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Re: [RFC PATCH v1] arch/x86: Livepatch: fix overflow check when computing ELF relocations


  • To: Bjoern Doebel <doebel@xxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
  • From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Date: Tue, 8 Mar 2022 17:26:28 +0100
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  • Cc: Michael Kurth <mku@xxxxxxxxx>, Martin Pohlack <mpohlack@xxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On Tue, Mar 08, 2022 at 05:15:33PM +0100, Roger Pau Monné wrote:
> On Tue, Mar 08, 2022 at 04:45:34PM +0100, Jan Beulich wrote:
> > On 08.03.2022 16:36, Bjoern Doebel wrote:
> > > --- a/xen/arch/x86/livepatch.c
> > > +++ b/xen/arch/x86/livepatch.c
> > > @@ -339,7 +339,7 @@ int arch_livepatch_perform_rela(struct livepatch_elf 
> > > *elf,
> > >  
> > >              val -= (uint64_t)dest;
> > >              *(int32_t *)dest = val;
> > 
> > Afaict after this assignment ...
> > 
> > > -            if ( (int64_t)val != *(int32_t *)dest )
> > > +            if ( (int32_t)val != *(int32_t *)dest )
> > 
> > ... this condition can never be false. The cast really wants to be
> > to int64_t, and the overflow you saw being reported is quite likely
> > for a different reason. But from the sole message you did quote
> > it's not really possible to figure what else is wrong.
> 
> It seems Linux has that check ifdef'ed [0], but there's no reference
> as to why it's that way (I've tracked it back to the x86-64 import on
> the historical tree, f3081f5b66a06175ff2dabfe885a53fb04e71076).
> 
> It's a 64bit relocation using a 32bit value, but it's unclear to me
> that modifying the top 32bits is not allowed/intended.

Urg, I've worded this very badly. It's a 64bit relocation using a
32bit value, but it's unclear to me that modifying the top 32bits is
not allowed/intended and fine to be dropped.

Thanks, Roger.



 


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