Re: [PATCH] virt: vbox: Only copy_from_user the request-header once

From: Hans de Goede
Date: Tue May 08 2018 - 10:08:07 EST


On 08-05-18 15:46, Hans de Goede wrote:
In vbg_misc_device_ioctl(), the header of the ioctl argument is copied from
the userspace pointer 'arg' and saved to the kernel object 'hdr'. Then the
'version', 'size_in', and 'size_out' fields of 'hdr' are verified.

Before this commit, after the checks a buffer for the entire request would
be allocated and then all data including the verified header would be
copied from the userspace 'arg' pointer again.

Given that the 'arg' pointer resides in userspace, a malicious userspace
process can race to change the data pointed to by 'arg' between the two
copies. By doing so, the user can bypass the verifications on the ioctl

This commit fixes this by using the already checked copy of the header
to fill the header part of the allocated buffer and only copying the
remainder of the data from userspace.

Reported-by: Wenwen Wang <wang6495@xxxxxxx>
Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>

Wenwen just send a v2 of his patch, lets go with his (identical)
version as he has done the hard work of tracking this down.



drivers/virt/vboxguest/vboxguest_linux.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/virt/vboxguest/vboxguest_linux.c b/drivers/virt/vboxguest/vboxguest_linux.c
index 398d22693234..6e2a9619192d 100644
--- a/drivers/virt/vboxguest/vboxguest_linux.c
+++ b/drivers/virt/vboxguest/vboxguest_linux.c
@@ -121,7 +121,9 @@ static long vbg_misc_device_ioctl(struct file *filp, unsigned int req,
if (!buf)
return -ENOMEM;
- if (copy_from_user(buf, (void *)arg, hdr.size_in)) {
+ *((struct vbg_ioctl_hdr *)buf) = hdr;
+ if (copy_from_user(buf + sizeof(hdr), (void *)arg + sizeof(hdr),
+ hdr.size_in - sizeof(hdr))) {
ret = -EFAULT;
goto out;