Messages in this thread | | | Date | Thu, 18 Jun 2009 17:43:31 +0200 | Subject | Re: [PATCH] ide-cd: prevent null pointer deref via cdrom_newpc_intr | From | Borislav Petkov <> |
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Hi,
On Thu, Jun 18, 2009 at 4:52 PM, Rainer Weikusat<rweikusat@mssgmbh.com> wrote: > Borislav Petkov <petkovbb@googlemail.com> writes: >> On Thu, Jun 18, 2009 at 3:48 PM, Rainer Weikusat<rweikusat@mssgmbh.com> wrote: >>> From: Rainer Weikusat <rweikusat@mssgmbh.com> >>> >>> With 2.6.30, the error handling code in cdrom_newpc_intr was changed >>> to deal with partial request failures by normally completing the 'good' >>> parts of a request and only 'error' the last (and presumably, >>> incompletely transferred) bio associated with a particular >>> request. This doesn't work for requests which don't have bios >>> associated with them ('GPCMD_READ_DISC_INFO'), because the first call >>> to ide_end_rq, done via ide_complete_rq in order to do the >>> partial completion part, returns with a code of zero for all non-bio >>> requests, causing the drive->hwif->rq pointer to be set to NULL. >> >> This is a bit misleading, it should be more like: "ide_complete_rq is >> called over ide_cd_error_cmd() to partially complete the rq but the rq >> is without a bio and the block layer does partial completion only for >> requests with bio's so this request is completed as a whole and the rq >> freed." > > Technically, this is not quite correct (assuming I haven't overlooked > something), because ide_cd_queue_pc still has a reference to the rq.
That doesn't matter because the OOPS happens after the command has been issued and _before_ ide_cd_queue_pc() gets to access the rq ptr again.
ide_cd_queue_pc() does blk_execute_rq() which does wait for completion of the request so it doesn't access the rq pointer before the IRQ handler has completed. Even if it would reach the blk_put_request part, the OOPS would have already happened.
ide_complete_rq simply does
blk_end_request |->blk_end_bidi_request |->blk_finish_request
after checking the rq->bio pointer through blk_update_bidi_request() and if the prior is NULL it does __blk_put_request in blk_finish_request and this is where the thing vanishes.
The second time ide_complete_rq() comes to run at the end of the IRQ handler the rq is already 0xdeadbeef :).
(this is all latest git but the old code (without the bidi stuff) did the same thing wrt to rq->bio).
-- Regards/Gruss, Boris
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