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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
[bwh: Backported to 4.19/5.4:
 - __FILL_RETURN_BUFFER takes an sp parameter
 - Open-code __FILL_RETURN_SLOT]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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 64f9755b40
commit 4e8d7039cf
1 changed files with 14 additions and 0 deletions
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14
arch/x86/include/asm/nospec-branch.h
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@ -44,6 +44,7 @@
* the optimal version — two calls, each with their own speculation
* trap should their return address end up getting used, in a loop.
*/
#ifdef CONFIG_X86_64
#define __FILL_RETURN_BUFFER(reg, nr, sp) \
mov $(nr/2), reg; \
771: \
@ -64,6 +65,19 @@
add $(BITS_PER_LONG/8) * nr, sp; \
/* barrier for jnz misprediction */ \
lfence;
#else
/*
* i386 doesn't unconditionally have LFENCE, as such it can't
* do a loop.
*/
#define __FILL_RETURN_BUFFER(reg, nr, sp) \
.rept nr; \
call 772f; \
int3; \
772:; \
.endr; \
add $(BITS_PER_LONG/8) * nr, sp;
#endif
#define __ISSUE_UNBALANCED_RET_GUARD(sp) \
call 881f; \