On 2018-11-14 03:46, William Kucharski wrote:
On Nov 14, 2018, at 4:09 AM, David Laight <David.Laight@xxxxxxxxxx>
wrote:
From: William Kucharski
Sent: 14 November 2018 10:35
On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres
<isaacm@xxxxxxxxxxxxxx> wrote:
diff --git a/mm/usercopy.c b/mm/usercopy.c
index 852eb4e..0293645 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -151,7 +151,7 @@ static inline void check_bogus_address(const
unsigned long ptr, unsigned long n,
bool to_user)
{
/* Reject if object wraps past end of memory. */
- if (ptr + n < ptr)
+ if (ptr + (n - 1) < ptr)
usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
I'm being paranoid, but is it possible this routine could ever be
passed "n" set to zero?
If so, it will erroneously abort indicating a wrapped address as (n -
1) wraps to ULONG_MAX.
Easily fixed via:
if ((n != 0) && (ptr + (n - 1) < ptr))
Ugg... you don't want a double test.
I'd guess that a length of zero is likely, but a usercopy that
includes
the highest address is going to be invalid because it is a kernel
address
(on most archs, and probably illegal on others).
What you really want to do is add 'ptr + len' and check the carry
flag.
The extra test is only a few extra instructions, but I understand the
concern. (Though I don't
know how you'd access the carry flag from C in a machine-independent
way. Also, for the
calculation to be correct you still need to check 'ptr + (len - 1)'
for the wrap.)
You could also theoretically call gcc's __builtin_uadd_overflow() if
you want to get carried away.
As I mentioned, I was just being paranoid, but the passed zero length
issue stood out to me.
William Kucharski
Hi William,
Thank you and David for your feedback. The check_bogus_address() routine
is only invoked from one place in the kernel, which is
__check_object_size(). Before invoking check_bogus_address,
__check_object_size ensures that n is non-zero, so it is not possible to
call this routine with n being 0. Therefore, we shouldn't run into the
scenario you described. Also, in the case where we are copying a page's
contents into a kernel space buffer and will not have that buffer
interacting with userspace at all, this change to that check should
still be valid, correct?
Thanks,
Isaac Manjarres
