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[PATCH 2/2] ocfs2: Replace strlcpy with strscpy

Azeem Shaikh posted 2 patches 2 years, 5 months ago
[PATCH 2/2] ocfs2: Replace strlcpy with strscpy
Posted by Azeem Shaikh 2 years, 5 months ago 
strlcpy() reads the entire source buffer first.
This read may exceed the destination size limit.
This is both inefficient and can lead to linear read
overflows if a source string is not NUL-terminated [1].
In an effort to remove strlcpy() completely [2], replace
strlcpy() here with strscpy().

Direct replacement is assumed to be safe here since
it's ok for `kernel_param_ops.get()` to return -errno [3].
This changes the behavior such that instead of silently ignoring the
case when sizeof(@buffer) < DLMFS_CAPABILITIES, we now return error.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89
[3] https://elixir.bootlin.com/linux/v6.5/source/include/linux/moduleparam.h#L52

Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
---
 fs/ocfs2/dlmfs/dlmfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ocfs2/dlmfs/dlmfs.c b/fs/ocfs2/dlmfs/dlmfs.c
index 33e529de93b2..b001eccdd2f3 100644
--- a/fs/ocfs2/dlmfs/dlmfs.c
+++ b/fs/ocfs2/dlmfs/dlmfs.c
@@ -80,7 +80,7 @@ static int param_set_dlmfs_capabilities(const char *val,
 static int param_get_dlmfs_capabilities(char *buffer,
 					const struct kernel_param *kp)
 {
-	return strlcpy(buffer, DLMFS_CAPABILITIES,
+	return strscpy(buffer, DLMFS_CAPABILITIES,
 		       strlen(DLMFS_CAPABILITIES) + 1);
 }
 static const struct kernel_param_ops dlmfs_capabilities_ops = {
--
2.42.0.283.g2d96d420d3-goog
Re: [PATCH 2/2] ocfs2: Replace strlcpy with strscpy
Posted by Kees Cook 2 years, 5 months ago 
On Wed, Aug 30, 2023 at 09:54:26PM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.
> This read may exceed the destination size limit.
> This is both inefficient and can lead to linear read
> overflows if a source string is not NUL-terminated [1].
> In an effort to remove strlcpy() completely [2], replace
> strlcpy() here with strscpy().
> 
> Direct replacement is assumed to be safe here since
> it's ok for `kernel_param_ops.get()` to return -errno [3].
> This changes the behavior such that instead of silently ignoring the
> case when sizeof(@buffer) < DLMFS_CAPABILITIES, we now return error.
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> [2] https://github.com/KSPP/linux/issues/89
> [3] https://elixir.bootlin.com/linux/v6.5/source/include/linux/moduleparam.h#L52
> 
> Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> ---
>  fs/ocfs2/dlmfs/dlmfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ocfs2/dlmfs/dlmfs.c b/fs/ocfs2/dlmfs/dlmfs.c
> index 33e529de93b2..b001eccdd2f3 100644
> --- a/fs/ocfs2/dlmfs/dlmfs.c
> +++ b/fs/ocfs2/dlmfs/dlmfs.c
> @@ -80,7 +80,7 @@ static int param_set_dlmfs_capabilities(const char *val,
>  static int param_get_dlmfs_capabilities(char *buffer,
>  					const struct kernel_param *kp)
>  {
> -	return strlcpy(buffer, DLMFS_CAPABILITIES,
> +	return strscpy(buffer, DLMFS_CAPABILITIES,
>  		       strlen(DLMFS_CAPABILITIES) + 1);
>  }

This is another case of "accidentally correct".


param->get() is hooked here, in the sysfs "show" callback:

static ssize_t param_attr_show(struct module_attribute *mattr,
                               struct module_kobject *mk, char *buf)
{
        int count;
        struct param_attribute *attribute = to_param_attr(mattr);

        if (!attribute->param->ops->get)
                return -EPERM;

        kernel_param_lock(mk->mod);
        count = attribute->param->ops->get(buf, attribute->param);
        kernel_param_unlock(mk->mod);
        return count;
}

Meaning ultimately this will show up here, if I'm reading names right:
/sys/module/ocfs/parameters/dlmfs_capabilities

Anyway, the "count" being returned would be quite bad if
DLMFS_CAPABILITIES were dynamic and larger than PAGE_SIZE (the size of
the sysfs buffer).

For this case, I would say replace strlcpy with sysfs_emit:

	return sysfs_emit(buffer, DLMFS_CAPABILITIES);

(Also, ew, existing code doesn't include a trailing "\n". Oh well.)

