Re: [PATCH v2] ACPI: NFIT: prevent underflow in acpi_nfit_ctl()

[Dan Carpenter <error27@gmail.com>]

On Thu, Jan 19, 2023 at 11:21:22AM -0500, Jeff Moyer wrote:
> Dan Carpenter <error27@gmail.com> writes:
> 
> > The concern here would be that "family" is negative and we pass a
> > negative value to test_bit() resulting in an out of bounds read
> > and potentially a crash.
> 
> I don't see how this can happen.  Do you have a particular scenario in
> mind?
> 

This is from static analysis.  My main thinking was:

1) The static checker says that this comes from the user.
2) Every upper bounds check should have a lower bounds check.
3) family is passed to array_index_nospec() so we must not trust it.

But looking closer today here is what the checker is concerned about:

	func = cmd_to_func(nfit_mem, cmd, call_pkg, &family);

Assume "nfit_mem" is NULL but "call_pkg" is non NULL (user input from
__nd_ioctl() or ars_get_status().  In that case family is unchecked user
input.

But probably, it's not possible for nfit_mem to be NULL in those caller
functions?

regards,
dan carpenter