[PATCH v1] arm64: mm: Permit PTE SW bits to change in live mappings

[Ryan Roberts]

Previously pgattr_change_is_safe() was overly-strict and complained
(e.g. "[  116.262743] __check_safe_pte_update: unsafe attribute change:
0x0560000043768fc3 -> 0x0160000043768fc3") if it saw any SW bits change
in a live PTE. There is no such restriction on SW bits in the Arm ARM.

Until now, no SW bits have been updated in live mappings via the
set_ptes() route. PTE_DIRTY would be updated live, but this is handled
by ptep_set_access_flags() which does not call pgattr_change_is_safe().
However, with the introduction of uffd-wp for arm64, there is core-mm
code that does ptep_get(); pte_clear_uffd_wp(); set_ptes(); which
triggers this false warning.

Silence this warning by masking out the SW bits during checks.

The bug isn't technically in the highlighted commit below, but that's
where bisecting would likely lead as its what made the bug user-visible.

Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
Fixes: 5b32510af77b ("arm64/mm: Add uffd write-protect support")
---

Hi All,

This applies on top of v6.10-rc4 and it would be good to land this as a hotfix
for v6.10 since its effectively fixing a bug in 5b32510af77b which was merged
for v6.10.

I've only been able to trigger this occasionally by running the mm uffd
selftests, when swap is configured to use a small (64M) zRam disk. With this fix
applied I can no longer trigger it.

Thanks,
Ryan

 arch/arm64/include/asm/pgtable-hwdef.h | 1 +
 arch/arm64/mm/mmu.c                    | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index 9943ff0af4c9..1f60aa1bc750 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -170,6 +170,7 @@
 #define PTE_CONT		(_AT(pteval_t, 1) << 52)	/* Contiguous range */
 #define PTE_PXN			(_AT(pteval_t, 1) << 53)	/* Privileged XN */
 #define PTE_UXN			(_AT(pteval_t, 1) << 54)	/* User XN */
+#define PTE_SWBITS_MASK		_AT(pteval_t, (BIT(63) | GENMASK(58, 55)))

 #define PTE_ADDR_LOW		(((_AT(pteval_t, 1) << (50 - PAGE_SHIFT)) - 1) << PAGE_SHIFT)
 #ifdef CONFIG_ARM64_PA_BITS_52
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index c927e9312f10..353ea5dc32b8 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -124,7 +124,8 @@ bool pgattr_change_is_safe(u64 old, u64 new)
 	 * The following mapping attributes may be updated in live
 	 * kernel mappings without the need for break-before-make.
 	 */
-	pteval_t mask = PTE_PXN | PTE_RDONLY | PTE_WRITE | PTE_NG;
+	pteval_t mask = PTE_PXN | PTE_RDONLY | PTE_WRITE | PTE_NG |
+			PTE_SWBITS_MASK;

 	/* creating or taking down mappings is always safe */
 	if (!pte_valid(__pte(old)) || !pte_valid(__pte(new)))
--
2.43.0