`strncpy` is deprecated for use on NUL-terminated destination strings
[1]. Which seems to be the case here due to the forceful setting of `buf`'s
tail to 0.
A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on its destination buffer argument which is
_not_ the case for `strncpy`!
In this case, there is some behavior being used in conjunction with
`strncpy` that `strscpy` already implements. This means we can drop some
of the extra stuff like `... -1` and `buf[len] = 0`
This should have no functional change and yet uses a more robust and
less ambiguous interface whilst reducing code complexity.
Link: http://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: [email protected]
Signed-off-by: Justin Stitt <[email protected]>
---
For reference, see a part of `strscpy`'s implementation here:
| /* Hit buffer length without finding a NUL; force NUL-termination. */
| if (res)
| dest[res-1] = '\0';
Note: compile tested
---
arch/arm64/kernel/idreg-override.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index 2fe2491b692c..482dc5c71e90 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -262,9 +262,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
if (!len)
return;
- len = min(len, ARRAY_SIZE(buf) - 1);
- strncpy(buf, cmdline, len);
- buf[len] = 0;
+ len = min(len, ARRAY_SIZE(buf));
+ strscpy(buf, cmdline, len);
if (strcmp(buf, "--") == 0)
return;
---
base-commit: 52a93d39b17dc7eb98b6aa3edb93943248e03b2f
change-id: 20230810-strncpy-arch-arm64-1f3d328bd9b8
Best regards,
--
Justin Stitt <[email protected]>
On Thu, Aug 10, 2023 at 12:25:37PM -0700, Justin Stitt wrote:
> On Thu, Aug 10, 2023 at 12:00 PM Kees Cook <[email protected]> wrote:
> >
> > On Thu, Aug 10, 2023 at 06:39:03PM +0000, Justin Stitt wrote:
> > > `strncpy` is deprecated for use on NUL-terminated destination strings
> > > [1]. Which seems to be the case here due to the forceful setting of `buf`'s
> > > tail to 0.
> >
> > Another note to include in these evaluations would be "does the
> > destination expect to be %NUL padded?". Here, it looks like no, as all
> > the routines "buf" is passed to expect a regular C string (padding
> > doesn't matter).
> >
> > >
> > > A suitable replacement is `strscpy` [2] due to the fact that it
> > > guarantees NUL-termination on its destination buffer argument which is
> > > _not_ the case for `strncpy`!
> > >
> > > In this case, there is some behavior being used in conjunction with
> > > `strncpy` that `strscpy` already implements. This means we can drop some
> > > of the extra stuff like `... -1` and `buf[len] = 0`
> > >
> > > This should have no functional change and yet uses a more robust and
> > > less ambiguous interface whilst reducing code complexity.
> > >
> > > Link: http://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
> > > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> > > Link: https://github.com/KSPP/linux/issues/90
> > > Cc: [email protected]
> > >
> > > Signed-off-by: Justin Stitt <[email protected]>
> > > ---
> > > For reference, see a part of `strscpy`'s implementation here:
> > >
> > > | /* Hit buffer length without finding a NUL; force NUL-termination. */
> > > | if (res)
> > > | dest[res-1] = '\0';
> > >
> > > Note: compile tested
> > > ---
> > > arch/arm64/kernel/idreg-override.c | 5 ++---
> > > 1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
> > > index 2fe2491b692c..482dc5c71e90 100644
> > > --- a/arch/arm64/kernel/idreg-override.c
> > > +++ b/arch/arm64/kernel/idreg-override.c
> > > @@ -262,9 +262,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
> > > if (!len)
> > > return;
> > >
> > > - len = min(len, ARRAY_SIZE(buf) - 1);
> > > - strncpy(buf, cmdline, len);
> > > - buf[len] = 0;
> > > + len = min(len, ARRAY_SIZE(buf));
> > > + strscpy(buf, cmdline, len);
> >
> > This, however, isn't correct: "cmdline" will be incremented by "leN"
> > later, and we want a count of the characters copied into "buf", even if
> > they're truncated. I think this should be:
> >
> > strscpy(buf, cmdline, ARRAY_SIZE(buf));
> > len = strlen(buf);
> >
> Thoughts on using the return value from `strscpy` here?
This code seems to silently accept truncation, so -E2BIG will cause a
problem if it only looks at the return value.
-Kees
--
Kees Cook
On Thu, Aug 10, 2023 at 12:58 PM Kees Cook <[email protected]> wrote:
>
> On Thu, Aug 10, 2023 at 12:25:37PM -0700, Justin Stitt wrote:
> > On Thu, Aug 10, 2023 at 12:00 PM Kees Cook <[email protected]> wrote:
> > >
> > > On Thu, Aug 10, 2023 at 06:39:03PM +0000, Justin Stitt wrote:
> > > > `strncpy` is deprecated for use on NUL-terminated destination strings
> > > > [1]. Which seems to be the case here due to the forceful setting of `buf`'s
> > > > tail to 0.
> > >
> > > Another note to include in these evaluations would be "does the
> > > destination expect to be %NUL padded?". Here, it looks like no, as all
> > > the routines "buf" is passed to expect a regular C string (padding
> > > doesn't matter).
> > >
> > > >
> > > > A suitable replacement is `strscpy` [2] due to the fact that it
> > > > guarantees NUL-termination on its destination buffer argument which is
> > > > _not_ the case for `strncpy`!
