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x86/nospec: Fix i386 RSB stuffing

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commit 332924973725e8cdcc783c175f68cf7e162cb9e5 upstream.

Turns out that i386 doesn't unconditionally have LFENCE, as such the
loop in __FILL_RETURN_BUFFER isn't actually speculation safe on such
chips.

Fixes: ba6e31af2be9 ("x86/speculation: Add LFENCE to RSB fill sequence")
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yv9tj9vbQ9nNlXoY@worktop.programming.kicks-ass.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Zijlstra
2022-08-19 13:01:35 +02:00
committed by Greg Kroah-Hartman
parent adee8f3082
commit e5796ff9ac
1 changed files with 12 additions and 0 deletions
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12
arch/x86/include/asm/nospec-branch.h
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@@ -50,6 +50,7 @@
* the optimal version — two calls, each with their own speculation * the optimal version — two calls, each with their own speculation
* trap should their return address end up getting used, in a loop. * trap should their return address end up getting used, in a loop.
*/ */
#ifdef CONFIG_X86_64
#define __FILL_RETURN_BUFFER(reg, nr) \ #define __FILL_RETURN_BUFFER(reg, nr) \
mov $(nr/2), reg; \ mov $(nr/2), reg; \
771: \ 771: \
@@ -60,6 +61,17 @@
jnz 771b; \ jnz 771b; \
/* barrier for jnz misprediction */ \ /* barrier for jnz misprediction */ \
lfence; lfence;
#else
/*
* i386 doesn't unconditionally have LFENCE, as such it can't
* do a loop.
*/
#define __FILL_RETURN_BUFFER(reg, nr) \
.rept nr; \
__FILL_RETURN_SLOT; \
.endr; \
add $(BITS_PER_LONG/8) * nr, %_ASM_SP;
#endif
/* /*
* Stuff a single RSB slot. * Stuff a single RSB slot.