Date: Fri, 15 Jun 2018 18:58:01 +0200 From: Jann Horn <jannh@...gle.com> To: Al Viro <viro@...iv.linux.org.uk> Cc: axboe@...nel.dk, fujita.tomonori@....ntt.co.jp, dgilbert@...erlog.com, jejb@...ux.vnet.ibm.com, martin.petersen@...cle.com, linux-block@...r.kernel.org, linux-scsi@...r.kernel.org, kernel list <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, security@...nel.org Subject: Re: [PATCH] sg, bsg: mitigate read/write abuse, block uaccess in release On Fri, Jun 15, 2018 at 6:49 PM Al Viro <viro@...iv.linux.org.uk> wrote: > > 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. > bsg_release() > bsg_put_device() > blk_complete_sgv4_hdr_rq() > ->complete_rq() > copy_to_user() > 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? Perhaps I should have split my patch into two parts; it consists of two somewhat related changes. The first change is that ->read() and ->write() violate the normal contract and, as a band-aid, should not be called in uaccess_kernel() context or with changed creds. The second change is an actual fix: AFAICS ->release() accidentally accessed userspace, which I've fixed using the added "cleaning_up" parameter.
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