Re: [XEN PATCH v2 1/3] libxl: Fix nul-termination of the return value of libxl_xen_console_read_line()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Aug 30, 2024 at 08:22:29PM +0100, Andrew Cooper wrote:
> On 29/08/2024 4:53 pm, Roger Pau Monné wrote:
> > On Fri, Aug 23, 2024 at 06:05:03PM +0100, Javi Merino wrote:
> >> When built with ASAN, "xl dmesg" crashes in the "printf("%s", line)"
> >> call in main_dmesg().  ASAN reports a heap buffer overflow: an
> >> off-by-one access to cr->buffer.
> >>
> >> The readconsole sysctl copies up to count characters into the buffer,
> >> but it does not add a null character at the end.  Despite the
> >> documentation of libxl_xen_console_read_line(), line_r is not
> >> nul-terminated if 16384 characters were copied to the buffer.
> >>
> >> Fix this by asking xc_readconsolering() to fill the buffer up to size
> >> - 1.  As the number of characters in the buffer is only needed in
> >> libxl_xen_console_read_line(), make it a local variable there instead
> >> of part of the libxl__xen_console_reader struct.
> > Instead of playing games with the buffer size in order to add an extra
> > NUL character, we could possibly use libxl_write_exactly(ctx,
> > STDOUT_FILENO,...) in main_dmesg(), using cr->count as the buffer
> > length?
> 
> Sadly no.
> 
> struct libxl__xen_console_reader (which has the count field) is a libxl
> private (opaque) type which `xl` can't access.

I was fooled by the libxl_xen_console_reader typedef.

> Otherwise this would be a oneline fix already...

Hm, maybe the easiest way is to introduce a new function that returns
the buffer and the number of characters?
(libxl_xen_console_dump_buffer() or similar?)

Thanks, Roger.