[PATCH v2 5/9] arm64: asm: Introduce test_sp_overflow macro

Kalesh Singh
2022-02-22T16:58:33Z

From: Quentin Perret <qperret@google.com>

The asm entry code in the kernel uses a trick to check if VMAP'd stacks
have overflowed by aligning them at THREAD_SHIFT * 2 granularity and
checking the SP's THREAD_SHIFT bit.

Protected KVM will soon make use of a similar trick to detect stack
overflows, so factor out the asm code in a re-usable macro.

Signed-off-by: Quentin Perret <qperret@google.com>
[Kalesh - Resolve minor conflicts]
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
arch/arm64/include/asm/assembler.h | 11 +++++++++++
arch/arm64/kernel/entry.S | 7 +------
2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index e8bd0af0141c..ad40eb0eee83 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -850,4 +850,15 @@ alternative_endif

#endif /* GNU_PROPERTY_AARCH64_FEATURE_1_DEFAULT */

+/*
+ * Test whether the SP has overflowed, without corrupting a GPR.
+ */
+.macro test_sp_overflow shift, label
+ add sp, sp, x0 // sp' = sp + x0
+ sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sp
+ tbnz x0, #\shift, \label
+ sub x0, sp, x0 // x0'' = sp' - x0' = (sp + x0) - sp = x0
+ sub sp, sp, x0 // sp'' = sp' - x0 = (sp + x0) - x0 = sp
+.endm
+
#endif /* __ASM_ASSEMBLER_H */
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 772ec2ecf488..ce99ee30c77e 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -53,15 +53,10 @@ alternative_else_nop_endif
sub sp, sp, #PT_REGS_SIZE
#ifdef CONFIG_VMAP_STACK
/*
- * Test whether the SP has overflowed, without corrupting a GPR.
* Task and IRQ stacks are aligned so that SP & (1 << THREAD_SHIFT)
* should always be zero.
*/
- add sp, sp, x0 // sp' = sp + x0
- sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sp
- tbnz x0, #THREAD_SHIFT, 0f
- sub x0, sp, x0 // x0'' = sp' - x0' = (sp + x0) - sp = x0
- sub sp, sp, x0 // sp'' = sp' - x0 = (sp + x0) - x0 = sp
+ test_sp_overflow THREAD_SHIFT, 0f
b el\el\ht\()_\regsize\()_\label

0:
--
2.35.1.473.g83b2b277ed-goog