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Re: [PATCH] spi: spidev: fix possible arithmetic overflow for multi-transfer message

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  Re: [PATCH] spi: spidev: fix possible arithmetic overflow for  multi-transfer message Dmitry Torokhov <dmitry.torokhov@gmail.com> - 2016-05-21 19:00 +0200
    Re: [PATCH] spi: spidev: fix possible arithmetic overflow for  multi-transfer message Ian Abbott <abbotti@mev.co.uk> - 2016-05-23 12:30 +0200
      Re: [PATCH] spi: spidev: fix possible arithmetic overflow for  multi-transfer message Dmitry Torokhov <dmitry.torokhov@gmail.com> - 2016-05-23 19:10 +0200
    Re: [PATCH] spi: spidev: fix possible arithmetic overflow for  multi-transfer message Mark Brown <broonie@kernel.org> - 2016-05-23 13:20 +0200
      Re: [PATCH] spi: spidev: fix possible arithmetic overflow for  multi-transfer message Ian Abbott <abbotti@mev.co.uk> - 2016-05-23 13:30 +0200

#1404870 — Re: [PATCH] spi: spidev: fix possible arithmetic overflow for multi-transfer message

On Mon, Mar 23, 2015 at 10:50 AM, Ian Abbott <abbotti@mev.co.uk> wrote:
> `spidev_message()` sums the lengths of the individual SPI transfers to
> determine the overall SPI message length.  It restricts the total
> length, returning an error if too long, but it does not check for
> arithmetic overflow.  For example, if the SPI message consisted of two
> transfers and the first has a length of 10 and the second has a length
> of (__u32)(-1), the total length would be seen as 9, even though the
> second transfer is actually very long.  If the second transfer specifies
> a null `rx_buf` and a non-null `tx_buf`, the `copy_from_user()` could
> overrun the spidev's pre-allocated tx buffer before it reaches an
> invalid user memory address.  Fix it by checking that neither the total
> nor the individual transfer lengths exceed the maximum allowed value.
>
> Thanks to Dan Carpenter for reporting the potential integer overflow.
>
> Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
> Cc: <stable@vger.kernel.org> # 4.0+
> ---
> This could be backported to kernels prior to 4.0, but the total and
> individual lengths would need to be checked against `bufsiz` instead of
> `INT_MAX`.
> ---
>  drivers/spi/spidev.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
> index bb6b3ab..23ad978 100644
> --- a/drivers/spi/spidev.c
> +++ b/drivers/spi/spidev.c
> @@ -249,9 +249,10 @@ static int spidev_message(struct spidev_data *spidev,
>                 total += k_tmp->len;
>                 /* Since the function returns the total length of transfers
>                  * on success, restrict the total to positive int values to
> -                * avoid the return value looking like an error.
> +                * avoid the return value looking like an error.  Also check
> +                * each transfer length to avoid arithmetic overflow.
>                  */
> -               if (total > INT_MAX) {
> +               if (total > INT_MAX || k_tmp->len > INT_MAX) {

What if total is INT_MAX - 2 and k_tmp->len is 3? What about total is
INT_MAX and k_tmp->len is INT_MAX as well? I think the proper check
should be:

if (total < k_tmp->len || total > INT_MAX) {
        ...
}

>                         status = -EMSGSIZE;
>                         goto done;
>                 }

Thanks.

