[PATCH 6.12 012/265] x86/fred: Correct speculative safety in fred_extint()

Greg Kroah-Hartman
2026-03-12T20:14:48Z

From: Andrew Cooper <andrew.cooper3@citrix.com>

[ Upstream commit aa280a08e7d8fae58557acc345b36b3dc329d595 ]

array_index_nospec() is no use if the result gets spilled to the stack, as
it makes the believed safe-under-speculation value subject to memory
predictions.

For all practical purposes, this means array_index_nospec() must be used in
the expression that accesses the array.

As the code currently stands, it's the wrong side of irqentry_enter(), and
'index' is put into %ebp across the function call.

Remove the index variable and reposition array_index_nospec(), so it's
calculated immediately before the array access.

Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260106131504.679932-1-andrew.cooper3@citrix.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/entry/entry_fred.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
index f004a4dc74c2d..563e439b743f2 100644
--- a/arch/x86/entry/entry_fred.c
+++ b/arch/x86/entry/entry_fred.c
@@ -159,8 +159,6 @@ void __init fred_complete_exception_setup(void)
static noinstr void fred_extint(struct pt_regs *regs)
{
unsigned int vector = regs->fred_ss.vector;
- unsigned int index = array_index_nospec(vector - FIRST_SYSTEM_VECTOR,
- NR_SYSTEM_VECTORS);

if (WARN_ON_ONCE(vector < FIRST_EXTERNAL_VECTOR))
return;
@@ -169,7 +167,8 @@ static noinstr void fred_extint(struct pt_regs *regs)
irqentry_state_t state = irqentry_enter(regs);

instrumentation_begin();
- sysvec_table[index](regs);
+ sysvec_table[array_index_nospec(vector - FIRST_SYSTEM_VECTOR,
+ NR_SYSTEM_VECTORS)](regs);
instrumentation_end();
irqentry_exit(regs, state);
} else {
--
2.51.0