

fs/exec.c static int copy_strings(int argc, struct user_arg_ptr argv, · line 492

Quoted 6 lines

fs/exec.c +1 -1

 		ret = -E2BIG;
-		if (!valid_arg_len(bprm, len))
+		if (len > max_len)
 			goto out;
 
 		/* We're going to work our way backwords. */

I'm struggling to find a reason to merge this - it churns code around rather pointlessly?

Quoted 1 line

...

That's fine. I originally went looking after a discussion with Mark about the weird strnlen_user semantics and this usage looked incorrect to me because it wasn't obviously checking >= MAX_ARG_STRLEN.

The rework I think makes it a bit more clear and passes the correct max length to strnlen_user. Its a bit odd to pass MAX_ARG_STRLEN and then check if it is longer than bprm->len, and I guess assumes that bprm->len is less than MAX_ARG_STRLEN.

Feel free to drop the patch if you think it is just churn.

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Quoted 11 lines · Ryan Mallon

Quoted 1 line

...

That's fine. I originally went looking after a discussion with Mark about the weird strnlen_user semantics and this usage looked incorrect to me because it wasn't obviously checking >= MAX_ARG_STRLEN.

The rework I think makes it a bit more clear and passes the correct max length to strnlen_user. Its a bit odd to pass MAX_ARG_STRLEN and then check if it is longer than bprm->len, and I guess assumes that bprm->len is less than MAX_ARG_STRLEN.

Quoted 1 line

Feel free to drop the patch if you think it is just churn.

As you are already busy with fs/exec.c and strnlen_user, could you please check, if the timerace (argv/envp corruption) using strnlen_user could also be fixed within the same patch (if bug still in kernel). See

https://bugzilla.kernel.org/show_bug.cgi?id=39222

The race occurs when one thread issues exec while another one with shared memory pages is flipping char/0-bytes of the args or envptrs, causing random split (number greater maxargs - no problem, silently droped)/fusion (greater MAX_ARG_STRLEN - might harm dumb programs) or removal of all args including program name on top of new stack when calling via binfmt handlers (might have influenced old ldlinux/libc pre 2009?? when handling suid binaries)