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authorThomas Gleixner <tglx@linutronix.de>2022-09-15 13:11:27 +0200
committerPeter Zijlstra <peterz@infradead.org>2022-10-17 16:41:15 +0200
commit5d8213864ade86b48fc492584ea86d65a62f892e (patch)
tree0005924a00492b018072967b39e46c6f6b511ba6 /arch/x86/kernel/asm-offsets.c
parent52354973573cc260ff2fc661cb28ff8eaa7b879b (diff)
x86/retbleed: Add SKL return thunk
To address the Intel SKL RSB underflow issue in software it's required to do call depth tracking. Provide a return thunk for call depth tracking on Intel SKL CPUs. The tracking does not use a counter. It uses uses arithmetic shift right on call entry and logical shift left on return. The depth tracking variable is initialized to 0x8000.... when the call depth is zero. The arithmetic shift right sign extends the MSB and saturates after the 12th call. The shift count is 5 so the tracking covers 12 nested calls. On return the variable is shifted left logically so it becomes zero again. CALL RET 0: 0x8000000000000000 0x0000000000000000 1: 0xfc00000000000000 0xf000000000000000 ... 11: 0xfffffffffffffff8 0xfffffffffffffc00 12: 0xffffffffffffffff 0xffffffffffffffe0 After a return buffer fill the depth is credited 12 calls before the next stuffing has to take place. There is a inaccuracy for situations like this: 10 calls 5 returns 3 calls 4 returns 3 calls .... The shift count might cause this to be off by one in either direction, but there is still a cushion vs. the RSB depth. The algorithm does not claim to be perfect, but it should obfuscate the problem enough to make exploitation extremly difficult. The theory behind this is: RSB is a stack with depth 16 which is filled on every call. On the return path speculation "pops" entries to speculate down the call chain. Once the speculative RSB is empty it switches to other predictors, e.g. the Branch History Buffer, which can be mistrained by user space and misguide the speculation path to a gadget. Call depth tracking is designed to break this speculation path by stuffing speculation trap calls into the RSB which are never getting a corresponding return executed. This stalls the prediction path until it gets resteered, The assumption is that stuffing at the 12th return is sufficient to break the speculation before it hits the underflow and the fallback to the other predictors. Testing confirms that it works. Johannes, one of the retbleed researchers. tried to attack this approach but failed. There is obviously no scientific proof that this will withstand future research progress, but all we can do right now is to speculate about it. The SAR/SHL usage was suggested by Andi Kleen. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20220915111147.890071690@infradead.org
Diffstat (limited to 'arch/x86/kernel/asm-offsets.c')
-rw-r--r--arch/x86/kernel/asm-offsets.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index a9824318e1c5..13afdbbee349 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -110,6 +110,9 @@ static void __used common(void)
OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
OFFSET(X86_top_of_stack, pcpu_hot, top_of_stack);
+#ifdef CONFIG_CALL_DEPTH_TRACKING
+ OFFSET(X86_call_depth, pcpu_hot, call_depth);
+#endif
if (IS_ENABLED(CONFIG_KVM_INTEL)) {
BLANK();