linux-kernel - Re: [PATCH] x86: uaccess: fix regression in unsafe_get

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Date:   Mon, 18 Feb 2019 11:15:44 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Arthur Gautier <baloo@...di.net>, Jann Horn <jannh@...gle.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        kernel list <linux-kernel@...r.kernel.org>,
        Pascal Bouchareine <pascal@...di.net>
Subject: Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user

On Mon, Feb 18, 2019 at 5:04 AM Thomas Gleixner <tglx@...utronix.de> wrote:

> > Another would be to have the buffer passed to flush_buffer() (i.e.
> > the callback of decompress_fn) allocated with 4 bytes of padding
> > past the part where the unpacked piece of data is placed for the
> > callback to find.  As in,
> >
> > diff --git a/lib/decompress_inflate.c b/lib/decompress_inflate.c
> > index 63b4b7eee138..ca3f7ecc9b35 100644
> > --- a/lib/decompress_inflate.c
> > +++ b/lib/decompress_inflate.c
> > @@ -48,7 +48,7 @@ STATIC int INIT __gunzip(unsigned char *buf, long len,
> >       rc = -1;
> >       if (flush) {
> >               out_len = 0x8000; /* 32 K */
> > -             out_buf = malloc(out_len);
> > +             out_buf = malloc(out_len + 4);
>
>   +8 actually.
>
> >       } else {
> >               if (!out_len)
> >                       out_len = ((size_t)~0) - (size_t)out_buf; /* no limit */
> >
> > for gunzip/decompress and similar ones for bzip2, etc.  The contents
> > layout doesn't have anything to do with that...
>
> Right. That works nicely.
>

This seems like it's just papering over the underlying problem: with
Jann's new checks in place, strncpy_from_user() is simply buggy.  Does
the patch below look decent?  It's only compile-tested, but it's
conceptually straightforward.  I was hoping I could get rid of the
check-maximum-address stuff, but it's needed for architectures where
the user range is adjacent to the kernel range (i.e. not x86_64).

Jann, I'm still unhappy that this code will write up to sizeof(long)-1
user-controlled garbage bytes in-bounds past the null-terminator in
the kernel buffer.  Do you think that's worth changing, too?  I don't
think it's a bug per se, but it seems like a nifty little wart for an
attacker to try to abuse.

On brief inspection, strnlen_user() does not have an equivalent bug.

View attachment "strncpy.patch" of type "text/x-patch" (1994 bytes)