-Kees

-- 
Kees Cook
Re: [PATCH 2/2] ocfs2: Replace strlcpy with strscpy
Posted by Azeem Shaikh 2 years, 5 months ago 
On Wed, Aug 30, 2023 at 7:06 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Aug 30, 2023 at 09:54:26PM +0000, Azeem Shaikh wrote:
> > strlcpy() reads the entire source buffer first.
> > This read may exceed the destination size limit.
> > This is both inefficient and can lead to linear read
> > overflows if a source string is not NUL-terminated [1].
> > In an effort to remove strlcpy() completely [2], replace
> > strlcpy() here with strscpy().
> >
> > Direct replacement is assumed to be safe here since
> > it's ok for `kernel_param_ops.get()` to return -errno [3].
> > This changes the behavior such that instead of silently ignoring the
> > case when sizeof(@buffer) < DLMFS_CAPABILITIES, we now return error.
> >
> > [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> > [2] https://github.com/KSPP/linux/issues/89
> > [3] https://elixir.bootlin.com/linux/v6.5/source/include/linux/moduleparam.h#L52
> >
> > Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> > ---
> >  fs/ocfs2/dlmfs/dlmfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/ocfs2/dlmfs/dlmfs.c b/fs/ocfs2/dlmfs/dlmfs.c
> > index 33e529de93b2..b001eccdd2f3 100644
> > --- a/fs/ocfs2/dlmfs/dlmfs.c
> > +++ b/fs/ocfs2/dlmfs/dlmfs.c
> > @@ -80,7 +80,7 @@ static int param_set_dlmfs_capabilities(const char *val,
> >  static int param_get_dlmfs_capabilities(char *buffer,
> >                                       const struct kernel_param *kp)
> >  {
> > -     return strlcpy(buffer, DLMFS_CAPABILITIES,
> > +     return strscpy(buffer, DLMFS_CAPABILITIES,
> >                      strlen(DLMFS_CAPABILITIES) + 1);
> >  }
>
> This is another case of "accidentally correct".
>
>
> param->get() is hooked here, in the sysfs "show" callback:
>
> static ssize_t param_attr_show(struct module_attribute *mattr,
>                                struct module_kobject *mk, char *buf)
> {
>         int count;
>         struct param_attribute *attribute = to_param_attr(mattr);
>
>         if (!attribute->param->ops->get)
>                 return -EPERM;
>
>         kernel_param_lock(mk->mod);
>         count = attribute->param->ops->get(buf, attribute->param);
>         kernel_param_unlock(mk->mod);
>         return count;
> }
>
> Meaning ultimately this will show up here, if I'm reading names right:
> /sys/module/ocfs/parameters/dlmfs_capabilities
>
> Anyway, the "count" being returned would be quite bad if
> DLMFS_CAPABILITIES were dynamic and larger than PAGE_SIZE (the size of
> the sysfs buffer).
>
> For this case, I would say replace strlcpy with sysfs_emit:
>
>         return sysfs_emit(buffer, DLMFS_CAPABILITIES);
>

