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Documentation/hw-vuln: Document the interaction between IBRS and STIBP

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commit e02b50ca44 upstream.

Explain why STIBP is needed with legacy IBRS as currently implemented
(KERNEL_IBRS) and why STIBP is not needed when enhanced IBRS is enabled.

Fixes: 7c693f54c8 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230227060541.1939092-2-kpsingh@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
KP Singh 2023-02-27 07:05:41 +01:00 committed by Greg Kroah-Hartman
parent 10543fb3c9
commit dfd3801d53
1 changed files with 16 additions and 5 deletions
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21
Documentation/admin-guide/hw-vuln/spectre.rst
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@ -479,8 +479,16 @@ Spectre variant 2
On Intel Skylake-era systems the mitigation covers most, but not all,
cases. See :ref:`[3] <spec_ref3>` for more details.
On CPUs with hardware mitigation for Spectre variant 2 (e.g. Enhanced
IBRS on x86), retpoline is automatically disabled at run time.
On CPUs with hardware mitigation for Spectre variant 2 (e.g. IBRS
or enhanced IBRS on x86), retpoline is automatically disabled at run time.
Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at
boot, by setting the IBRS bit, and they're automatically protected against
Spectre v2 variant attacks, including cross-thread branch target injections
on SMT systems (STIBP). In other words, eIBRS enables STIBP too.
Legacy IBRS systems clear the IBRS bit on exit to userspace and
therefore explicitly enable STIBP for that
The retpoline mitigation is turned on by default on vulnerable
CPUs. It can be forced on or off by the administrator
@ -504,9 +512,12 @@ Spectre variant 2
For Spectre variant 2 mitigation, individual user programs
can be compiled with return trampolines for indirect branches.
This protects them from consuming poisoned entries in the branch
target buffer left by malicious software. Alternatively, the
programs can disable their indirect branch speculation via prctl()
(See :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`).
target buffer left by malicious software.
On legacy IBRS systems, at return to userspace, implicit STIBP is disabled
because the kernel clears the IBRS bit. In this case, the userspace programs
can disable indirect branch speculation via prctl() (See
:ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`).
On x86, this will turn on STIBP to guard against attacks from the
sibling thread when the user program is running, and use IBPB to
flush the branch target buffer when switching to/from the program.