sys: don't hold uts_sem while accessing userspace memory
commit 42a0cc3478584d4d63f68f2f5af021ddbea771fa upstream.
Holding uts_sem as a writer while accessing userspace memory allows a
namespace admin to stall all processes that attempt to take uts_sem.
Instead, move data through stack buffers and don't access userspace memory
while uts_sem is held.
Cc: stable@vger.kernel.org
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/kernel/utsname_sysctl.c b/kernel/utsname_sysctl.c
index c8eac43..d2b3b29 100644
--- a/kernel/utsname_sysctl.c
+++ b/kernel/utsname_sysctl.c
@@ -17,7 +17,7 @@
#ifdef CONFIG_PROC_SYSCTL
-static void *get_uts(struct ctl_table *table, int write)
+static void *get_uts(struct ctl_table *table)
{
char *which = table->data;
struct uts_namespace *uts_ns;
@@ -25,21 +25,9 @@
uts_ns = current->nsproxy->uts_ns;
which = (which - (char *)&init_uts_ns) + (char *)uts_ns;
- if (!write)
- down_read(&uts_sem);
- else
- down_write(&uts_sem);
return which;
}
-static void put_uts(struct ctl_table *table, int write, void *which)
-{
- if (!write)
- up_read(&uts_sem);
- else
- up_write(&uts_sem);
-}
-
/*
* Special case of dostring for the UTS structure. This has locks
* to observe. Should this be in kernel/sys.c ????
@@ -49,13 +37,34 @@
{
struct ctl_table uts_table;
int r;
- memcpy(&uts_table, table, sizeof(uts_table));
- uts_table.data = get_uts(table, write);
- r = proc_dostring(&uts_table, write, buffer, lenp, ppos);
- put_uts(table, write, uts_table.data);
+ char tmp_data[__NEW_UTS_LEN + 1];
- if (write)
+ memcpy(&uts_table, table, sizeof(uts_table));
+ uts_table.data = tmp_data;
+
+ /*
+ * Buffer the value in tmp_data so that proc_dostring() can be called
+ * without holding any locks.
+ * We also need to read the original value in the write==1 case to
+ * support partial writes.
+ */
+ down_read(&uts_sem);
+ memcpy(tmp_data, get_uts(table), sizeof(tmp_data));
+ up_read(&uts_sem);
+ r = proc_dostring(&uts_table, write, buffer, lenp, ppos);
+
+ if (write) {
+ /*
+ * Write back the new value.
+ * Note that, since we dropped uts_sem, the result can
+ * theoretically be incorrect if there are two parallel writes
+ * at non-zero offsets to the same sysctl.
+ */
+ down_write(&uts_sem);
+ memcpy(get_uts(table), tmp_data, sizeof(tmp_data));
+ up_write(&uts_sem);
proc_sys_poll_notify(table->poll);
+ }
return r;
}