On x86_64, all returns to usermode go through prepare_exit_to_usermode(), with the sole exception of do_nmi(). This even includes machine checks -- this was added several years ago to support MCE recovery. Update the documentation.
Cc: stable@vger.kernel.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Borislav Petkov bp@suse.de Cc: Frederic Weisbecker frederic@kernel.org Cc: Jon Masters jcm@redhat.com Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user") Signed-off-by: Andy Lutomirski luto@kernel.org --- Documentation/x86/mds.rst | 39 +++++++-------------------------------- 1 file changed, 7 insertions(+), 32 deletions(-)
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst index 0dc812bb9249..5d4330be200f 100644 --- a/Documentation/x86/mds.rst +++ b/Documentation/x86/mds.rst @@ -142,38 +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. - - - 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. - - - 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. + 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(). + + (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