[PATCH 2/3] cpuset: zero malloc - revert the old cpuset fix

[Paul Jackson]

From: Paul Jackson <pj@xxxxxxx>

Second of three -- This one undoes the first one.  Apply before
the container patch that changes cpusets to use containers,
in order to avoid a patch conflict.

The cpuset code to present a list of tasks using a cpuset to
user space could write to an array that it had kmalloc'd,
after a kmalloc request of zero size.

The problem was that the code didn't check for writes past
the allocated end of the array until -after- the first write.

This is a race condition that is likely rare -- it would only
show up if a cpuset went from being empty to having a task
in it, during the brief time between the allocation and the
first write.

Prior to roughly 2.6.22 kernels, this was also a benign
problem, because a zero kmalloc returned a few usable bytes
anyway, and no harm was done with the bogus write.

With the 2.6.22 kernel changes to make issue a warning if
code tries to write to the location returned from a zero
size allocation, this problem is no longer benign.  This
cpuset code would occassionally trigger that warning.

The fix is trivial -- check before storing into the array,
not after, whether the array is big enough to hold the store.

Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: "Serge E. Hallyn" <serue@xxxxxxxxxx>
Cc: Balbir Singh <balbir@xxxxxxxxxx>
Cc: Dave Hansen <haveblue@xxxxxxxxxx>
Cc: Herbert Poetzl <herbert@xxxxxxxxxxxx>
Cc: Kirill Korotaev <dev@xxxxxxxxxx>
Cc: Paul Menage <menage@xxxxxxxxxx>
Cc: Srivatsa Vaddagiri <vatsa@xxxxxxxxxx>
Cc: Christoph Lameter <clameter@xxxxxxx>

Signed-off-by: Paul Jackson <pj@xxxxxxx>

---

Andrew,

This is the second of three in the set - it reverses the
first one, to avoid colliding with the container patches.

Apply somewhere before, or entirely merge into:
    containersv10-share-css_group-arrays-between-tasks-with-same-container-memberships.patch


 kernel/cpuset.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 2.6.22-rc4-mm2.orig/kernel/cpuset.c	2007-06-08 14:57:59.091074320 -0700
+++ 2.6.22-rc4-mm2/kernel/cpuset.c	2007-06-08 15:34:24.336172342 -0700
@@ -1620,9 +1620,9 @@ static int pid_array_load(pid_t *pidarra
 
 	do_each_thread(g, p) {
 		if (p->cpuset == cs) {
+			pidarray[n++] = p->pid;
 			if (unlikely(n == npids))
 				goto array_full;
-			pidarray[n++] = p->pid;
 		}
 	} while_each_thread(g, p);
 

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@xxxxxxx> 1.650.933.1373
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