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authorLinus Torvalds <torvalds@linux-foundation.org>2019-05-16 11:02:27 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-05-16 11:02:27 -0700
commitd396360acdf7e57edcd9e2d080343b0353d65d63 (patch)
tree735baf5df67bceb65af02728948494f2451cbefd /Documentation
parentb2c3dda6f8f06d825b9b6099f57b906c774141c0 (diff)
parent9d8d0294e78a164d407133dea05caf4b84247d6a (diff)
Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull x86 fixes from Ingo Molnar: "Misc fixes and updates: - a handful of MDS documentation/comment updates - a cleanup related to hweight interfaces - a SEV guest fix for large pages - a kprobes LTO fix - and a final cleanup commit for vDSO HPET support removal" * 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: x86/speculation/mds: Improve CPU buffer clear documentation x86/speculation/mds: Revert CPU buffer clear on double fault exit x86/kconfig: Disable CONFIG_GENERIC_HWEIGHT and remove __HAVE_ARCH_SW_HWEIGHT x86/mm: Do not use set_{pud, pmd}_safe() when splitting a large page x86/kprobes: Make trampoline_handler() global and visible x86/vdso: Remove hpet_page from vDSO
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/x86/mds.rst44
1 files changed, 6 insertions, 38 deletions
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 534e9baa4e1d..5d4330be200f 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -142,45 +142,13 @@ Mitigation points
mds_user_clear.
The mitigation is invoked in prepare_exit_to_usermode() which covers
- most of the kernel to user space transitions. There are a few exceptions
- which are not invoking prepare_exit_to_usermode() on return to user
- space. These exceptions use the paranoid exit code.
+ all but one of the kernel to user space transitions. The exception
+ is when we return from a Non Maskable Interrupt (NMI), which is
+ handled directly in do_nmi().
- - Non Maskable Interrupt (NMI):
-
- Access to sensible data like keys, credentials in the NMI context is
- mostly theoretical: The CPU can do prefetching or execute a
- misspeculated code path and thereby fetching data which might end up
- leaking through a buffer.
-
- But for mounting other attacks the kernel stack address of the task is
- already valuable information. So in full mitigation mode, the NMI is
- mitigated on the return from do_nmi() to provide almost complete
- coverage.
-
- - Double fault (#DF):
-
- A double fault is usually fatal, but the ESPFIX workaround, which can
- be triggered from user space through modify_ldt(2) is a recoverable
- double fault. #DF uses the paranoid exit path, so explicit mitigation
- in the double fault handler is required.
-
- - Machine Check Exception (#MC):
-
- Another corner case is a #MC which hits between the CPU buffer clear
- invocation and the actual return to user. As this still is in kernel
- space it takes the paranoid exit path which does not clear the CPU
- buffers. So the #MC handler repopulates the buffers to some
- extent. Machine checks are not reliably controllable and the window is
- extremly small so mitigation would just tick a checkbox that this
- theoretical corner case is covered. To keep the amount of special
- cases small, ignore #MC.
-
- - Debug Exception (#DB):
-
- This takes the paranoid exit path only when the INT1 breakpoint is in
- kernel space. #DB on a user space address takes the regular exit path,
- so no extra mitigation required.
+ (The reason that NMI is special is that prepare_exit_to_usermode() can
+ enable IRQs. In NMI context, NMIs are blocked, and we don't want to
+ enable IRQs with NMIs blocked.)
2. C-State transition