-- 
Dmitry

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#1405214

On 21/05/16 17:50, Dmitry Torokhov wrote:
> On Mon, Mar 23, 2015 at 10:50 AM, Ian Abbott <abbotti@mev.co.uk> wrote:
>> `spidev_message()` sums the lengths of the individual SPI transfers to
>> determine the overall SPI message length.  It restricts the total
>> length, returning an error if too long, but it does not check for
>> arithmetic overflow.  For example, if the SPI message consisted of two
>> transfers and the first has a length of 10 and the second has a length
>> of (__u32)(-1), the total length would be seen as 9, even though the
>> second transfer is actually very long.  If the second transfer specifies
>> a null `rx_buf` and a non-null `tx_buf`, the `copy_from_user()` could
>> overrun the spidev's pre-allocated tx buffer before it reaches an
>> invalid user memory address.  Fix it by checking that neither the total
>> nor the individual transfer lengths exceed the maximum allowed value.
>>
>> Thanks to Dan Carpenter for reporting the potential integer overflow.
>>
>> Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
>> Cc: <stable@vger.kernel.org> # 4.0+
>> ---
>> This could be backported to kernels prior to 4.0, but the total and
>> individual lengths would need to be checked against `bufsiz` instead of
>> `INT_MAX`.
>> ---
>>   drivers/spi/spidev.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
>> index bb6b3ab..23ad978 100644
>> --- a/drivers/spi/spidev.c
>> +++ b/drivers/spi/spidev.c
>> @@ -249,9 +249,10 @@ static int spidev_message(struct spidev_data *spidev,
>>                  total += k_tmp->len;
>>                  /* Since the function returns the total length of transfers
>>                   * on success, restrict the total to positive int values to
>> -                * avoid the return value looking like an error.
>> +                * avoid the return value looking like an error.  Also check
>> +                * each transfer length to avoid arithmetic overflow.
>>                   */
>> -               if (total > INT_MAX) {
>> +               if (total > INT_MAX || k_tmp->len > INT_MAX) {
>
> What if total is INT_MAX - 2 and k_tmp->len is 3? What about total is
> INT_MAX and k_tmp->len is INT_MAX as well? I think the proper check

In your questions, I assume you are referring to the values of 'total' 
before the addition.  I'll call the values 'old_total' and 'new_total' 
(with the same type as 'total', i.e. 'unsigned int').  Note that total 
(and old_total, and new_total) and 'k_tmp->len' have range UINT_MAX, or 
2*INT_MAX+1.

Before the addition, we know that old_total <= INT_MAX (otherwise the 
loop would have errored out already), but k_tmp->len can have any value 
from 0 to UINT_MAX.  After the addition, new_total can have any value 
from 0 to UINT_MAX, and might be less than old_total.  new_total can 
only be less than old_total if old_total + k_tmp->len > UINT_MAX, and 
here I am referring to proper addition, not addition modulo UINT_MAX+1. 
  Rearranging, new_total will be less than old_total if k_tmp->len > 
UINT_MAX - old_total.  Since the maximum value of old_total is INT_MAX, 
the lowest possible value of k_tmp->len that could cause new_total to be 
less than old_total is UINT_MAX - INT_MAX, or INT_MAX+1.  That is what 
the second part of the 'if' test is detecting.

> should be:
>
> if (total < k_tmp->len || total > INT_MAX) {
>          ...
> }
>

That also works.

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@mev.co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-

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#1405519

On Mon, May 23, 2016 at 11:20:35AM +0100, Ian Abbott wrote:
> On 21/05/16 17:50, Dmitry Torokhov wrote:
> >On Mon, Mar 23, 2015 at 10:50 AM, Ian Abbott <abbotti@mev.co.uk> wrote:
> >>`spidev_message()` sums the lengths of the individual SPI transfers to
> >>determine the overall SPI message length.  It restricts the total
> >>length, returning an error if too long, but it does not check for
> >>arithmetic overflow.  For example, if the SPI message consisted of two
> >>transfers and the first has a length of 10 and the second has a length
> >>of (__u32)(-1), the total length would be seen as 9, even though the
> >>second transfer is actually very long.  If the second transfer specifies
> >>a null `rx_buf` and a non-null `tx_buf`, the `copy_from_user()` could
> >>overrun the spidev's pre-allocated tx buffer before it reaches an
> >>invalid user memory address.  Fix it by checking that neither the total
> >>nor the individual transfer lengths exceed the maximum allowed value.
> >>
> >>Thanks to Dan Carpenter for reporting the potential integer overflow.
> >>
> >>Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
> >>Cc: <stable@vger.kernel.org> # 4.0+
> >>---
> >>This could be backported to kernels prior to 4.0, but the total and
> >>individual lengths would need to be checked against `bufsiz` instead of
> >>`INT_MAX`.
> >>---
> >>  drivers/spi/spidev.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
> >>index bb6b3ab..23ad978 100644
> >>--- a/drivers/spi/spidev.c
> >>+++ b/drivers/spi/spidev.c
> >>@@ -249,9 +249,10 @@ static int spidev_message(struct spidev_data *spidev,
> >>                 total += k_tmp->len;
> >>                 /* Since the function returns the total length of transfers
> >>                  * on success, restrict the total to positive int values to
> >>-                * avoid the return value looking like an error.
> >>+                * avoid the return value looking like an error.  Also check
> >>+                * each transfer length to avoid arithmetic overflow.
> >>                  */
> >>-               if (total > INT_MAX) {
> >>+               if (total > INT_MAX || k_tmp->len > INT_MAX) {
> >
> >What if total is INT_MAX - 2 and k_tmp->len is 3? What about total is
> >INT_MAX and k_tmp->len is INT_MAX as well? I think the proper check
> 
> In your questions, I assume you are referring to the values of
> 'total' before the addition.  I'll call the values 'old_total' and

