[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [PATCH] xen-blkback: fix compatibility bug with single page rings
> -----Original Message----- > From: Jan Beulich <jbeulich@xxxxxxxx> > Sent: 27 January 2021 10:57 > To: Paul Durrant <paul@xxxxxxx> > Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Konrad Rzeszutek Wilk > <konrad.wilk@xxxxxxxxxx>; Roger Pau > Monné <roger.pau@xxxxxxxxxx>; Jens Axboe <axboe@xxxxxxxxx>; Dongli Zhang > <dongli.zhang@xxxxxxxxxx>; > linux-kernel@xxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; > xen-devel@xxxxxxxxxxxxxxxxxxxx > Subject: Re: [PATCH] xen-blkback: fix compatibility bug with single page rings > > On 27.01.2021 11:30, Paul Durrant wrote: > > From: Paul Durrant <pdurrant@xxxxxxxxxx> > > > > Prior to commit 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid > > inconsistent xenstore 'ring-page-order' set by malicious blkfront"), the > > behaviour of xen-blkback when connecting to a frontend was: > > > > - read 'ring-page-order' > > - if not present then expect a single page ring specified by 'ring-ref' > > - else expect a ring specified by 'ring-refX' where X is between 0 and > > 1 << ring-page-order > > > > This was correct behaviour, but was broken by the afforementioned commit to > > become: > > > > - read 'ring-page-order' > > - if not present then expect a single page ring > > - expect a ring specified by 'ring-refX' where X is between 0 and > > 1 << ring-page-order > > - if that didn't work then see if there's a single page ring specified by > > 'ring-ref' > > > > This incorrect behaviour works most of the time but fails when a frontend > > that sets 'ring-page-order' is unloaded and replaced by one that does not > > because, instead of reading 'ring-ref', xen-blkback will read the stale > > 'ring-ref0' left around by the previous frontend will try to map the wrong > > grant reference. > > > > This patch restores the original behaviour. > > Isn't this only the 2nd of a pair of fixes that's needed, the > first being the drivers, upon being unloaded, to fully clean up > after itself? Any stale key left may lead to confusion upon > re-use of the containing directory. In a backend we shouldn't be relying on, nor really expect IMO, a frontend to clean up after itself. Any backend should know *exactly* what xenstore nodes it’s looking for from a frontend. Paul > > Jan
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