Balbir Singh <sbl...@amazon.com> writes:
> Implement a mechanism to selectively flush the L1D cache. The goal is to
> allow tasks that are paranoid due to the recent snoop assisted data sampling
> vulnerabilites, to flush their L1D on being switched out. This protects
> their data from being snooped or leaked via side channels after the task
> has context switched out.
>
> There are two scenarios we might want to protect against, a task leaving
> the CPU with data still in L1D (which is the main concern of this patch),
> the second scenario is a malicious task coming in (not so well trusted)
> for which we want to clean up the cache before it starts. Only the case
> for the former is addressed.
>
> A new thread_info flag TIF_SPEC_FLUSH_L1D is added to track tasks which
> opt-into L1D flushing. cpu_tlbstate.last_user_mm_spec is used to convert
> the TIF flags into mm state (per cpu via last_user_mm_spec) in
> cond_mitigation(), which then used to do decide when to call flush_l1d().
>
> Add prctl()'s to opt-in to the L1D cache on context switch out, the
> existing mechanisms of tracking prev_mm via cpu_tlbstate is
> reused to track state of the tasks and to flush the L1D cache.
> The prctl interface is generic and can be ported over to other
> architectures.
>
> Suggested-by: Thomas Gleixner <t...@linutronix.de>
> Signed-off-by: Balbir Singh <sbl...@amazon.com>
> Reviewed-by: Kees Cook <keesc...@chromium.org>
> ---
> arch/x86/include/asm/thread_info.h | 7 ++++-
> arch/x86/mm/tlb.c | 44 ++++++++++++++++++++++++++++--
> include/uapi/linux/prctl.h | 4 +++
> kernel/sys.c | 20 ++++++++++++++
> 4 files changed, 72 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/thread_info.h
> b/arch/x86/include/asm/thread_info.h
> index 8de8ceccb8bc..67de693d9ba1 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -84,7 +84,7 @@ struct thread_info {
> #define TIF_SYSCALL_AUDIT 7 /* syscall auditing active */
> #define TIF_SECCOMP 8 /* secure computing */
> #define TIF_SPEC_IB 9 /* Indirect branch speculation
> mitigation */
> -#define TIF_SPEC_FORCE_UPDATE 10 /* Force speculation MSR update
> in context switch */
> +#define TIF_SPEC_FLUSH_L1D 10 /* Flush L1D on mm switches (processes)
> */
> #define TIF_USER_RETURN_NOTIFY 11 /* notify kernel of userspace
> return */
> #define TIF_UPROBE 12 /* breakpointed or singlestepping */
> #define TIF_PATCH_PENDING 13 /* pending live patching update */
> @@ -96,6 +96,7 @@ struct thread_info {
> #define TIF_MEMDIE 20 /* is terminating due to OOM killer */
> #define TIF_POLLING_NRFLAG 21 /* idle is polling for TIF_NEED_RESCHED
> */
> #define TIF_IO_BITMAP 22 /* uses I/O bitmap */
> +#define TIF_SPEC_FORCE_UPDATE 23 /* Force speculation MSR update
> in context switch */
> #define TIF_FORCED_TF 24 /* true if TF in eflags
> artificially */
> #define TIF_BLOCKSTEP 25 /* set when we want
> DEBUGCTLMSR_BTF */
> #define TIF_LAZY_MMU_UPDATES 27 /* task is updating the mmu lazily */
> @@ -132,6 +133,7 @@ struct thread_info {
> #define _TIF_ADDR32 (1 << TIF_ADDR32)
> #define _TIF_X32 (1 << TIF_X32)
> #define _TIF_FSCHECK (1 << TIF_FSCHECK)
> +#define _TIF_SPEC_FLUSH_L1D (1 << TIF_SPEC_FLUSH_L1D)
Bah. These defines are ordered in the same way as the TIF defines....
> /*
> - * Bits to mangle the TIF_SPEC_IB state into the mm pointer which is
> + * Bits to mangle the TIF_SPEC_* state into the mm pointer which is
> * stored in cpu_tlb_state.last_user_mm_spec.
> */
> #define LAST_USER_MM_IBPB 0x1UL
> -#define LAST_USER_MM_SPEC_MASK (LAST_USER_MM_IBPB)
> +#define LAST_USER_MM_L1D_FLUSH 0x2UL
> +#define LAST_USER_MM_SPEC_MASK (LAST_USER_MM_IBPB |
> LAST_USER_MM_L1D_FLUSH)
You lost
+ BUILD_BUG_ON(TIF_SPEC_FLUSH_L1D != TIF_SPEC_IB + 1);
from patch I gave you.
