[ David added to CC ]
On Wed, 14 Nov 2018, Eric Biggers wrote:
From: Eric Biggers ebiggers@google.com
When a UHID_CREATE command is written to the uhid char device, a copy_from_user() is done from a user pointer embedded in the command. When the address limit is KERNEL_DS, e.g. as is the case during sys_sendfile(), this can read from kernel memory. Alternatively, information can be leaked from a setuid binary that is tricked to write to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
No other commands in uhid_char_write() are affected by this bug and UHID_CREATE is marked as "obsolete", so apply the restriction to UHID_CREATE only rather than to uhid_char_write() entirely.
Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess helpers fault on kernel addresses"), allowing this bug to be found.
Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events") Cc: stable@vger.kernel.org # v3.6+ Cc: Jann Horn jannh@google.com Cc: Andy Lutomirski luto@kernel.org Signed-off-by: Eric Biggers ebiggers@google.com
Thanks for the patch. I however believe the fix below is more generic, and would prefer taking that one in case noone sees any major flaw in that I've overlooked. Thanks.
From: David Herrmann dh.herrmann@gmail.com Subject: [PATCH] HID: uhid: prevent splice(2)
The kernel has a default implementation of splice(2) for writing from a pipe into an arbitrary file. This behavior can be overriden by providing an f_op.splice_write() callback.
Unfortunately, the default implementation of splice_write() takes page by page from the source pipe, calls kmap() and passes the mapped page as kernel-address to f_op.write(). Thus, it uses standard write(2) to implement splice(2). However, since the page is kernel-mapped, they have to `set_fs(get_ds())`. This is mostly fine, but UHID takes command-streams through write(2), and thus it might interpret the data taken as pointers. If called with KERNEL_DS, you can trick UHID to allow kernel-space pointers as well.
As a simple fix, prevent splice(2) on UHID. It is unsecure, but it is also non-functional. We need a linear mapping of the input in UHID, so chunked input from splice(2) makes no sense, anyway.
Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com Signed-off-by: David Herrmann dh.herrmann@gmail.com --- drivers/hid/uhid.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c index 3c5507313606..fefedc0b4dc6 100644 --- a/drivers/hid/uhid.c +++ b/drivers/hid/uhid.c @@ -753,6 +753,15 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer, return ret ? ret : count; }
+static ssize_t uhid_char_splice_write(struct pipe_inode_info *pipe, + struct file *out, + loff_t *ppos, + size_t len, + unsigned int flags) +{ + return -EOPNOTSUPP; +} + static __poll_t uhid_char_poll(struct file *file, poll_table *wait) { struct uhid_device *uhid = file->private_data; @@ -771,6 +780,7 @@ static const struct file_operations uhid_fops = { .release = uhid_char_release, .read = uhid_char_read, .write = uhid_char_write, + .splice_write = uhid_char_splice_write, .poll = uhid_char_poll, .llseek = no_llseek, };