Re: [PATCH] sg, bsg: mitigate read/write abuse, block uaccess in release

From: Al Viro
Date: Fri Jun 15 2018 - 12:49:45 EST

On Fri, Jun 15, 2018 at 05:23:35PM +0200, Jann Horn wrote:
> As Al Viro noted in commit 128394eff343 ("sg_write()/bsg_write() is not fit
> to be called under KERNEL_DS"), sg and bsg improperly access userspace
> memory outside the provided buffer, permitting kernel memory corruption via
> splice().
> But they don't just do it on ->write(), also on ->read() and (in the case
> of bsg) even on ->release().
> As a band-aid, make sure that the ->read() and ->write() handlers can not
> be called in weird contexts (kernel context or credentials different from
> file opener), like for ib_safe_file_access().
> Also, completely prevent user memory accesses from ->release().

Band-aid it is, and a bloody awful one, at that. What the hell is going on
in bsg_put_device() and can it _ever_ hit that call chain? I.e.
If it can, the whole thing is FUBAR by design - ->release() may bloody well
be called in a context that has no userspace at all.

This is completely insane; what's going on there?