This reverts commit 336fd4f5f25157e9e8bd50e898a1bbcd99eaea46.
Please note that `strlcpy()` does *NOT* do what you think it does.
strlcpy() *ALWAYS* reads the full input string, regardless of the
'length' parameter. That is, if the input is not zero-terminated,
strlcpy() will *READ* beyond input boundaries. It does this, because it
always returns the size it *would* copy if the target was big enough,
not the truncated size it actually copied.
The original code was perfectly fine. The hid device is
zero-initialized and the strncpy() functions copied up to n-1
characters. The result is always zero-terminated this way.
This is the third time someone tried to replace strncpy with strlcpy in
this function, and gets it wrong. I now added a comment that should at
least make people reconsider.
Signed-off-by: David Herrmann <[email protected]>
---
drivers/hid/uhid.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index fefedc0b4dc6..0dfdd0ac7120 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -496,12 +496,13 @@ static int uhid_dev_create2(struct uhid_device *uhid,
goto err_free;
}
- len = min(sizeof(hid->name), sizeof(ev->u.create2.name));
- strlcpy(hid->name, ev->u.create2.name, len);
- len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys));
- strlcpy(hid->phys, ev->u.create2.phys, len);
- len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq));
- strlcpy(hid->uniq, ev->u.create2.uniq, len);
+ /* @hid is zero-initialized, strncpy() is correct, strlcpy() not */
+ len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
+ strncpy(hid->name, ev->u.create2.name, len);
+ len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
+ strncpy(hid->phys, ev->u.create2.phys, len);
+ len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
+ strncpy(hid->uniq, ev->u.create2.uniq, len);
hid->ll_driver = &uhid_hid_driver;
hid->bus = ev->u.create2.bus;
--
2.19.1
On 11/14/18 5:16 AM, David Herrmann wrote:
> This reverts commit 336fd4f5f25157e9e8bd50e898a1bbcd99eaea46.
>
> Please note that `strlcpy()` does *NOT* do what you think it does.
> strlcpy() *ALWAYS* reads the full input string, regardless of the
> 'length' parameter. That is, if the input is not zero-terminated,
> strlcpy() will *READ* beyond input boundaries. It does this, because it
> always returns the size it *would* copy if the target was big enough,
> not the truncated size it actually copied.
>
> The original code was perfectly fine. The hid device is
> zero-initialized and the strncpy() functions copied up to n-1
> characters. The result is always zero-terminated this way.
>
> This is the third time someone tried to replace strncpy with strlcpy in
> this function, and gets it wrong. I now added a comment that should at
> least make people reconsider.
>
Can we switch to strscpy instead? This will quiet gcc and avoid the
issues with strlcpy.
> Signed-off-by: David Herrmann <[email protected]>
> ---
> drivers/hid/uhid.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index fefedc0b4dc6..0dfdd0ac7120 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -496,12 +496,13 @@ static int uhid_dev_create2(struct uhid_device *uhid,
> goto err_free;
> }
>
> - len = min(sizeof(hid->name), sizeof(ev->u.create2.name));
> - strlcpy(hid->name, ev->u.create2.name, len);
> - len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys));
> - strlcpy(hid->phys, ev->u.create2.phys, len);
> - len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq));
> - strlcpy(hid->uniq, ev->u.create2.uniq, len);
> + /* @hid is zero-initialized, strncpy() is correct, strlcpy() not */
> + len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
> + strncpy(hid->name, ev->u.create2.name, len);
> + len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
> + strncpy(hid->phys, ev->u.create2.phys, len);
> + len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
> + strncpy(hid->uniq, ev->u.create2.uniq, len);
>
> hid->ll_driver = &uhid_hid_driver;
> hid->bus = ev->u.create2.bus;
>
On Wed, Nov 14, 2018 at 9:40 AM, Laura Abbott <[email protected]> wrote:
> On 11/14/18 5:16 AM, David Herrmann wrote:
>>
>> This reverts commit 336fd4f5f25157e9e8bd50e898a1bbcd99eaea46.
