+cc Andy
On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers ebiggers@kernel.org wrote:
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 sendfile(), this can read from kernel memory. Therefore, UHID_CREATE must not be allowed in this case.
For consistency and to make sure all current and future uhid commands are covered, apply the restriction to uhid_char_write() as a whole rather than to UHID_CREATE specifically.
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
Wheeeee, it found something! :)
Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events") Cc: stable@vger.kernel.org # v3.6+ Cc: Jann Horn jannh@google.com Signed-off-by: Eric Biggers ebiggers@google.com
drivers/hid/uhid.c | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c index 3c55073136064..e94c5e248b56e 100644 --- a/drivers/hid/uhid.c +++ b/drivers/hid/uhid.c @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer, int ret; size_t len;
if (uaccess_kernel()) { /* payload may contain a __user pointer */pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",__func__, task_tgid_vnr(current), current->comm);return -EACCES;}
If this file can conceivably be opened by a process that doesn't have root privileges, this check should be something along the lines of ib_safe_file_access() or sg_check_file_access().
Checking for uaccess_kernel() prevents the symptom that syzkaller notices - a user being able to cause a kernel memory access -, but it doesn't deal with the case where a user opens a file descriptor to this device and tricks a more privileged process into writing into it (e.g. by passing it to a suid binary as stdout or stderr).
Looking closer, I wonder whether this kind of behavior is limited to the UHID_CREATE request, which has a comment on it saying "/* Obsolete! Use UHID_CREATE2. */". If we could keep this kind of ugly kludge away from the code paths you're supposed to be using, that would be nice...