Sorry, yes, for some reason I was thinking we are checking before
performing addition. Ignore me.

> 'new_total' (with the same type as 'total', i.e. 'unsigned int').
> Note that total (and old_total, and new_total) and 'k_tmp->len' have
> range UINT_MAX, or 2*INT_MAX+1.
> 
> Before the addition, we know that old_total <= INT_MAX (otherwise
> the loop would have errored out already), but k_tmp->len can have
> any value from 0 to UINT_MAX.  After the addition, new_total can
> have any value from 0 to UINT_MAX, and might be less than old_total.
> new_total can only be less than old_total if old_total + k_tmp->len
> > UINT_MAX, and here I am referring to proper addition, not addition
> modulo UINT_MAX+1.  Rearranging, new_total will be less than
> old_total if k_tmp->len > UINT_MAX - old_total.  Since the maximum
> value of old_total is INT_MAX, the lowest possible value of
> k_tmp->len that could cause new_total to be less than old_total is
> UINT_MAX - INT_MAX, or INT_MAX+1.  That is what the second part of
> the 'if' test is detecting.
> 
> >should be:
> >
> >if (total < k_tmp->len || total > INT_MAX) {
> >         ...
> >}
> >
> 
> That also works.
> 
> -- 
> -=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@mev.co.uk> )=-
> -=(                          Web: http://www.mev.co.uk/  )=-

-- 
Dmitry

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#1405264

[Multipart message — attachments visible in raw view] — view raw

On Sat, May 21, 2016 at 09:50:35AM -0700, Dmitry Torokhov wrote:
> On Mon, Mar 23, 2015 at 10:50 AM, Ian Abbott <abbotti@mev.co.uk> wrote:
> > `spidev_message()` sums the lengths of the individual SPI transfers to
> > determine the overall SPI message length.  It restricts the total
> > length, returning an error if too long, but it does not check for

As documented in SubmittingPatches please send patches to the
maintainers for the code you would like to change.  The normal kernel
workflow is that people apply patches from their inboxes, if they aren't
copied they are likely to not see the patch at all and it is much more
difficult to apply patches.

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#1405269

On 23/05/16 12:15, Mark Brown wrote:
> On Sat, May 21, 2016 at 09:50:35AM -0700, Dmitry Torokhov wrote:
>> On Mon, Mar 23, 2015 at 10:50 AM, Ian Abbott <abbotti@mev.co.uk> wrote:
>>> `spidev_message()` sums the lengths of the individual SPI transfers to
>>> determine the overall SPI message length.  It restricts the total
>>> length, returning an error if too long, but it does not check for
>
> As documented in SubmittingPatches please send patches to the
> maintainers for the code you would like to change.  The normal kernel
> workflow is that people apply patches from their inboxes, if they aren't
> copied they are likely to not see the patch at all and it is much more
> difficult to apply patches.
>

In this case, Dmitry was replying to my patch that has already been 
applied over a year ago. :)

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@mev.co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-

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