linux-kernel - [PATCH] arm64: mm: prevent context switching during idmap is mapped

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Message-Id: <20251202004223.108388-1-leo.yeo@luaberry.com>
Date: Tue,  2 Dec 2025 00:42:23 +0000
From: Jeongin Yeo <leo.yeo@...berry.com>
To: catalin.marinas@....com,
	will@...nel.org
Cc: ryan.roberts@....com,
	akpm@...ux-foundation.org,
	david@...nel.org,
	kevin.brodsky@....com,
	dev.jain@....com,
	quic_zhenhuah@...cinc.com,
	lorenzo.stoakes@...cle.com,
	yang@...amperecomputing.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] arm64: mm: prevent context switching during idmap is mapped

 cpu_install_idmap replaces ttbr0 to install idmap. replace_phys uses
 idmap_cpu_replace_ttbr1 which is located in idmap,
 thus it requires ttbr0 to point idmap. 

 However, if a kernel task has switched out right after
 cpu_install_idmap, when kernel task is loaded, its ttbr0 is
 mapped to reserved_pg_dir which doesn't contain any idmap
 functions.
 Therefore, the kernel task cannot find any functions to run,
 causing memory fault.

 As far as I can verify from the current arm64 tree,
 no in-tree caller invokes this function from a preemptible context.

 Nevertheless, future work might allow this function to be called in
 preemptive state. Encapsulating the idmap sequence with
 preempt_disable() and preempt_enable() prevents this possible problem.

Signed-off-by: Jeongin Yeo <leo.yeo@...berry.com>
---
 arch/arm64/mm/mmu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 2ba01dc8ef82..c3da836ebe8d 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -29,6 +29,7 @@
 #include <linux/mm_inline.h>
 #include <linux/pagewalk.h>
 #include <linux/stop_machine.h>
+#include <linux/preempt.h>

 #include <asm/barrier.h>
 #include <asm/cputype.h>
@@ -2136,7 +2137,7 @@ void __cpu_replace_ttbr1(pgd_t *pgdp, bool cnp)
 		ttbr1 |= TTBR_CNP_BIT;

 	replace_phys = (void *)__pa_symbol(idmap_cpu_replace_ttbr1);
-
+	preempt_disable();
 	cpu_install_idmap();

 	/*
@@ -2148,6 +2149,7 @@ void __cpu_replace_ttbr1(pgd_t *pgdp, bool cnp)
 	local_daif_restore(daif);

 	cpu_uninstall_idmap();
+	preempt_enable();
 }

 #ifdef CONFIG_ARCH_HAS_PKEYS
-- 
2.43.0