[PATCH v2] arm64: mm: Defer read-only remap of data/bss linear alias

[PATCH v2] arm64: mm: Defer read-only remap of data/bss linear alias

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@kernel.org>

Since commit

  f2ba877402e5 ("arm64: mm: Map the kernel data/bss read-only in the linear map")

the linear alias of the .data and .bss regions is remapped read-only
early during the boot. (Note that a subsequent patch to unmap this
region entirely was reverted just before the v7.2 merge window, and will
be brought back in an improved form for the v7.3 cycle)

Fuad reports that in some cases, the KVM init code may apply relocations
to variables that reside in .data, and does so via the linear map. This
means that remapping .data read-only beforehand is a bad idea, and
results in an early boot crash.

These variables in .data are only present when CONFIG_NVHE_EL2_DEBUG or
CONFIG_NVHE_EL2_TRACING are enabled, which is why it was not spotted in
testing.

So move the remap to mark_rodata_ro(), which is a reasonable place to
put this, and ensures that it happens much later during the boot. It
also means that rodata=off is now taken into account, and so the linear
alias will remain writable in that case.

Fixes: f2ba877402e5 ("arm64: mm: Map the kernel data/bss read-only in the linear map")
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Fuad Tabba < fuad.tabba@linux.dev>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
v2: improve changelog, add Fuad's R-b

 arch/arm64/mm/mmu.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 9f354971b7e4..1f7eca86b5c1 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1198,11 +1198,6 @@ static void __init map_mem(void)
 		__map_memblock(start, end, pgprot_tagged(PAGE_KERNEL),
 			       flags);
 	}
-
-	/* Map the kernel data/bss read-only in the linear map */
-	__map_memblock(init_end, kernel_end, PAGE_KERNEL_RO, flags);
-	flush_tlb_kernel_range((unsigned long)lm_alias(__init_end),
-			       (unsigned long)lm_alias(__bss_stop));
 }
 
 void mark_rodata_ro(void)
@@ -1221,6 +1216,12 @@ void mark_rodata_ro(void)
 	update_mapping_prot(__pa_symbol(_text), (unsigned long)_text,
 			    (unsigned long)_stext - (unsigned long)_text,
 			    PAGE_KERNEL_RO);
+
+	/* Map the kernel data/bss read-only in the linear map */
+	update_mapping_prot(__pa_symbol(__init_end),
+			    (unsigned long)lm_alias(__init_end),
+			    (unsigned long)__bss_stop - (unsigned long)__init_end,
+			    PAGE_KERNEL_RO);
 }
 
 static void __init declare_vma(struct vm_struct *vma,
-- 
2.55.0.rc0.799.gd6f94ed593-goog