Subject: Re: linux-next: build warnings (buffer size is not provably correct)

From: Arjan van de Ven <[email-masked]>
Date: Thu, 8 Oct 2009 06:42:21 -0700

On Thu, 8 Oct 2009 15:47:10 +1100
Stephen Rothwell <[email-masked]> wrote:

> Hi all,
> 
> Today's linux-next build (i386 defconfig) produced these warnings:
> 
> In file included from arch/x86/include/asm/uaccess.h:572,
>                  from kernel/capability.c:18:
> arch/x86/include/asm/uaccess_32.h: In function 'sys_capset':

the following needs to go somewhere... Andrew ?

From: Arjan van de Ven <[email-masked]>
Subject: [PATCH 7/9] Simplify bound checks in capabilities for copy_from_user
CC: James Morris <[email-masked]>

The capabilities syscall has a copy_from_user() call where gcc currently
cannot prove to itself that the copy is always within bounds.

This patch adds a very explicity bound check to prove to gcc that 
this copy_from_user cannot overflow its destination buffer.

Signed-off-by: Arjan van de Ven <[email-masked]>
Acked-by: James Morris <[email-masked]>

diff --git a/kernel/capability.c b/kernel/capability.c
index 4e17041..204f11f 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -238,7 +241,7 @@ SYSCALL_DEFINE2(capget, cap_user_header_t, header, cap_user_data_t, dataptr)
 SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
 {
 	struct __user_cap_data_struct kdata[_KERNEL_CAPABILITY_U32S];
-	unsigned i, tocopy;
+	unsigned i, tocopy, copybytes;
 	kernel_cap_t inheritable, permitted, effective;
 	struct cred *new;
 	int ret;
@@ -255,8 +258,11 @@ SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
 	if (pid != 0 && pid != task_pid_vnr(current))
 		return -EPERM;
 
-	if (copy_from_user(&kdata, data,
-			   tocopy * sizeof(struct __user_cap_data_struct)))
+	copybytes = tocopy * sizeof(struct __user_cap_data_struct);
+	if (copybytes > sizeof(kdata))
+		return -EFAULT;
+
+	if (copy_from_user(&kdata, data, copybytes))
 		return -EFAULT;
 
 	for (i = 0; i < tocopy; i++) {

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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