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Subject[PATCH] sysctl: protect poll() in entries that may go away
Date
Protect code accessing ctl_table by grabbing the header with
grab_header() and after releasing with sysctl_head_finish(). This is
needed if poll() is called in entries created by modules: currently only
hostname and domainname support poll(), but this bug may be triggered
when/if modules use it and if user called poll() in a file that doesn't
support it.

Dave Jones reported the following when using a syscall fuzzer while
hibernating/resuming:

RIP: 0010:[<ffffffff81233e3e>] [<ffffffff81233e3e>] proc_sys_poll+0x4e/0x90
RAX: 0000000000000145 RBX: ffff88020cab6940 RCX: 0000000000000000
RDX: ffffffff81233df0 RSI: 6b6b6b6b6b6b6b6b RDI: ffff88020cab6940
[ ... ]
Code: 00 48 89 fb 48 89 f1 48 8b 40 30 4c 8b 60 e8 b8 45 01 00 00 49 83
7c 24 28 00 74 2e 49 8b 74 24 30 48 85 f6 74 24 48 85 c9 75 32 <8b> 16
b8 45 01 00 00 48 63 d2 49 39 d5 74 10 8b 06 48 98 48 89
If an entry goes away while we are polling() it, ctl_table may not exist
anymore.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
---
fs/proc/proc_sysctl.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index a6b6217..53c3bce 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -188,20 +188,32 @@ static ssize_t proc_sys_write(struct file *filp, const char __user *buf,

static int proc_sys_open(struct inode *inode, struct file *filp)
{
+ struct ctl_table_header *head = grab_header(inode);
struct ctl_table *table = PROC_I(inode)->sysctl_entry;

+ /* sysctl was unregistered */
+ if (IS_ERR(head))
+ return PTR_ERR(head);
+
if (table->poll)
filp->private_data = proc_sys_poll_event(table->poll);

+ sysctl_head_finish(head);
+
return 0;
}

static unsigned int proc_sys_poll(struct file *filp, poll_table *wait)
{
struct inode *inode = filp->f_path.dentry->d_inode;
+ struct ctl_table_header *head = grab_header(inode);
struct ctl_table *table = PROC_I(inode)->sysctl_entry;
- unsigned long event = (unsigned long)filp->private_data;
unsigned int ret = DEFAULT_POLLMASK;
+ unsigned long event;
+
+ /* sysctl was unregistered */
+ if (IS_ERR(head))
+ return POLLERR | POLLHUP;

if (!table->proc_handler)
goto out;
@@ -209,6 +221,7 @@ static unsigned int proc_sys_poll(struct file *filp, poll_table *wait)
if (!table->poll)
goto out;

+ event = (unsigned long)filp->private_data;
poll_wait(filp, &table->poll->wait, wait);

if (event != atomic_read(&table->poll->event)) {
@@ -217,6 +230,8 @@ static unsigned int proc_sys_poll(struct file *filp, poll_table *wait)
}

out:
+ sysctl_head_finish(head);
+
return ret;
}

--
1.7.9.4


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