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author | Linus Torvalds <torvalds@linux-foundation.org> | 2019-05-16 11:02:27 -0700 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2019-05-16 11:02:27 -0700 |
commit | d396360acdf7e57edcd9e2d080343b0353d65d63 (patch) | |
tree | 735baf5df67bceb65af02728948494f2451cbefd /Documentation | |
parent | b2c3dda6f8f06d825b9b6099f57b906c774141c0 (diff) | |
parent | 9d8d0294e78a164d407133dea05caf4b84247d6a (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.rst | 44 |
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 |