[PATCH v2 1/5] fs: use raw_copy_from_user() to copy mount() data

[Peter Collingbourne]

On Tue, Nov 23, 2021 at 2:09 AM Alexander Potapenko <glider at google.com> wrote:
>
> On Tue, Nov 23, 2021 at 8:51 AM Dmitry Vyukov <dvyukov at google.com> wrote:
> >
> > On Tue, 23 Nov 2021 at 06:17, Peter Collingbourne <pcc at google.com> wrote:
> > >
> > > With uaccess logging the contract is that the kernel must not report
> > > accessing more data than necessary, as this can lead to false positive
> > > reports in downstream consumers. This generally works out of the box
> > > when instrumenting copy_{from,to}_user(), but with the data argument
> > > to mount() we use copy_from_user() to copy PAGE_SIZE bytes (or as
> > > much as we can, if the PAGE_SIZE sized access failed) and figure out
> > > later how much we actually need.
> > >
> > > To prevent this from leading to a false positive report, use
> > > raw_copy_from_user(), which will prevent the access from being logged.
> > > Recall that it is valid for the kernel to report accessing less
> > > data than it actually accessed, as uaccess logging is a best-effort
> > > mechanism for reporting uaccesses.
> > >
> > > Link: https://linux-review.googlesource.com/id/I5629b92a725c817acd9a861288338dd605cafee6
> > > Signed-off-by: Peter Collingbourne <pcc at google.com>
> > > ---
> > >  fs/namespace.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/namespace.c b/fs/namespace.c
> > > index 659a8f39c61a..695b30e391f0 100644
> > > --- a/fs/namespace.c
> > > +++ b/fs/namespace.c
> > > @@ -3197,7 +3197,12 @@ static void *copy_mount_options(const void __user * data)
> > >         if (!copy)
> > >                 return ERR_PTR(-ENOMEM);
> > >
> > > -       left = copy_from_user(copy, data, PAGE_SIZE);
> > > +       /*
> > > +        * Use raw_copy_from_user to avoid reporting overly large accesses in
> > > +        * the uaccess buffer, as this can lead to false positive reports in
> > > +        * downstream consumers.
> > > +        */
> > > +       left = raw_copy_from_user(copy, data, PAGE_SIZE);
>
> I don't really like the idea of using raw_copy_from_user() anywhere.
> Right now users of instrumented.h can more or less assume they see all
> usercopy events, and removing the copy_from_user() call from here
> looks like a regression.
>
> Cannot the usercopy logger decide whether it wants to log the access
> based on the size (e.g. skip accesses >= PAGE_SIZE)?
> Will it help if we can instrument both sides of copy_from_user() (see
> the code here: https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/14103/4)?
>
> If not, maybe we can disable/enable uaccess logging for the current
> task around these accesses?

This seems reasonable, done in v3.

Peter