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x86/nospec: Unwreck the RSB stuffing

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commit 4e3aa9238277597c6c7624f302d81a7b568b6f2d upstream.

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.

Cc: stable@vger.kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/YvuNdDWoUZSBjYcm@worktop.programming.kicks-ass.net
[bwh: Backported to 5.10: adjust context]
Signed-off-by: Ben Hutchings <benh@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Zijlstra 2022-08-16 14:28:36 +02:00 committed by Greg Kroah-Hartman
parent 895428ee12
commit adee8f3082
1 changed files with 39 additions and 41 deletions
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80
arch/x86/include/asm/nospec-branch.h
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@ -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__
@ -120,28 +131,15 @@
#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=ALT_NOT(X86_FEATURE_ALWAYS)
ALTERNATIVE_2 "jmp .Lskip_rsb_\@", \
__stringify(__FILL_RETURN_BUFFER(\reg,\nr)), \ftr, \
__stringify(__FILL_ONE_RETURN), \ftr2
.Lskip_rsb_\@:
.endm