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Subject[PATCHv5 13/21] arm64: zero GPRs upon entry from EL0
Date
We can zero GPRs x0 - x29 upon entry from EL0 to make it harder for
userspace to control values consumed by speculative gadgets.

We don't blat x30, since this is stashed much later, and we'll blat it
before invoking C code.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
---
arch/arm64/kernel/entry.S | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 22b240da949b..d1440f84668b 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -53,6 +53,12 @@
#endif
.endm

+ .macro clear_gp_regs
+ .irp n,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29
+ mov x\n, xzr
+ .endr
+ .endm
+
/*
* Bad Abort numbers
*-----------------
@@ -169,6 +175,7 @@ alternative_cb_end
stp x28, x29, [sp, #16 * 14]

.if \el == 0
+ clear_gp_regs
mrs x21, sp_el0
ldr_this_cpu tsk, __entry_task, x20 // Ensure MDSCR_EL1.SS is clear,
ldr x19, [tsk, #TSK_TI_FLAGS] // since we can unmask debug
@@ -176,7 +183,6 @@ alternative_cb_end

apply_ssbd 1, x22, x23

- mov x29, xzr // fp pointed to user-space
.else
add x21, sp, #S_FRAME_SIZE
get_thread_info tsk
--
2.11.0
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