Reviewed-by: Eric Dong <eric.d...@intel.com> > -----Original Message----- > From: Wu, Hao A > Sent: Tuesday, September 25, 2018 2:13 PM > To: edk2-devel@lists.01.org > Cc: Wu, Hao A <hao.a...@intel.com>; Laszlo Ersek <ler...@redhat.com>; > Yao, Jiewen <jiewen....@intel.com>; Kinney, Michael D > <michael.d.kin...@intel.com>; Dong, Eric <eric.d...@intel.com> > Subject: [PATCH v2 5/5] UefiCpuPkg/PiSmmCpuDxeSmm: [CVE-2017-5753] > Fix bounds check bypass > > REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1194 > > Speculative execution is used by processor to avoid having to wait for > data to arrive from memory, or for previous operations to finish, the > processor may speculate as to what will be executed. > > If the speculation is incorrect, the speculatively executed instructions > might leave hints such as which memory locations have been brought into > cache. Malicious actors can use the bounds check bypass method (code > gadgets with controlled external inputs) to infer data values that have > been used in speculative operations to reveal secrets which should not > otherwise be accessed. > > It is possible for SMI handler(s) to call EFI_SMM_CPU_PROTOCOL service > ReadSaveState() and use the content in the 'CommBuffer' (controlled > external inputs) as the 'CpuIndex'. So this commit will insert AsmLfence > API to mitigate the bounds check bypass issue within SmmReadSaveState(). > > For SmmReadSaveState(): > > The 'CpuIndex' will be passed into function ReadSaveStateRegister(). And > then in to ReadSaveStateRegisterByIndex(). > > With the call: > ReadSaveStateRegisterByIndex ( > CpuIndex, > SMM_SAVE_STATE_REGISTER_IOMISC_INDEX, > sizeof(IoMisc.Uint32), > &IoMisc.Uint32 > ); > > The 'IoMisc' can be a cross boundary access during speculative execution. > Later, 'IoMisc' is used as the index to access buffers 'mSmmCpuIoWidth' > and 'mSmmCpuIoType'. One can observe which part of the content within > those buffers was brought into cache to possibly reveal the value of > 'IoMisc'. > > Hence, this commit adds a AsmLfence() after the check of 'CpuIndex' > within function SmmReadSaveState() to prevent the speculative execution. > > A more detailed explanation of the purpose of commit is under the > 'Bounds check bypass mitigation' section of the below link: > https://software.intel.com/security-software-guidance/insights/host- > firmware-speculative-execution-side-channel-mitigation > > And the document at: > https://software.intel.com/security-software-guidance/api- > app/sites/default/files/337879-analyzing-potential-bounds-Check-bypass- > vulnerabilities.pdf > > Cc: Laszlo Ersek <ler...@redhat.com> > Cc: Jiewen Yao <jiewen....@intel.com> > Cc: Michael D Kinney <michael.d.kin...@intel.com> > Cc: Eric Dong <eric.d...@intel.com> > Contributed-under: TianoCore Contribution Agreement 1.1 > Signed-off-by: Hao Wu <hao.a...@intel.com> > > cb pismm > --- > UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c > b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c > index fbf74e8d90..19979d5418 100644 > --- a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c > +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c > @@ -237,6 +237,11 @@ SmmReadSaveState ( > if ((CpuIndex >= gSmst->NumberOfCpus) || (Buffer == NULL)) { > return EFI_INVALID_PARAMETER; > } > + // > + // The AsmLfence() call here is to ensure the above check for the > CpuIndex > + // has been completed before the execution of subsequent codes. > + // > + AsmLfence (); > > // > // Check for special EFI_SMM_SAVE_STATE_REGISTER_PROCESSOR_ID > -- > 2.12.0.windows.1
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