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SubjectPATCH: 2.4.22-pre7 drivers/i2c/i2c-dev.c user/kernel bug and mem leak
From
Date
The user kernel bug was discovered by the Berkeley-developed static
analysis tool, CQual, developed by Jeff Foster, John Kodumal, and many
others. The mem leak bug just happened to be in the wrong place at the
wrong time. :)

The user/kernel bug
-------------------

int i2cdev_ioctl (struct inode *inode, struct file *file, unsigned int cmd,
unsigned long arg)
{

<snip>

case I2C_RDWR:
if (copy_from_user(&rdwr_arg,
(struct i2c_rdwr_ioctl_data *)arg,
sizeof(rdwr_arg)))
return -EFAULT;

<snip>

for( i=0; i<rdwr_arg.nmsgs; i++ )
{

<snip>

if(copy_from_user(rdwr_pa[i].buf,
rdwr_arg.msgs[i].buf,
rdwr_pa[i].len))
{
res = -EFAULT;
break;
}
}

<snip>

After the first copy_from_user(), rdwr_arg.msgs is a pointer under user
control. Thus evaluating the expression "rdwr_arg.msgs[i].buf" at the
bottom of the for-loop requires an unsafe dereference.

The mem leak
------------

int i2cdev_ioctl (struct inode *inode, struct file *file, unsigned int cmd,
unsigned long arg)
{

<snip>

case I2C_RDWR:

<snip>

for( i=0; i<rdwr_arg.nmsgs; i++ )
{

<snip>

rdwr_pa[i].buf = kmalloc(rdwr_pa[i].len, GFP_KERNEL);
if(rdwr_pa[i].buf == NULL)
{
res = -ENOMEM;
break;
}
if(copy_from_user(rdwr_pa[i].buf,
rdwr_arg.msgs[i].buf,
rdwr_pa[i].len))
{
res = -EFAULT;
break;
}
}
if (res < 0) {
int j;
for (j = 0; j < i; ++j)
kfree(rdwr_pa[j].buf);
kfree(rdwr_pa);
return res;
}

<snip>

Notice the for-loop may exit with rdwr_pa[i].buf allocated if the
copy_from_user() fails. The cleanup code doesn't handle this case.
Thus a malicious user could make the kernel lose up to 8KB per ioctl.

The following patch fixes both bugs. There's a couple of lines with
changed whitespace, but I think the changes make it more consistent (and
fixing them would be a hassle). Thanks for looking into this, and sorry
if I got anything wrong.

Best,
Rob

--- drivers/i2c/i2c-dev.c.orig Tue Jul 22 11:26:28 2003
+++ drivers/i2c/i2c-dev.c Wed Jul 23 12:55:57 2003
@@ -219,6 +219,7 @@
struct i2c_smbus_ioctl_data data_arg;
union i2c_smbus_data temp;
struct i2c_msg *rdwr_pa;
+ struct i2c_msg *tmp_pa;
int i,datasize,res;
unsigned long funcs;

@@ -265,10 +266,21 @@

if (rdwr_pa == NULL) return -ENOMEM;

+ tmp_pa = (struct i2c_msg *)
+ kmalloc(rdwr_arg.nmsgs * sizeof(struct i2c_msg),
+ GFP_KERNEL);
+
+ if (tmp_pa == NULL)
+ {
+ kfree(rdwr_pa);
+ return -ENOMEM;
+ }
+
res = 0;
for( i=0; i<rdwr_arg.nmsgs; i++ )
{
- if(copy_from_user(&(rdwr_pa[i]),
+ rdwr_pa[i].buf = NULL;
+ if(copy_from_user(&tmp_pa[i],
&(rdwr_arg.msgs[i]),
sizeof(rdwr_pa[i])))
{
@@ -276,10 +288,13 @@
break;
}
/* Limit the size of the message to a sane amount */
- if (rdwr_pa[i].len > 8192) {
+ if (tmp_pa[i].len > 8192) {
res = -EINVAL;
break;
}
+ rdwr_pa[i].addr = tmp_pa[i].addr;
+ rdwr_pa[i].flags = tmp_pa[i].flags;
+ rdwr_pa[i].len = tmp_pa[i].len;
rdwr_pa[i].buf = kmalloc(rdwr_pa[i].len, GFP_KERNEL);
if(rdwr_pa[i].buf == NULL)
{
@@ -287,18 +302,20 @@
break;
}
if(copy_from_user(rdwr_pa[i].buf,
- rdwr_arg.msgs[i].buf,
+ tmp_pa[i].buf,
rdwr_pa[i].len))
{
- res = -EFAULT;
+ res = -EFAULT;
break;
}
}
if (res < 0) {
int j;
- for (j = 0; j < i; ++j)
- kfree(rdwr_pa[j].buf);
+ for (j = 0; j <= i; ++j)
+ if (rdwr_pa[j].buf)
+ kfree(rdwr_pa[j].buf);
kfree(rdwr_pa);
+ kfree(tmp_pa);
return res;
}
if (!res)
@@ -312,7 +329,7 @@
if( res>=0 && (rdwr_pa[i].flags & I2C_M_RD))
{
if(copy_to_user(
- rdwr_arg.msgs[i].buf,
+ tmp_pa[i].buf,
rdwr_pa[i].buf,
rdwr_pa[i].len))
{
@@ -322,6 +339,7 @@
kfree(rdwr_pa[i].buf);
}
kfree(rdwr_pa);
+ kfree(tmp_pa);
return res;

case I2C_SMBUS:
-
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