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Re: [RFC PATCH v1] arch/x86: Livepatch: fix overflow check when computing ELF relocations
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 8 Mar 2022 17:15:33 +0100
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- Cc: Bjoern Doebel <doebel@xxxxxxxxx>, 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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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.
Regards, Roger.
[0] https://elixir.bootlin.com/linux/latest/source/arch/x86/kernel/module.c#L190
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