> > > >
> > > > In this case, there is some behavior being used in conjunction with
> > > > `strncpy` that `strscpy` already implements. This means we can drop some
> > > > of the extra stuff like `... -1` and `buf[len] = 0`
> > > >
> > > > This should have no functional change and yet uses a more robust and
> > > > less ambiguous interface whilst reducing code complexity.
> > > >
> > > > Link: http://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
> > > > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> > > > Link: https://github.com/KSPP/linux/issues/90
> > > > Cc: [email protected]
> > > >
> > > > Signed-off-by: Justin Stitt <[email protected]>
> > > > ---
> > > > For reference, see a part of `strscpy`'s implementation here:
> > > >
> > > > | /* Hit buffer length without finding a NUL; force NUL-termination. */
> > > > | if (res)
> > > > | dest[res-1] = '\0';
> > > >
> > > > Note: compile tested
> > > > ---
> > > > arch/arm64/kernel/idreg-override.c | 5 ++---
> > > > 1 file changed, 2 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
> > > > index 2fe2491b692c..482dc5c71e90 100644
> > > > --- a/arch/arm64/kernel/idreg-override.c
> > > > +++ b/arch/arm64/kernel/idreg-override.c
> > > > @@ -262,9 +262,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
> > > > if (!len)
> > > > return;
> > > >
> > > > - len = min(len, ARRAY_SIZE(buf) - 1);
> > > > - strncpy(buf, cmdline, len);
> > > > - buf[len] = 0;
> > > > + len = min(len, ARRAY_SIZE(buf));
> > > > + strscpy(buf, cmdline, len);
Perhaps keeping the `... - 1` is good because we then don't have to
check strlen immediately after. This does still silently truncate but
didn't the previous `strncpy` also do that?
> > >
> > > This, however, isn't correct: "cmdline" will be incremented by "leN"
> > > later, and we want a count of the characters copied into "buf", even if
> > > they're truncated. I think this should be:
> > >
> > > strscpy(buf, cmdline, ARRAY_SIZE(buf));
> > > len = strlen(buf);
> > >
> > Thoughts on using the return value from `strscpy` here?
>
> This code seems to silently accept truncation, so -E2BIG will cause a
> problem if it only looks at the return value.
>
> -Kees
>
> --
> Kees Cook
On August 10, 2023 2:17:41 PM PDT, Justin Stitt <[email protected]> wrote:
>On Thu, Aug 10, 2023 at 12:58 PM Kees Cook <[email protected]> wrote:
>>
>> On Thu, Aug 10, 2023 at 12:25:37PM -0700, Justin Stitt wrote:
>> > On Thu, Aug 10, 2023 at 12:00 PM Kees Cook <[email protected]> wrote:
>> > >
>> > > On Thu, Aug 10, 2023 at 06:39:03PM +0000, Justin Stitt wrote:
>> > > > `strncpy` is deprecated for use on NUL-terminated destination strings
>> > > > [1]. Which seems to be the case here due to the forceful setting of `buf`'s
>> > > > tail to 0.
>> > >
>> > > Another note to include in these evaluations would be "does the
>> > > destination expect to be %NUL padded?". Here, it looks like no, as all
>> > > the routines "buf" is passed to expect a regular C string (padding
>> > > doesn't matter).
>> > >
>> > > >
>> > > > A suitable replacement is `strscpy` [2] due to the fact that it
>> > > > guarantees NUL-termination on its destination buffer argument which is
>> > > > _not_ the case for `strncpy`!
>> > > >
>> > > > In this case, there is some behavior being used in conjunction with
>> > > > `strncpy` that `strscpy` already implements. This means we can drop some
>> > > > of the extra stuff like `... -1` and `buf[len] = 0`
>> > > >
>> > > > This should have no functional change and yet uses a more robust and
>> > > > less ambiguous interface whilst reducing code complexity.
>> > > >
>> > > > Link: http://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
>> > > > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
>> > > > Link: https://github.com/KSPP/linux/issues/90
>> > > > Cc: [email protected]
>> > > >
>> > > > Signed-off-by: Justin Stitt <[email protected]>
>> > > > ---
>> > > > For reference, see a part of `strscpy`'s implementation here:
>> > > >
>> > > > | /* Hit buffer length without finding a NUL; force NUL-termination. */
>> > > > | if (res)
>> > > > | dest[res-1] = '\0';
>> > > >
>> > > > Note: compile tested
>> > > > ---
>> > > > arch/arm64/kernel/idreg-override.c | 5 ++---
>> > > > 1 file changed, 2 insertions(+), 3 deletions(-)
>> > > >
>> > > > diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
>> > > > index 2fe2491b692c..482dc5c71e90 100644
>> > > > --- a/arch/arm64/kernel/idreg-override.c
>> > > > +++ b/arch/arm64/kernel/idreg-override.c
>> > > > @@ -262,9 +262,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
>> > > > if (!len)
>> > > > return;
>> > > >
>> > > > - len = min(len, ARRAY_SIZE(buf) - 1);
>> > > > - strncpy(buf, cmdline, len);
>> > > > - buf[len] = 0;
>> > > > + len = min(len, ARRAY_SIZE(buf));
>> > > > + strscpy(buf, cmdline, len);
>Perhaps keeping the `... - 1` is good because we then don't have to
>check strlen immediately after. This does still silently truncate but
>didn't the previous `strncpy` also do that?
Ah, actually there's no need to get too tricky here. This should be behaviorally identical:
len = strscpy(buf, cmdline, ARRAY_SIZE(buf));
if (len == -E2BIG)
len = ARRAY_SIZE(buf) - 1;
-Kees
--
Kees Cook