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Subject[PATCH 4.4 20/37] kaiser: PCID 0 for kernel and 128 for user
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Hugh Dickins <hughd@google.com>


Why was 4 chosen for kernel PCID and 6 for user PCID?
No good reason in a backport where PCIDs are only used for Kaiser.

If we continue with those, then we shall need to add Andy Lutomirski's
4.13 commit 6c690ee1039b ("x86/mm: Split read_cr3() into read_cr3_pa()
and __read_cr3()"), which deals with the problem of read_cr3() callers
finding stray bits in the cr3 that they expected to be page-aligned;
and for hibernation, his 4.14 commit f34902c5c6c0 ("x86/hibernate/64:
Mask off CR3's PCID bits in the saved CR3").

But if 0 is used for kernel PCID, then there's no need to add in those
commits - whenever the kernel looks, it sees 0 in the lower bits; and
0 for kernel seems an obvious choice.

And I naughtily propose 128 for user PCID. Because there's a place
in _SWITCH_TO_USER_CR3 where it takes note of the need for TLB FLUSH,
but needs to reset that to NOFLUSH for the next occasion. Currently
it does so with a "movb $(0x80)" into the high byte of the per-cpu
quadword, but that will cause a machine without PCID support to crash.
Now, if %al just happened to have 0x80 in it at that point, on a
machine with PCID support, but 0 on a machine without PCID support...

(That will go badly wrong once the pgd can be at a physical address
above 2^56, but even with 5-level paging, physical goes up to 2^52.)

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/kaiser.h | 19 ++++++++++++-------
arch/x86/include/asm/pgtable_types.h | 7 ++++---
arch/x86/mm/tlb.c | 3 +++
3 files changed, 19 insertions(+), 10 deletions(-)

--- a/arch/x86/include/asm/kaiser.h
+++ b/arch/x86/include/asm/kaiser.h
@@ -29,14 +29,19 @@ orq X86_CR3_PCID_KERN_VAR, \reg
movq \reg, %cr3
.endm

-.macro _SWITCH_TO_USER_CR3 reg
+.macro _SWITCH_TO_USER_CR3 reg regb
+/*
+ * regb must be the low byte portion of reg: because we have arranged
+ * for the low byte of the user PCID to serve as the high byte of NOFLUSH
+ * (0x80 for each when PCID is enabled, or 0x00 when PCID and NOFLUSH are
+ * not enabled): so that the one register can update both memory and cr3.
+ */
movq %cr3, \reg
andq $(~(X86_CR3_PCID_ASID_MASK | KAISER_SHADOW_PGD_OFFSET)), \reg
orq PER_CPU_VAR(X86_CR3_PCID_USER_VAR), \reg
js 9f
-// FLUSH this time, reset to NOFLUSH for next time
-// But if nopcid? Consider using 0x80 for user pcid?
-movb $(0x80), PER_CPU_VAR(X86_CR3_PCID_USER_VAR+7)
+/* FLUSH this time, reset to NOFLUSH for next time (if PCID enabled) */
+movb \regb, PER_CPU_VAR(X86_CR3_PCID_USER_VAR+7)
9:
movq \reg, %cr3
.endm
@@ -49,7 +54,7 @@ popq %rax

.macro SWITCH_USER_CR3
pushq %rax
-_SWITCH_TO_USER_CR3 %rax
+_SWITCH_TO_USER_CR3 %rax %al
popq %rax
.endm

@@ -61,7 +66,7 @@ movq PER_CPU_VAR(unsafe_stack_register_b

.macro SWITCH_USER_CR3_NO_STACK
movq %rax, PER_CPU_VAR(unsafe_stack_register_backup)
-_SWITCH_TO_USER_CR3 %rax
+_SWITCH_TO_USER_CR3 %rax %al
movq PER_CPU_VAR(unsafe_stack_register_backup), %rax
.endm

@@ -69,7 +74,7 @@ movq PER_CPU_VAR(unsafe_stack_register_b

.macro SWITCH_KERNEL_CR3 reg
.endm
-.macro SWITCH_USER_CR3 reg
+.macro SWITCH_USER_CR3 reg regb
.endm
.macro SWITCH_USER_CR3_NO_STACK
.endm
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -111,16 +111,17 @@

/* Mask for all the PCID-related bits in CR3: */
#define X86_CR3_PCID_MASK (X86_CR3_PCID_NOFLUSH | X86_CR3_PCID_ASID_MASK)
+#define X86_CR3_PCID_ASID_KERN (_AC(0x0,UL))
+
#if defined(CONFIG_KAISER) && defined(CONFIG_X86_64)
-#define X86_CR3_PCID_ASID_KERN (_AC(0x4,UL))
-#define X86_CR3_PCID_ASID_USER (_AC(0x6,UL))
+/* Let X86_CR3_PCID_ASID_USER be usable for the X86_CR3_PCID_NOFLUSH bit */
+#define X86_CR3_PCID_ASID_USER (_AC(0x80,UL))

#define X86_CR3_PCID_KERN_FLUSH (X86_CR3_PCID_ASID_KERN)
#define X86_CR3_PCID_USER_FLUSH (X86_CR3_PCID_ASID_USER)
#define X86_CR3_PCID_KERN_NOFLUSH (X86_CR3_PCID_NOFLUSH | X86_CR3_PCID_ASID_KERN)
#define X86_CR3_PCID_USER_NOFLUSH (X86_CR3_PCID_NOFLUSH | X86_CR3_PCID_ASID_USER)
#else
-#define X86_CR3_PCID_ASID_KERN (_AC(0x0,UL))
#define X86_CR3_PCID_ASID_USER (_AC(0x0,UL))
/*
* PCIDs are unsupported on 32-bit and none of these bits can be
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -50,6 +50,9 @@ static void load_new_mm_cr3(pgd_t *pgdir
* invpcid_flush_single_context(X86_CR3_PCID_ASID_USER) could
* do it here, but can only be used if X86_FEATURE_INVPCID is
* available - and many machines support pcid without invpcid.
+ *
+ * The line below is a no-op: X86_CR3_PCID_KERN_FLUSH is now 0;
+ * but keep that line in there in case something changes.
*/
new_mm_cr3 |= X86_CR3_PCID_KERN_FLUSH;
kaiser_flush_tlb_on_return_to_user();

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