>>
>> Please note that `strlcpy()` does *NOT* do what you think it does.
>> strlcpy() *ALWAYS* reads the full input string, regardless of the
>> 'length' parameter. That is, if the input is not zero-terminated,
>> strlcpy() will *READ* beyond input boundaries. It does this, because it
>> always returns the size it *would* copy if the target was big enough,
>> not the truncated size it actually copied.
>>
>> The original code was perfectly fine. The hid device is
>> zero-initialized and the strncpy() functions copied up to n-1
>> characters. The result is always zero-terminated this way.
>>
>> This is the third time someone tried to replace strncpy with strlcpy in
>> this function, and gets it wrong. I now added a comment that should at
>> least make people reconsider.
>>
>
> Can we switch to strscpy instead? This will quiet gcc and avoid the
> issues with strlcpy.
Yes please: it looks like these strings are expected to be NUL
terminated, so strscpy() without the "- 1" and min() logic would be
the correct solution here. If @hid is already zero, then this would
just be:
strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));
If they are NOT NUL terminated, then keep using strncpy() but mark the
fields in the struct with the __nonstring attribute.
-Kees
>
>
>> Signed-off-by: David Herrmann <[email protected]>
>> ---
>> drivers/hid/uhid.c | 13 +++++++------
>> 1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
>> index fefedc0b4dc6..0dfdd0ac7120 100644
>> --- a/drivers/hid/uhid.c
>> +++ b/drivers/hid/uhid.c
>> @@ -496,12 +496,13 @@ static int uhid_dev_create2(struct uhid_device
>> *uhid,
>> goto err_free;
>> }
>> - len = min(sizeof(hid->name), sizeof(ev->u.create2.name));
>> - strlcpy(hid->name, ev->u.create2.name, len);
>> - len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys));
>> - strlcpy(hid->phys, ev->u.create2.phys, len);
>> - len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq));
>> - strlcpy(hid->uniq, ev->u.create2.uniq, len);
>> + /* @hid is zero-initialized, strncpy() is correct, strlcpy() not
>> */
>> + len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
>> + strncpy(hid->name, ev->u.create2.name, len);
>> + len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
>> + strncpy(hid->phys, ev->u.create2.phys, len);
>> + len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
>> + strncpy(hid->uniq, ev->u.create2.uniq, len);
>> hid->ll_driver = &uhid_hid_driver;
>> hid->bus = ev->u.create2.bus;
>>
>
--
Kees Cook
Hi
On Thu, Nov 15, 2018 at 12:09 AM Kees Cook <[email protected]> wrote:
> On Wed, Nov 14, 2018 at 9:40 AM, Laura Abbott <[email protected]> wrote:
[...]
> > Can we switch to strscpy instead? This will quiet gcc and avoid the
> > issues with strlcpy.
>
> Yes please: it looks like these strings are expected to be NUL
> terminated, so strscpy() without the "- 1" and min() logic would be
> the correct solution here.
"the correct solution"? To my knowledge the original code was correct
as well. Am I missing something?
> If @hid is already zero, then this would
> just be:
>
> strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
> strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
> strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));
>
> If they are NOT NUL terminated, then keep using strncpy() but mark the
> fields in the struct with the __nonstring attribute.
They are supposed to be NUL terminated, but for compatibility reasons
we allow them to be not. So I don't think your proposal is safe.
Thanks
David
On Thu, Nov 15, 2018 at 5:55 AM, David Herrmann <[email protected]> wrote:
> Hi
>
> On Thu, Nov 15, 2018 at 12:09 AM Kees Cook <[email protected]> wrote:
>> On Wed, Nov 14, 2018 at 9:40 AM, Laura Abbott <[email protected]> wrote:
> [...]
>> > Can we switch to strscpy instead? This will quiet gcc and avoid the
>> > issues with strlcpy.
