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authorNicholas Piggin <npiggin@gmail.com>2020-11-17 08:59:13 +0300
committerMichael Ellerman <mpe@ellerman.id.au>2020-11-19 15:47:18 +0300
commit9a32a7e78bd0cd9a9b6332cbdc345ee5ffd0c5de (patch)
treee9f7d6a2b72ddc59cb30cc87a36e635c2e0ab243 /Documentation
parentf79643787e0a0762d2409b7b8334e83f22d85695 (diff)
downloadlinux-9a32a7e78bd0cd9a9b6332cbdc345ee5ffd0c5de.tar.xz
powerpc/64s: flush L1D after user accesses
IBM Power9 processors can speculatively operate on data in the L1 cache before it has been completely validated, via a way-prediction mechanism. It is not possible for an attacker to determine the contents of impermissible memory using this method, since these systems implement a combination of hardware and software security measures to prevent scenarios where protected data could be leaked. However these measures don't address the scenario where an attacker induces the operating system to speculatively execute instructions using data that the attacker controls. This can be used for example to speculatively bypass "kernel user access prevention" techniques, as discovered by Anthony Steinhauser of Google's Safeside Project. This is not an attack by itself, but there is a possibility it could be used in conjunction with side-channels or other weaknesses in the privileged code to construct an attack. This issue can be mitigated by flushing the L1 cache between privilege boundaries of concern. This patch flushes the L1 cache after user accesses. This is part of the fix for CVE-2020-4788. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Daniel Axtens <dja@axtens.net> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/admin-guide/kernel-parameters.txt4
1 files changed, 4 insertions, 0 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 383889fe771f..44fde25bb221 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2859,6 +2859,7 @@
tsx_async_abort=off [X86]
kvm.nx_huge_pages=off [X86]
no_entry_flush [PPC]
+ no_uaccess_flush [PPC]
Exceptions:
This does not have any effect on
@@ -3238,6 +3239,9 @@
nospec_store_bypass_disable
[HW] Disable all mitigations for the Speculative Store Bypass vulnerability
+ no_uaccess_flush
+ [PPC] Don't flush the L1-D cache after accessing user data.
+
noxsave [BUGS=X86] Disables x86 extended register state save
and restore using xsave. The kernel will fallback to
enabling legacy floating-point and sse state.