[PATCH v5 07/21] arm64: head: split off idmap creation code

Ard Biesheuvel
2022-06-24T15:09:36Z

Split off the creation of the ID map page tables, so that we can avoid
running it again unnecessarily when KASLR is in effect (which only
randomizes the virtual placement). This will permit us to drop some
explicit cache maintenance to the PoC which was necessary because the
cache invalidation being performed on some global variables might
otherwise clobber unrelated variables that happen to share a cacheline.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/arm64/kernel/head.S | 101 ++++++++++----------
1 file changed, 52 insertions(+), 49 deletions(-)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index eb54c0289c8a..1cbc52097bf9 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -84,7 +84,7 @@
* Register Scope Purpose
* x21 primary_entry() .. start_kernel() FDT pointer passed at boot in x0
* x23 primary_entry() .. start_kernel() physical misalignment/KASLR offset
- * x28 __create_page_tables() callee preserved temp register
+ * x28 clear_page_tables() callee preserved temp register
* x19/x20 __primary_switch() callee preserved temp registers
* x24 __primary_switch() .. relocate_kernel() current RELR displacement
*/
@@ -94,7 +94,10 @@ SYM_CODE_START(primary_entry)
adrp x23, __PHYS_OFFSET
and x23, x23, MIN_KIMG_ALIGN - 1 // KASLR offset, defaults to 0
bl set_cpu_boot_mode_flag
- bl __create_page_tables
+ bl clear_page_tables
+ bl create_idmap
+ bl create_kernel_mapping
+
/*
* The following calls CPU setup code, see arch/arm64/mm/proc.S for
* details.
@@ -122,6 +125,35 @@ SYM_CODE_START_LOCAL(preserve_boot_args)
b dcache_inval_poc // tail call
SYM_CODE_END(preserve_boot_args)

+SYM_FUNC_START_LOCAL(clear_page_tables)
+ mov x28, lr
+
+ /*
+ * Invalidate the init page tables to avoid potential dirty cache lines
+ * being evicted. Other page tables are allocated in rodata as part of
+ * the kernel image, and thus are clean to the PoC per the boot
+ * protocol.
+ */
+ adrp x0, init_pg_dir
+ adrp x1, init_pg_end
+ bl dcache_inval_poc
+
+ /*
+ * Clear the init page tables.
+ */
+ adrp x0, init_pg_dir
+ adrp x1, init_pg_end
+ sub x1, x1, x0
+1: stp xzr, xzr, [x0], #16
+ stp xzr, xzr, [x0], #16
+ stp xzr, xzr, [x0], #16
+ stp xzr, xzr, [x0], #16
+ subs x1, x1, #64
+ b.ne 1b
+
+ ret x28
+SYM_FUNC_END(clear_page_tables)
+
/*
* Macro to populate page table entries, these entries can be pointers to the next level
* or last level entries pointing to physical memory.
@@ -231,44 +263,8 @@ SYM_CODE_END(preserve_boot_args)
populate_entries \tbl, \rtbl, \istart, \iend, \flags, #SWAPPER_BLOCK_SIZE, \tmp
.endm

-/*
- * Setup the initial page tables. We only setup the barest amount which is
- * required to get the kernel running. The following sections are required:
- * - identity mapping to enable the MMU (low address, TTBR0)
- * - first few MB of the kernel linear mapping to jump to once the MMU has
- * been enabled
- */
-SYM_FUNC_START_LOCAL(__create_page_tables)
- mov x28, lr

- /*
- * Invalidate the init page tables to avoid potential dirty cache lines
- * being evicted. Other page tables are allocated in rodata as part of
- * the kernel image, and thus are clean to the PoC per the boot
- * protocol.
- */
- adrp x0, init_pg_dir
- adrp x1, init_pg_end
- bl dcache_inval_poc
-
- /*
- * Clear the init page tables.
- */
- adrp x0, init_pg_dir
- adrp x1, init_pg_end
- sub x1, x1, x0
-1: stp xzr, xzr, [x0], #16
- stp xzr, xzr, [x0], #16
- stp xzr, xzr, [x0], #16
- stp xzr, xzr, [x0], #16
- subs x1, x1, #64
- b.ne 1b
-
- mov x7, SWAPPER_MM_MMUFLAGS
-
- /*
- * Create the identity mapping.
- */
+SYM_FUNC_START_LOCAL(create_idmap)
adrp x0, idmap_pg_dir
adrp x3, __idmap_text_start // __pa(__idmap_text_start)

@@ -319,12 +315,23 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
*/
#endif
adr_l x6, __idmap_text_end // __pa(__idmap_text_end)
+ mov x7, SWAPPER_MM_MMUFLAGS

map_memory x0, x1, x3, x6, x7, x3, IDMAP_PGD_ORDER, x10, x11, x12, x13, x14, EXTRA_SHIFT

/*
- * Map the kernel image (starting with PHYS_OFFSET).
+ * Since the page tables have been populated with non-cacheable
+ * accesses (MMU disabled), invalidate those tables again to
+ * remove any speculatively loaded cache lines.
*/
+ dmb sy
+
+ adrp x0, idmap_pg_dir
+ adrp x1, idmap_pg_end
+ b dcache_inval_poc // tail call
+SYM_FUNC_END(create_idmap)
+
+SYM_FUNC_START_LOCAL(create_kernel_mapping)
adrp x0, init_pg_dir
mov_q x5, KIMAGE_VADDR // compile time __va(_text)
add x5, x5, x23 // add KASLR displacement
@@ -332,6 +339,7 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
adrp x3, _text // runtime __pa(_text)
sub x6, x6, x3 // _end - _text
add x6, x6, x5 // runtime __va(_end)
+ mov x7, SWAPPER_MM_MMUFLAGS

map_memory x0, x1, x5, x6, x7, x3, (VA_BITS - PGDIR_SHIFT), x10, x11, x12, x13, x14

@@ -342,16 +350,10 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
*/
dmb sy

- adrp x0, idmap_pg_dir
- adrp x1, idmap_pg_end
- bl dcache_inval_poc
-
adrp x0, init_pg_dir
adrp x1, init_pg_end
- bl dcache_inval_poc
-
- ret x28
-SYM_FUNC_END(__create_page_tables)
+ b dcache_inval_poc // tail call
+SYM_FUNC_END(create_kernel_mapping)

/*
* Initialize CPU registers with task-specific and cpu-specific context.
@@ -836,7 +838,8 @@ SYM_FUNC_START_LOCAL(__primary_switch)
pre_disable_mmu_workaround
msr sctlr_el1, x20 // disable the MMU
isb
- bl __create_page_tables // recreate kernel mapping
+ bl clear_page_tables
+ bl create_kernel_mapping // Recreate kernel mapping

tlbi vmalle1 // Remove any stale TLB entries
dsb nsh