Re: [edk2-devel] [PATCH] MdeModulePkg/SMM: Support to unregister SMI handler inside SMI handler

["Zhiguang Liu" <zhiguang.liu@intel.com>]

Hi Laszlo,

We talked about how to unregister SMI handler inside other SMI handler.
And the conclusion was there is no such usage so we don't need support it.
However, I find some platform does need to unregister SMI handler inside other SMI handler.

The design you introduced below is great to meet the needs.
If you don't have time to implement and have no concern, I can implement your design and send the patch out.

Thanks
Zhiguang

> -----Original Message-----
> From: Laszlo Ersek <lersek@redhat.com>
> Sent: Thursday, January 25, 2024 6:48 PM
> To: devel@edk2.groups.io; Liu, Zhiguang <zhiguang.liu@intel.com>
> Cc: Liming Gao <gaoliming@byosoft.com.cn>; Wu, Jiaxin
> <jiaxin.wu@intel.com>; Ni, Ray <ray.ni@intel.com>; Ard Biesheuvel
> <ardb+tianocore@kernel.org>; Sami Mujawar <Sami.Mujawar@arm.com>
> Subject: Re: [edk2-devel] [PATCH] MdeModulePkg/SMM: Support to
> unregister SMI handler inside SMI handler
> 
> On 1/24/24 05:03, Zhiguang Liu wrote:
> > To support unregister SMI handler inside SMI handler itself, get next
> > node before SMI handler is executed, since LIST_ENTRY that Link points
> > to may be freed if unregister SMI handler in SMI handler itself.
> >
> > Cc: Liming Gao <gaoliming@byosoft.com.cn>
> > Cc: Jiaxin Wu <jiaxin.wu@intel.com>
> > Cc: Ray Ni <ray.ni@intel.com>
> > Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>
> > ---
> >  MdeModulePkg/Core/PiSmmCore/Smi.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/MdeModulePkg/Core/PiSmmCore/Smi.c
> > b/MdeModulePkg/Core/PiSmmCore/Smi.c
> > index 2985f989c3..a75e52b1ae 100644
> > --- a/MdeModulePkg/Core/PiSmmCore/Smi.c
> > +++ b/MdeModulePkg/Core/PiSmmCore/Smi.c
> > @@ -134,8 +134,14 @@ SmiManage (
> >
> >    Head = &SmiEntry->SmiHandlers;
> >
> > -  for (Link = Head->ForwardLink; Link != Head; Link =
> > Link->ForwardLink) {
> > +  for (Link = Head->ForwardLink; Link != Head;) {
> >      SmiHandler = CR (Link, SMI_HANDLER, Link, SMI_HANDLER_SIGNATURE);
> > +    //
> > +    // To support unregiser SMI handler inside SMI handler itself,
> > +    // get next node before handler is executed, since LIST_ENTRY that
> > +    // Link points to may be freed if unregister SMI handler.
> > +    //
> > +    Link = Link->ForwardLink;
> >
> >      Status = SmiHandler->Handler (
> >                             (EFI_HANDLE)SmiHandler,
> 
> I've had a further thought here.
> 
> (1) This patch may enable an SMI handler to unregister *itself*, that is,
> through the following call chain
> 
>   SmiManage()
>     SmiHandler->Handler()
>       SmiHandlerUnRegister()
> 
> but it still does not ensure *general iterator safety*.
> 
> Assume that an SMM driver registers two handlers, handler A and handler B.
> Assume the DispatchHandles for these are stored in global variables in the
> driver. Then, assume that "handler A" (for whatever reason, under some
> circumstances) unregisters "handler B".
> 
> That could still lead to a use-after-free in the SmiManage() loop; is that right?
> 
> If that driver scenario is plausible, then something like the following would be
> needed:
> 
> - a global variable of enum type in "MdeModulePkg/Core/PiSmmCore/Smi.c",
> with three possible values (NotManaging=0, Managing=1, CleanupNeeded=2).
> 
> - a new "BOOLEAN Deleted" field in SMI_HANDLER
> 
> - all loops in "Smi.c" iterating over SMI_HANDLERs would have to skip handlers
> that have been marked as Deleted
> 
> - in SmiManage(), set the state to Managing (=1) before the loop. After the
> loop, check if the state is CleanupNeeded (=2); if so, add an extra pass for
> actually deleting SMI_HANDLERs from the list that have been marked Deleted.
> Finally (regardless of the state being Managing (=1) or CleanupNeeded (=2)),
> reset the state to NotManaging (=0).
> 
> - in SmiHandlerUnRegister(), if the state is NotManaging (=0), delete the
> handler immediately. Otherwise (i.e., when the state is Managing
> (=1) or CleanupNeeded (=2)), set the state to CleanupNeeded (=2), and only
> mark the handler as Deleted.
> 
> The idea is to inform SmiHandlerUnRegister() whether it's running or not
> running on the stack of SmiManage(), and to postpone SMI_HANDLER
> deletion until after the loop finishes in the former case.
> 
> I'm not sure if real life SmiHandlerUnRegister() usage is worth this
> complication, however.
> 
> (2) Independently: does the same issue exist for the StMM core? I seem to be
> discovering the same problem in MmiManage()
> [StandaloneMmPkg/Core/Mmi.c].
> 
> So, whatever patch we add to the SMM_CORE, should likely be reflected to
> MM_CORE_STANDALONE too. Adding Ard and Sami to the CC list.
> 
> Thanks
> Laszlo



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