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Re: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-enhanced functions
- To: Bjoern Doebel <doebel@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- Date: Wed, 9 Mar 2022 17:12:05 +0000
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- 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>
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- Thread-topic: [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>
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