Re: [Cocci] [PATCH] coccicheck: add a test for repeat copy_from_user

[Vaishali Thakkar]

On Tuesday 27 December 2016 11:51 PM, Julia Lawall wrote:
> I totally dropped the ball on this.  Many thanks to Vaishali for
> resurrecting it.
>
> Some changes are suggested below.
>
> On Tue, 26 Apr 2016, Kees Cook wrote:
>
> > This is usually a sign of a resized request. This adds a check for
> > potential races or confusions. The check isn't 100% accurate, so it
> > needs some manual review.
> >
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  scripts/coccinelle/tests/reusercopy.cocci | 36 +++++++++++++++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> >  create mode 100644 scripts/coccinelle/tests/reusercopy.cocci
> >
> > diff --git a/scripts/coccinelle/tests/reusercopy.cocci b/scripts/coccinelle/tests/reusercopy.cocci
> > new file mode 100644
> > index 000000000000..53645de8ae95
> > --- /dev/null
> > +++ b/scripts/coccinelle/tests/reusercopy.cocci
> > @@ -0,0 +1,36 @@
> > +/// Recopying from the same user buffer frequently indicates a pattern of
> > +/// Reading a size header, allocating, and then re-reading an entire
> > +/// structure. If the structure's size is not re-validated, this can lead
> > +/// to structure or data size confusions.
> > +///
> > +// Confidence: Moderate
> > +// Copyright: (C) 2016 Kees Cook, Google. License: GPLv2.
> > +// URL: http://coccinelle.lip6.fr/
> > +// Comments:
> > +// Options: -no_includes -include_headers
>
> The options could be: --no-include --include-headers
>
> Actually, Coccinelle supports both, but it only officially supports the
> -- versions.
>
> > +
> > +virtual report
> > +virtual org
>
> Add, the following for the *s:
>
> virtual context
>
> Then add the following rule:
>
> @ok@
> position p;
> expression src,dest;
> @@
>
> copy_from_user@p(&dest, src, sizeof(dest))
>
> > +
> > +@cfu_twice@
> > +position p;
>
> Change this to:
>
> position p != ok.p;
>
> > +identifier src;
> > +expression dest1, dest2, size1, size2, offset;
> > +@@
> > +
> > +*copy_from_user(dest1, src, size1)
> > + ... when != src = offset
> > +     when != src += offset

Here, may be we should add few more lines from Pengfei's
script to avoid th potential FPs.

> Add the following lines:
>
>      when != if (size2 > e1 || ...) { ... return ...; }
>      when != if (size2 > e1 || ...) { ... size2 = e2 ... }
>
> These changes drop cases where the last argument to copy_from_usr is the
> size of the first argument, which seems safe enough, and where there is a
> test on the size value that can either update it or abort the function.
> These changes only eliminate false positives, as far as I could tell.
>
> If it would be more convenient, I could just send the complete revised
> patch, or whatever seems convenient.

I was also thinking that probably we should also add other user space 
memory API functions. May be get_user and strncpy_from_user. Although 
I'm not sure how common it is to find such patterns for both of these 
functions.

> thanks,
> julia
>
> > +*copy_from_user@p(dest2, src, size2)
> > +
> > +@script:python depends on org@
> > +p << cfu_twice.p;
> > +@@
> > +
> > +cocci.print_main("potentially dangerous second copy_from_user()",p)
> > +
> > +@script:python depends on report@
> > +p << cfu_twice.p;
> > +@@
> > +
> > +coccilib.report.print_report(p[0],"potentially dangerous second copy_from_user()")
> > --
> > 2.6.3
> >
> >
> > --
> > Kees Cook
> > Chrome OS & Brillo Security
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