commit baf8361e54 upstream.
MDS mitigation requires clearing the CPU buffers before returning to
user. This needs to be done late in the exit-to-user path. Current
location of VERW leaves a possibility of kernel data ending up in CPU
buffers for memory accesses done after VERW such as:
1. Kernel data accessed by an NMI between VERW and return-to-user can
remain in CPU buffers since NMI returning to kernel does not
execute VERW to clear CPU buffers.
2. Alyssa reported that after VERW is executed,
CONFIG_GCC_PLUGIN_STACKLEAK=y scrubs the stack used by a system
call. Memory accesses during stack scrubbing can move kernel stack
contents into CPU buffers.
3. When caller saved registers are restored after a return from
function executing VERW, the kernel stack accesses can remain in
CPU buffers(since they occur after VERW).
To fix this VERW needs to be moved very late in exit-to-user path.
In preparation for moving VERW to entry/exit asm code, create macros
that can be used in asm. Also make VERW patching depend on a new feature
flag X86_FEATURE_CLEAR_CPU_BUF.
Reported-by: Alyssa Milburn <alyssa.milburn@intel.com>
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20240213-delay-verw-v8-1-a6216d83edb7%40linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
jmp2ret mitigates the easy-to-attack case at relatively low overhead.
It mitigates the long speculation windows after a mispredicted RET, but
it does not mitigate the short speculation window from arbitrary
instruction boundaries.
On Zen2, there is a chicken bit which needs setting, which mitigates
"arbitrary instruction boundaries" down to just "basic block boundaries".
But there is no fix for the short speculation window on basic block
boundaries, other than to flush the entire BTB to evict all attacker
predictions.
On the spectrum of "fast & blurry" -> "safe", there is (on top of STIBP
or no-SMT):
1) Nothing System wide open
2) jmp2ret May stop a script kiddy
3) jmp2ret+chickenbit Raises the bar rather further
4) IBPB Only thing which can count as "safe".
Tentative numbers put IBPB-on-entry at a 2.5x hit on Zen2, and a 10x hit
on Zen1 according to lmbench.
[ bp: Fixup feature bit comments, document option, 32-bit build fix. ]
Suggested-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
