Re: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-enhanced functions

["Doebel, Bjoern" <doebel@xxxxxxxxx>]

On 17.03.22 11:00, Jiamei Xie wrote:
> > -----Original Message-----
> > From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> > Jiamei Xie
> > Sent: 2022年3月17日 17:17
> > To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>; Bjoern Doebel
> > <doebel@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: Michael Kurth <mku@xxxxxxxxx>; Martin Pohlack
> > <mpohlack@xxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
> > Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Konrad Rzeszutek Wilk
> > <konrad.wilk@xxxxxxxxxx>
> > Subject: RE: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-
> > enhanced functions
> >
> > Hi  Bjoern,
> >
> > > -----Original Message-----
> > > From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> > > Ross Lagerwall
> > > Sent: 2022年3月10日 1:12
> > > To: Bjoern Doebel <doebel@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > > Cc: Michael Kurth <mku@xxxxxxxxx>; Martin Pohlack
> > > <mpohlack@xxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
> > > Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Konrad Rzeszutek Wilk
> > > <konrad.wilk@xxxxxxxxxx>
> > > Subject: Re: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-
> > > enhanced functions
> > >
> > > > From: Bjoern Doebel <doebel@xxxxxxxxx>
> > > > Sent: Wednesday, March 9, 2022 2:53 PM
> > > > To: xen-devel@xxxxxxxxxxxxxxxxxxxx <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> > > > Cc: Michael Kurth <mku@xxxxxxxxx>; Martin Pohlack
> > > <mpohlack@xxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
> > > Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Bjoern Doebel
> > > <doebel@xxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>;
> > > Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> > > > Subject: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-
> > > enhanced functions
> > > > Xen enabled CET for supporting architectures. The control flow aspect of
> > > > CET expects functions that can be called indirectly (i.e., via function
> > > > pointers) to start with an ENDBR64 instruction. Otherwise a control flow
> > > > exception is raised.
> > > >
> > > > This expectation breaks livepatching flows because we patch functions by
> > > > overwriting their first 5 bytes with a JMP + <offset>, thus breaking the
> > > > ENDBR64. We fix this by checking the start of a patched function for
> > > > being ENDBR64. In the positive case we move the livepatch JMP to start
> > > > behind the ENDBR64 instruction.
> > > >
> > > > To avoid having to guess the ENDBR64 offset again on patch reversal
> > > > (which might race with other mechanisms adding/removing ENDBR
> > > > dynamically), use the livepatch metadata to store the computed offset
> > > > along with the saved bytes of the overwritten function.
> > > >
> > > > Signed-off-by: Bjoern Doebel <doebel@xxxxxxxxx>
> > > > Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > > CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> > > Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> > Tested-by: Jiamei xie <jiamei.xie@xxxxxxx>
> >
> > Cheers,
> > Jiamei
> Sorry I forgot to add the scope I tested in last email. I tested it on armv8a. 
> It worked fine and  didn't break arm.
> Tested-by: Jiamei xie <jiamei.xie@xxxxxxx>
Thanks Jiamei!

As Jan already pointed out there's a v6 patch out already. It is only 
cosmetically different from this one. Unless you insist, I'd not roll a 
v7 only to add this tag?
Bjoern



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