>>
>> Yes please: it looks like these strings are expected to be NUL
>> terminated, so strscpy() without the "- 1" and min() logic would be
>> the correct solution here.
>
> "the correct solution"? To my knowledge the original code was correct
> as well. Am I missing something?
So, yes, no one should use strlcpy():
https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
And while I think nothing was technically wrong with the strncpy()
usage in the original version, I think strncpy() should only be used
for __nonstring cases:
https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
>
>> If @hid is already zero, then this would
>> just be:
>>
>> strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
>> strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
>> strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));
>>
>> If they are NOT NUL terminated, then keep using strncpy() but mark the
>> fields in the struct with the __nonstring attribute.
>
> They are supposed to be NUL terminated, but for compatibility reasons
> we allow them to be not. So I don't think your proposal is safe.
I was originally thinking only about the the hid->* strings, so I was
confused by this answer (they appear to always be NUL-terminated).
Then I thought you meant that ev->u.create2.* strings may not be
terminated, but I stayed confused. :)
The original code was:
len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
strncpy(hid->name, ev->u.create2.name, len);
If sizeof(hid->name) is smaller than sizeof(ev->u.create2.name), it
made sure than hid->name kept a trailing NUL.
If sizeof(ev->u.create2.name) is smaller than sizeof(hid->name), it
made sure than the last byte of ev->u.create2.name was ignored, and by
definition, hid->name would be NUL-terminated.
So you're converting from a potentially unterminated string into a
terminated string... (ev->u.create2.name maybe needs to be marked
__nonstring?)
The most you can write is sizeof(dest) - 1 but you must not read more
than sizeof(source). So I see that if the destination is smaller than
the source, you cannot represent these conditions correctly to
strscpy(). (And, I would argue, you can't with strncpy() either.)
However, they're all exactly the same size, so none of this matters,
and I think strscpy() would be the most sensible. And maybe you could
enforce the size checking:
BUILD_BUG_ON(sizeof(hid->name) != sizeof(ev->u.create2.name));
strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
etc...
--
Kees Cook
On Wed, 14 Nov 2018, David Herrmann wrote:
> This reverts commit 336fd4f5f25157e9e8bd50e898a1bbcd99eaea46.
>
> Please note that `strlcpy()` does *NOT* do what you think it does.
> strlcpy() *ALWAYS* reads the full input string, regardless of the
> 'length' parameter. That is, if the input is not zero-terminated,
> strlcpy() will *READ* beyond input boundaries. It does this, because it
> always returns the size it *would* copy if the target was big enough,
> not the truncated size it actually copied.
>
> The original code was perfectly fine. The hid device is
> zero-initialized and the strncpy() functions copied up to n-1
> characters. The result is always zero-terminated this way.
>
> This is the third time someone tried to replace strncpy with strlcpy in
> this function, and gets it wrong. I now added a comment that should at
> least make people reconsider.
>
> Signed-off-by: David Herrmann <[email protected]>
> ---
> drivers/hid/uhid.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index fefedc0b4dc6..0dfdd0ac7120 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -496,12 +496,13 @@ static int uhid_dev_create2(struct uhid_device *uhid,
> goto err_free;
> }
>
> - len = min(sizeof(hid->name), sizeof(ev->u.create2.name));
> - strlcpy(hid->name, ev->u.create2.name, len);
> - len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys));
> - strlcpy(hid->phys, ev->u.create2.phys, len);
> - len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq));
> - strlcpy(hid->uniq, ev->u.create2.uniq, len);
> + /* @hid is zero-initialized, strncpy() is correct, strlcpy() not */
> + len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
> + strncpy(hid->name, ev->u.create2.name, len);
> + len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
> + strncpy(hid->phys, ev->u.create2.phys, len);
> + len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
> + strncpy(hid->uniq, ev->u.create2.uniq, len);
Applied, thanks.
--
Jiri Kosina
SUSE Labs