Currently, when checking to see if accessing n bytes starting at
address "ptr" will cause a wraparound in the memory addresses,
the check in check_bogus_address() adds an extra byte, which is
incorrect, as the range of addresses that will be accessed is
[ptr, ptr + (n - 1)].
This can lead to incorrectly detecting a wraparound in the
memory address, when trying to read 4 KB from memory that is
mapped to the the last possible page in the virtual address
space, when in fact, accessing that range of memory would not
cause a wraparound to occur.
Use the memory range that will actually be accessed when
considering if accessing a certain amount of bytes will cause
the memory address to wrap around.
Change-Id: I2563a5988e41122727ede17180f365e999b953e6
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Co-Developed-by: Prasad Sodagudi <[email protected]>
Signed-off-by: Prasad Sodagudi <[email protected]>
Signed-off-by: Isaac J. Manjarres <[email protected]>
Cc: [email protected]
---
mm/usercopy.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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);
/* Reject if NULL or ZERO-allocation. */
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project
> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <[email protected]> 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))
William Kucharski
From: William Kucharski
> Sent: 14 November 2018 10:35
>
> > On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <[email protected]> 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.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
> On Nov 14, 2018, at 4:09 AM, David Laight <[email protected]> wrote:
>
> From: William Kucharski
>> Sent: 14 November 2018 10:35
>>
>>> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <[email protected]> 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
On 2018-11-14 03:46, William Kucharski wrote:
>> On Nov 14, 2018, at 4:09 AM, David Laight <[email protected]>
>> wrote:
>>
>> From: William Kucharski
>>> Sent: 14 November 2018 10:35
>>>
>>>> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres
>>>> <[email protected]> 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
> On Nov 14, 2018, at 10:32 AM, [email protected] wrote:
>
> 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?
Having fixed more than one bug resulting from a "only called in one place" routine later being called elsewhere,
I am wary, but ultimately it's likely not worth the performance hit of a check or BUG_ON().
It's a generic math check for overflow, so it should work with any address.
Reviewed-by: William Kucharski <[email protected]>
On Wed, Nov 14, 2018 at 4:35 AM, William Kucharski
<[email protected]> wrote:
>
>
>> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <[email protected]> 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?
It's a single-use inline, and zero is tested just before getting called:
/* Skip all tests if size is zero. */
if (!n)
return;
/* Check for invalid addresses. */
check_bogus_address((const unsigned long)ptr, n, to_user);
>
> 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))
Agreed. Thanks for noticing this!
-Kees
--
Kees Cook
On Tue, Nov 13, 2018 at 6:51 PM, Isaac J. Manjarres
<[email protected]> wrote:
> Currently, when checking to see if accessing n bytes starting at
> address "ptr" will cause a wraparound in the memory addresses,
> the check in check_bogus_address() adds an extra byte, which is
> incorrect, as the range of addresses that will be accessed is
> [ptr, ptr + (n - 1)].
>
> This can lead to incorrectly detecting a wraparound in the
> memory address, when trying to read 4 KB from memory that is
> mapped to the the last possible page in the virtual address
> space, when in fact, accessing that range of memory would not
> cause a wraparound to occur.
I'm kind of surprised anything is using the -4K memory range -- this
is ERR_PTR() area and I'd expect there to be an explicit unallocated
memory hole here.
>
> Use the memory range that will actually be accessed when
> considering if accessing a certain amount of bytes will cause
> the memory address to wrap around.
>
> Change-Id: I2563a5988e41122727ede17180f365e999b953e6
> Fixes: f5509cc18daa ("mm: Hardened usercopy")
> Co-Developed-by: Prasad Sodagudi <[email protected]>
> Signed-off-by: Prasad Sodagudi <[email protected]>
> Signed-off-by: Isaac J. Manjarres <[email protected]>
> Cc: [email protected]
Regardless, I'll take it in my tree if akpm doesn't grab it first. :)
Acked-by: Kees Cook <[email protected]>
-Kees
> ---
> mm/usercopy.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> 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);
>
> /* Reject if NULL or ZERO-allocation. */
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>
--
Kees Cook