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sys: don't hold uts_sem while accessing userspace memory

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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: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
This commit is contained in:
Jann Horn
2018-06-25 18:34:10 +02:00
committed by Eric W. Biederman
parent 5820f140ed
commit 42a0cc3478
5 changed files with 121 additions and 112 deletions
Download Patch File Download Diff File Expand all files Collapse all files

45
kernel/utsname_sysctl.c
View File

@@ -18,7 +18,7 @@
#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;
@@ -26,21 +26,9 @@ static void *get_uts(struct ctl_table *table, int write)
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 ????
@@ -50,13 +38,34 @@ static int proc_do_uts_string(struct ctl_table *table, int write,
{
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;
}