Re: [PATCH] HID: uhid: prevent uhid_char_write() under KERNEL_DS
From: Jann Horn <jannh@google.com>
Date: 2018-11-14 18:19:09
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lkml, stable
+cc Andy On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers [off-list ref] 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.comWheeeee, it found something! :)
quoted hunk ↗ jump to hunk
Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events") Cc: <redacted> # v3.6+ Cc: Jann Horn <jannh@google.com> Signed-off-by: Eric Biggers <redacted> --- 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...