Thanks, sending out a v2 for this. Out of curiosity - why sysfs_emit?
Is it because DLMFS_CAPABILITIES is a hard-coded string?
Re: [PATCH 2/2] ocfs2: Replace strlcpy with strscpy
Posted by Kees Cook 2 years, 5 months ago 
On Thu, Aug 31, 2023 at 03:28:32PM -0400, Azeem Shaikh wrote:
> On Wed, Aug 30, 2023 at 7:06 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Wed, Aug 30, 2023 at 09:54:26PM +0000, Azeem Shaikh wrote:
> > > strlcpy() reads the entire source buffer first.
> > > This read may exceed the destination size limit.
> > > This is both inefficient and can lead to linear read
> > > overflows if a source string is not NUL-terminated [1].
> > > In an effort to remove strlcpy() completely [2], replace
> > > strlcpy() here with strscpy().
> > >
> > > Direct replacement is assumed to be safe here since
> > > it's ok for `kernel_param_ops.get()` to return -errno [3].
> > > This changes the behavior such that instead of silently ignoring the
> > > case when sizeof(@buffer) < DLMFS_CAPABILITIES, we now return error.
> > >
> > > [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> > > [2] https://github.com/KSPP/linux/issues/89
> > > [3] https://elixir.bootlin.com/linux/v6.5/source/include/linux/moduleparam.h#L52
> > >
> > > Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> > > ---
> > >  fs/ocfs2/dlmfs/dlmfs.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/fs/ocfs2/dlmfs/dlmfs.c b/fs/ocfs2/dlmfs/dlmfs.c
> > > index 33e529de93b2..b001eccdd2f3 100644
> > > --- a/fs/ocfs2/dlmfs/dlmfs.c
> > > +++ b/fs/ocfs2/dlmfs/dlmfs.c
> > > @@ -80,7 +80,7 @@ static int param_set_dlmfs_capabilities(const char *val,
> > >  static int param_get_dlmfs_capabilities(char *buffer,
> > >                                       const struct kernel_param *kp)
> > >  {
> > > -     return strlcpy(buffer, DLMFS_CAPABILITIES,
> > > +     return strscpy(buffer, DLMFS_CAPABILITIES,
> > >                      strlen(DLMFS_CAPABILITIES) + 1);
> > >  }
> >
> > This is another case of "accidentally correct".
> >
> >
> > param->get() is hooked here, in the sysfs "show" callback:
> >
> > static ssize_t param_attr_show(struct module_attribute *mattr,
> >                                struct module_kobject *mk, char *buf)
> > {
> >         int count;
> >         struct param_attribute *attribute = to_param_attr(mattr);
> >
> >         if (!attribute->param->ops->get)
> >                 return -EPERM;
> >
> >         kernel_param_lock(mk->mod);
> >         count = attribute->param->ops->get(buf, attribute->param);
> >         kernel_param_unlock(mk->mod);
> >         return count;
> > }
> >
> > Meaning ultimately this will show up here, if I'm reading names right:
> > /sys/module/ocfs/parameters/dlmfs_capabilities
> >
> > Anyway, the "count" being returned would be quite bad if
> > DLMFS_CAPABILITIES were dynamic and larger than PAGE_SIZE (the size of
> > the sysfs buffer).
> >
> > For this case, I would say replace strlcpy with sysfs_emit:
> >
> >         return sysfs_emit(buffer, DLMFS_CAPABILITIES);
> >
> 
> Thanks, sending out a v2 for this. Out of curiosity - why sysfs_emit?
> Is it because DLMFS_CAPABILITIES is a hard-coded string?

It's basically that sysfs_emit() Does The Right Thing for these
callbacks: it tracks the offset of the buffer and checks for buffer
overflow. (All the sysfs callback work on a page-aligned page-sized
buffer, and the API lacks any length info -- it's "understood" to be
page-sized.) So, since this is doing a string copy into what is
ultimately a sysfs buffer, it's best to use sysfs_emit().

-- 
Kees Cook
Re: [PATCH 2/2] ocfs2: Replace strlcpy with strscpy
Posted by Justin Stitt 2 years, 5 months ago 
On Wed, Aug 30, 2023 at 09:54:26PM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.
> This read may exceed the destination size limit.
> This is both inefficient and can lead to linear read
> overflows if a source string is not NUL-terminated [1].
> In an effort to remove strlcpy() completely [2], replace
> strlcpy() here with strscpy().
>
> Direct replacement is assumed to be safe here since
> it's ok for `kernel_param_ops.get()` to return -errno [3].
> This changes the behavior such that instead of silently ignoring the
> case when sizeof(@buffer) < DLMFS_CAPABILITIES, we now return error.

Not super familiar with the semantics of `kernel_param_ops.get()` but do
note that strscpy can only return -E2BIG and not -ERRNO specifically. Is
this still OK?

>
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> [2] https://github.com/KSPP/linux/issues/89
> [3] https://elixir.bootlin.com/linux/v6.5/source/include/linux/moduleparam.h#L52
>
> Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> ---
>  fs/ocfs2/dlmfs/dlmfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ocfs2/dlmfs/dlmfs.c b/fs/ocfs2/dlmfs/dlmfs.c
> index 33e529de93b2..b001eccdd2f3 100644
> --- a/fs/ocfs2/dlmfs/dlmfs.c
> +++ b/fs/ocfs2/dlmfs/dlmfs.c
> @@ -80,7 +80,7 @@ static int param_set_dlmfs_capabilities(const char *val,
>  static int param_get_dlmfs_capabilities(char *buffer,
>  					const struct kernel_param *kp)
>  {
> -	return strlcpy(buffer, DLMFS_CAPABILITIES,
> +	return strscpy(buffer, DLMFS_CAPABILITIES,
>  		       strlen(DLMFS_CAPABILITIES) + 1);
>  }
>  static const struct kernel_param_ops dlmfs_capabilities_ops = {
> --
> 2.42.0.283.g2d96d420d3-goog
>

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