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Date:   Sat, 16 Feb 2019 23:25:18 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jann Horn <jannh@google.com>
cc:     baloo@gandi.net, Andy Lutomirski <luto@amacapital.net>,
        the arch/x86 maintainers <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        kernel list <linux-kernel@vger.kernel.org>,
        Pascal Bouchareine <pascal@gandi.net>
Subject: Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user
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On Sat, 16 Feb 2019, Thomas Gleixner wrote:

> On Sat, 16 Feb 2019, Jann Horn wrote:
> > On Sat, Feb 16, 2019 at 12:59 AM <baloo@gandi.net> wrote:
> > > When extracting an initramfs, a filename may be near an allocation boundary.
> > > Should that happen, strncopy_from_user will invoke unsafe_get_user which
> > > may cross the allocation boundary. Should that happen, unsafe_get_user will
> > > trigger a page fault, and strncopy_from_user would then bailout to
> > > byte_at_a_time behavior.
> > >
> > > unsafe_get_user is unsafe by nature, and rely on pagefault to detect boundaries.
> > > After 9da3f2b74054 ("x86/fault: BUG() when uaccess helpers fault on kernel addresses")
> > > it may no longer rely on pagefault as the new page fault handler would
> > > trigger a BUG().
> > >
> > > This commit allows unsafe_get_user to explicitly trigger pagefaults and
> > > handle them directly with the error target label.
> > 
> > Oof. So basically the init code is full of things that just call
> > syscalls instead of using VFS functions (which don't actually exist
> > for everything), and the VFS syscalls use getname_flags(), which uses
> > strncpy_from_user(), which can access out-of-bounds pages on
> > architectures that set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, and
> > that in summary means that all the init code is potentially prone to
> > tripping over this?
> 
> Not all init code. It should be only the initramfs decompression.
> 
> > I don't particularly like this approach to fixing it, but I also don't
> > have any better ideas, so I guess unless someone else has a bright
> > idea, this patch might have to go in.
> 
> So we know that this happens in the context of decompress() which calls
> flush_buffer() for every chunk. flush_buffer() gets the start_address and
> the length. We also know that the fault can only happen within:
> 
>     start_address <= fault_address < start_address + length + 8;
> 
> So something like the untested workaround below should cover the initramfs
> oddity and avoid to weaken the protection for all other cases.

The other even simpler solution would be to force these functions into the
byte at a time code path during init. Too tired to hack that up now.

Thanks,

	tglx