Replying here, because obviously there's no actual posting of this patch... :/
--- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -118,13 +118,28 @@ #endif .endm +.macro ISSUE_UNBALANCED_RET_GUARD
- ANNOTATE_INTRA_FUNCTION_CALL
- call .Lunbalanced_ret_guard_@
- int3
+.Lunbalanced_ret_guard_@:
- add $(BITS_PER_LONG/8), %_ASM_SP
- lfence
+.endm
- /*
*/
- A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
- monstrosity above, manually.
-.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req +.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2 +.ifb \ftr2 ALTERNATIVE "jmp .Lskip_rsb_@", "", \ftr +.else
- ALTERNATIVE_2 "jmp .Lskip_rsb_@", "", \ftr, "jmp .Lunbalanced_@", \ftr2
+.endif __FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP) +.Lunbalanced_@:
- ISSUE_UNBALANCED_RET_GUARD
.Lskip_rsb_@: .endm
(/me deletes all the swear words and starts over)
This must absolutely be the most horrible patch you could come up with, no? I suppose that's the price of me taking PTO :-(
Could you please test this; I've only compiled it.
--- Subject: x86/nospec: Unwreck the RSB stuffing
Commit 2b1299322016 ("x86/speculation: Add RSB VM Exit protections") made a right mess of the RSB stuffing, rewrite the whole thing to not suck.
Thanks to Andrew for the enlightening comment about Post-Barrier RSB things so we can make this code less magical.
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org --- cpufeatures.h | 2 + nospec-branch.h | 80 +++++++++++++++++++++++++++----------------------------- 2 files changed, 41 insertions(+), 41 deletions(-)
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h index 235dc85c91c3..1a31ae6d758b 100644 --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -420,6 +420,8 @@ #define X86_FEATURE_V_TSC_AUX (19*32+ 9) /* "" Virtual TSC_AUX */ #define X86_FEATURE_SME_COHERENT (19*32+10) /* "" AMD hardware-enforced cache coherency */
+#define X86_FEATURE_NEVER (-1) /* "" Logical complement of ALWAYS */ + /* * BUG word(s) */ diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index e64fd20778b6..336f8e8cebf8 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -35,33 +35,44 @@ #define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */
/* + * Common helper for __FILL_RETURN_BUFFER and __FILL_ONE_RETURN. + */ +#define __FILL_RETURN_SLOT \ + ANNOTATE_INTRA_FUNCTION_CALL; \ + call 772f; \ + int3; \ +772: + +/* + * Stuff the entire RSB. + * * Google experimented with loop-unrolling and this turned out to be * the optimal version - two calls, each with their own speculation * trap should their return address end up getting used, in a loop. */ -#define __FILL_RETURN_BUFFER(reg, nr, sp) \ - mov $(nr/2), reg; \ -771: \ - ANNOTATE_INTRA_FUNCTION_CALL; \ - call 772f; \ -773: /* speculation trap */ \ - UNWIND_HINT_EMPTY; \ - pause; \ - lfence; \ - jmp 773b; \ -772: \ - ANNOTATE_INTRA_FUNCTION_CALL; \ - call 774f; \ -775: /* speculation trap */ \ - UNWIND_HINT_EMPTY; \ - pause; \ - lfence; \ - jmp 775b; \ -774: \ - add $(BITS_PER_LONG/8) * 2, sp; \ - dec reg; \ - jnz 771b; \ - /* barrier for jnz misprediction */ \ +#define __FILL_RETURN_BUFFER(reg, nr) \ + mov $(nr/2), reg; \ +771: \ + __FILL_RETURN_SLOT \ + __FILL_RETURN_SLOT \ + add $(BITS_PER_LONG/8) * 2, %_ASM_SP; \ + dec reg; \ + jnz 771b; \ + /* barrier for jnz misprediction */ \ + lfence; + +/* + * Stuff a single RSB slot. + * + * To mitigate Post-Barrier RSB speculation, one CALL instruction must be + * forced to retire before letting a RET instruction execute. + * + * On PBRSB-vulnerable CPUs, it is not safe for a RET to be executed + * before this point. + */ +#define __FILL_ONE_RETURN \ + __FILL_RETURN_SLOT \ + add $(BITS_PER_LONG/8), %_ASM_SP; \ lfence;
#ifdef __ASSEMBLY__ @@ -132,28 +143,15 @@ #endif .endm
-.macro ISSUE_UNBALANCED_RET_GUARD - ANNOTATE_INTRA_FUNCTION_CALL - call .Lunbalanced_ret_guard_@ - int3 -.Lunbalanced_ret_guard_@: - add $(BITS_PER_LONG/8), %_ASM_SP - lfence -.endm - /* * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP * monstrosity above, manually. */ -.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2 -.ifb \ftr2 - ALTERNATIVE "jmp .Lskip_rsb_@", "", \ftr -.else - ALTERNATIVE_2 "jmp .Lskip_rsb_@", "", \ftr, "jmp .Lunbalanced_@", \ftr2 -.endif - __FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP) -.Lunbalanced_@: - ISSUE_UNBALANCED_RET_GUARD +.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2=X86_FEATURE_NEVER + ALTERNATIVE_2 "jmp .Lskip_rsb_@", \ + __stringify(__FILL_RETURN_BUFFER(\reg,\nr)), \ftr, \ + __stringify(__FILL_ONE_RETURN), \ftr2 + .Lskip_rsb_@: .endm