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Re: [PATCH v2] livepatch: account for patch offset when applying NOP patch
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 31 Mar 2022 08:42:47 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, Konrad Wilk <konrad.wilk@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Bjoern Doebel <doebel@xxxxxxxxx>
- Delivery-date: Thu, 31 Mar 2022 06:42:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.03.2022 19:04, Roger Pau Monné wrote:
> On Wed, Mar 30, 2022 at 01:05:31PM +0200,>> --- a/xen/arch/x86/livepatch.c
>> +++ b/xen/arch/x86/livepatch.c
>> @@ -157,9 +157,15 @@ void noinline arch_livepatch_apply(struc
>> * loaded hotpatch (to avoid racing against other fixups adding/removing
>> * ENDBR64 or similar instructions).
>> */
>> - if ( is_endbr64(old_ptr) || is_endbr64_poison(func->old_addr) )
>> + if ( len >= ENDBR64_LEN &&
>
> Sorry, didn't realize before, but shouldn't this check be using
> old_size instead of len (which is based on new_size)?
Yes and no: In principle yes, but with len == func->new_size in the NOP
case, and with arch_livepatch_verify_func() guaranteeing new_size <=
old_size, the check is still fine for that case. Plus: If new_size was
less than 4 _but_ there's an ENDBR64 at the start, what would we do? I
think there's more that needs fixing in this regard. So I guess I'll
make a v3 with this extra fix dropped and with the livepatch_insn_len()
invocation simply moved. After all the primary goal is to get the
stable trees unstuck.
Jan
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