Re: [PATCH] x86/Hyper-V: Fix overflow issue in the fill_gva_list()

[Tianyu Lan]

On Sat, Aug 31, 2019 at 1:41 AM Michael Kelley <mikelley@xxxxxxxxxxxxx> wrote:
>
> From: lantianyu1986@xxxxxxxxx  Sent: Thursday, August 29, 2019 11:16 PM
> >
> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> >
> > fill_gva_list() populates gva list and adds offset
> > HV_TLB_FLUSH_UNIT(0x1000000) to variable "cur"
> > in the each loop. When diff between "end" and "cur" is
> > less than HV_TLB_FLUSH_UNIT, the gva entry should
> > be the last one and the loop should be end.
> >
> > If cur is equal or greater than 0xFF000000 on 32-bit
> > mode, "cur" will be overflow after adding HV_TLB_FLUSH_UNIT.
> > Its value will be wrapped and less than "end". fill_gva_list()
> > falls into an infinite loop and fill gva list out of
> > border finally.
> >
> > Set "cur" to be "end" to make loop end when diff is
> > less than HV_TLB_FLUSH_UNIT and add HV_TLB_FLUSH_UNIT to
> > "cur" when diff is equal or greater than HV_TLB_FLUSH_UNIT.
> > Fix the overflow issue.
>
> Let me suggest simplifying the commit message a bit.  It
> doesn't need to describe every line of the code change.   I think
> it should also make clear that the same problem could occur on
> 64-bit systems with the right "start" address.  My suggestion:
>
> When the 'start' parameter is >=  0xFF000000 on 32-bit
> systems, or >= 0xFFFFFFFF'FF000000 on 64-bit systems,
> fill_gva_list gets into an infinite loop.  With such inputs,
> 'cur' overflows after adding HV_TLB_FLUSH_UNIT and always
> compares as less than end.  Memory is filled with guest virtual
> addresses until the system crashes
>
> Fix this by never incrementing 'cur' to be larger than 'end'.
>
> >
> > Reported-by: Jong Hyun Park <park.jonghyun@xxxxxxxxxxxx>
> > Signed-off-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> > Fixes: 2ffd9e33ce4a ("x86/hyper-v: Use hypercall for remote
> > TLB flush")
>
> The "Fixes:" line needs to not wrap.  It's exempt from the
> "wrap at 75 columns" rule in order to simplify parsing scripts.
>
> The code itself looks good.

Hi Michael:
       Thanks for suggestion. Update commit log in V2.
-- 
Best regards
Tianyu Lan