Subject: RE: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c"

From: David Laight <[email-masked]>
Date: Thu, 22 Oct 2020 16:35:17 +0000

From: Christoph Hellwig
> Sent: 22 October 2020 14:24
> 
> On Thu, Oct 22, 2020 at 11:36:40AM +0200, David Hildenbrand wrote:
> > My thinking: if the compiler that calls import_iovec() has garbage in
> > the upper 32 bit
> >
> > a) gcc will zero it out and not rely on it being zero.
> > b) clang will not zero it out, assuming it is zero.
> >
> > But
> >
> > a) will zero it out when calling the !inlined variant
> > b) clang will zero it out when calling the !inlined variant
> >
> > When inlining, b) strikes. We access garbage. That would mean that we
> > have calling code that's not generated by clang/gcc IIUC.
> 
> Most callchains of import_iovec start with the assembly syscall wrappers.

Wait...
readv(2) defines:
	ssize_t readv(int fd, const struct iovec *iov, int iovcnt);

But the syscall is defined as:

SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec,
                unsigned long, vlen)
{
        return do_readv(fd, vec, vlen, 0);
}

I'm guessing that nothing actually masks the high bits that come
from an application that is compiled with clang?

The vlen is 'unsigned long' through the first few calls.
So unless there is a non-inlined function than takes vlen
as 'int' the high garbage bits from userspace are kept.

Which makes it a bug in the kernel C syscall wrappers.
They need to explicitly mask the high bits of 32bit
arguments on arm64 but not x86-64.

What does the ARM EABI say about register parameters?

	David

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