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

[Alexander Potapenko <glider@google.com>]

On Tue, Nov 23, 2021 at 8:51 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Tue, 23 Nov 2021 at 06:17, Peter Collingbourne <pcc@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@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 will skip KASAN/etc checks as well, right? I guess it is fine b/c
> this affects just this place and the code looks safe (famous last
> words :)) and we can refine it in future.
> But I wonder about false positives under KMSAN. However, we probably
> can add an explicit KMSAN annotation to mark it as initialised.
> Alex?
>
> >         /*
> >          * Not all architectures have an exact copy_from_user(). Resort to
> > --
> > 2.34.0.rc2.393.gf8c9666880-goog
> >



-- 
Alexander Potapenko
Software Engineer

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