Re: [PATCH mm] maccess: fix strncpy_from_user_nofault empty string handling

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Apr 17, 2025 at 1:44 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Apr 17, 2025 at 8:28 AM Mykyta Yatsenko
> <mykyta.yatsenko5@gmail.com> wrote:
> >
> > From: Mykyta Yatsenko <yatsenko@meta.com>
> >
> > strncpy_from_user_nofault should return the length of the copied string
> > including the trailing NUL, but if the argument unsafe_addr points to
> > an empty string ({'\0'}), the return value is 0.
> >
> > This happens as strncpy_from_user copies terminal symbol into dst
> > and returns 0 (as expected), but strncpy_from_user_nofault does not
> > modify ret as it is not equal to count and not greater than 0, so 0 is
> > returned, which contradicts the contract.
> >
> > Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> > ---
> >  kernel/trace/trace_events_filter.c | 10 ++++++++--
> >  mm/maccess.c                       |  2 +-
> >  2 files changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> > index 0993dfc1c5c1..86b7e5a4e235 100644
> > --- a/kernel/trace/trace_events_filter.c
> > +++ b/kernel/trace/trace_events_filter.c
> > @@ -800,6 +800,7 @@ static __always_inline char *test_string(char *str)
> >  {
> >         struct ustring_buffer *ubuf;
> >         char *kstr;
> > +       int cnt;
> >
> >         if (!ustring_per_cpu)
> >                 return NULL;
> > @@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str)
> >         kstr = ubuf->buffer;
> >
> >         /* For safety, do not trust the string pointer */
> > -       if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
> > +       cnt = strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE);
> > +       /* Return null if empty string or error */
> > +       if (cnt <= 1)
> >                 return NULL;
>
> I wouldn't touch this part and leave it up to Steven to fix (if he
> agrees it needs fixing). Current logic seems wrong already, as it
> won't correctly handle -EFAULT. And, on the other hand, there is
> nothing wrong or special about empty string, so I don't think it needs
> special handling. Let's drop these changes in trace_events_filter.c?

Actually, you are not even touching strncpy_from_kernel_nofault(), so
yeah, definitely let's not do this (at least not in this patch).

>
> >         return kstr;
> >  }
> > @@ -818,6 +821,7 @@ static __always_inline char *test_ustring(char *str)
> >         struct ustring_buffer *ubuf;
> >         char __user *ustr;
> >         char *kstr;
> > +       int cnt;
> >
> >         if (!ustring_per_cpu)
> >                 return NULL;
> > @@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str)
> >
> >         /* user space address? */
> >         ustr = (char __user *)str;
> > -       if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
> > +       cnt = strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE);
> > +       /* Return null if empty string or error */
> > +       if (cnt <= 1)
> >                 return NULL;
>
> ditto
>
> >
> >         return kstr;
> > diff --git a/mm/maccess.c b/mm/maccess.c
> > index 8f0906180a94..831b4dd7296c 100644
> > --- a/mm/maccess.c
> > +++ b/mm/maccess.c
> > @@ -196,7 +196,7 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
> >         if (ret >= count) {
> >                 ret = count;
> >                 dst[ret - 1] = '\0';
> > -       } else if (ret > 0) {
> > +       } else if (ret >= 0) {
> >                 ret++;
> >         }
> >
>
> This part looks good and does indeed fix the issue. Good catch!
>
> Reviewed-by: Andrii Nakryiko <andrii@kernel.org>
>
> > --
> > 2.49.0
> >