One of the patches
in the upcoming 18.104.22.168
stable kernel release is a fix for a security vulnerability designated as
CVE-2006-1055. It makes a small change to the code which implements the
ability to write to sysfs attributes; with this change, the maximum amount
of data which can be written to an attribute is PAGE_SIZE-1
or 4095 on most systems. Since last June, the limit had simply been
, allowing a full page to be written.
Since the page is zeroed before being filled, this change ensures that the
data coming from user space will be null-terminated when it is passed to
the specific sysfs store() function. Without that assurance, that
function might have proceeded merrily off the end of the one-page buffer,
accessing data which did not come from user space and possibly overwriting
buffers elsewhere. The possibility of this happening was enough to raise
security fears and motivate a quick fix.
The interesting thing is that the prototype for the store()
ssize_t (*store)(struct kobject *kobj, struct attribute *attr,
const char *buffer, size_t size);
The size parameter is the amount of user data being passed in.
So, one might ask, why bother null-terminating the buffer, when its size
has already been made available to the receiving code? Certain developers,
whose code was receiving 4096-byte data via sysfs attributes, have, indeed,
asked that question.
The question was answered, in one way, in the message featured in the quote of the week. More
diplomatically, one might say that, regardless of how the interface was
designed, a number of sysfs attribute implementations have be coded on the
assumption that the incoming data will be null-terminated. So they do not
bother to check the length of that data, and they will do bad things in the
absence of the expected terminator.
With the 22.214.171.124 patch, the situation will be fixed and those
implementations made safe again. But it is hard not to be a little nervous
about the situation. If there is carelessly-written code in the tree,
there may be other issues with it as well, and the return of
null-termination may not help much. It would be nicer if there were a way
to verify that the interfaces were being used correctly. In the mean time,
people writing sysfs interfaces - each of which is an interface to user
space and a possible target of attack - may want to look a little more
carefully at their code before submitting it.
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