Re: [PATCH] x86/pti: Fix !PCID and sanitize defines

Laura Abbott <labbott@xxxxxxxxxx> · Mon, 15 Jan 2018 07:14:59 -0800

On 01/13/2018 03:23 PM, Thomas Gleixner wrote:
The switch to the user space page tables in the low level ASM code sets
unconditionally bit 12 and bit 11 of CR3. Bit 12 is switching the base
address of the page directory to the user part, bit 11 is switching the
PCID to the PCID associated with the user page tables.

This fails on a machine which lacks PCID support because bit 11 is set in
CR3. Bit 11 is reserved when PCID is inactive.

While the Intel SDM claims that the reserved bits are ignored when PCID is
disabled, the AMD APM states that they should be cleared.

This went unnoticed as the AMD APM was not checked when the code was
developed and reviewed and test systems with Intel CPUs never failed to
boot. The report is against a Centos 6 host where the guest fails to boot,
so it's not yet clear whether this is a virt issue or can happen on real
hardware too, but thats irrelevant as the AMD APM clearly ask for clearing
the reserved bits.

Make sure that on non PCID machines bit 11 is not set by the page table
switching code.

Andy suggested to rename the related bits and masks so they are clearly
describing what they should be used for, which is done as well for clarity.

That split could have been done with alternatives but the macro hell is
horrible and ugly. This can be done on top if someone cares to remove the
extra orq. For now it's a straight forward fix.


Original reporter confirmed it fixes the problem. Thanks for the
prompt response.

Laura

Fixes: 6fd166aae78c ("x86/mm: Use/Fix PCID to optimize user/kernel switches")
Reported-by: Laura Abbott <labbott@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Willy Tarreau <w@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

---
  arch/x86/entry/calling.h               |   36 +++++++++++++++++----------------
  arch/x86/include/asm/processor-flags.h |    2 -
  arch/x86/include/asm/tlbflush.h        |    6 ++---
  3 files changed, 23 insertions(+), 21 deletions(-)

--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -198,8 +198,11 @@ For 32-bit we have the following convent
   * PAGE_TABLE_ISOLATION PGDs are 8k.  Flip bit 12 to switch between the two
   * halves:
   */
-#define PTI_SWITCH_PGTABLES_MASK	(1<<PAGE_SHIFT)
-#define PTI_SWITCH_MASK		(PTI_SWITCH_PGTABLES_MASK|(1<<X86_CR3_PTI_SWITCH_BIT))
+#define PTI_USER_PGTABLE_BIT		PAGE_SHIFT
+#define PTI_USER_PGTABLE_MASK		(1 << PTI_USER_PGTABLE_BIT)
+#define PTI_USER_PCID_BIT		X86_CR3_PTI_PCID_USER_BIT
+#define PTI_USER_PCID_MASK		(1 << PTI_USER_PCID_BIT)
+#define PTI_USER_PGTABLE_AND_PCID_MASK  (PTI_USER_PCID_MASK | PTI_USER_PGTABLE_MASK)
  
  .macro SET_NOFLUSH_BIT	reg:req
  	bts	$X86_CR3_PCID_NOFLUSH_BIT, \reg
@@ -208,7 +211,7 @@ For 32-bit we have the following convent
  .macro ADJUST_KERNEL_CR3 reg:req
  	ALTERNATIVE "", "SET_NOFLUSH_BIT \reg", X86_FEATURE_PCID
  	/* Clear PCID and "PAGE_TABLE_ISOLATION bit", point CR3 at kernel pagetables: */
-	andq    $(~PTI_SWITCH_MASK), \reg
+	andq    $(~PTI_USER_PGTABLE_AND_PCID_MASK), \reg
  .endm
  
  .macro SWITCH_TO_KERNEL_CR3 scratch_reg:req
@@ -239,15 +242,19 @@ For 32-bit we have the following convent
  	/* Flush needed, clear the bit */
  	btr	\scratch_reg, THIS_CPU_user_pcid_flush_mask
  	movq	\scratch_reg2, \scratch_reg
-	jmp	.Lwrcr3_\@
+	jmp	.Lwrcr3_pcid_\@
  
  .Lnoflush_\@:
  	movq	\scratch_reg2, \scratch_reg
  	SET_NOFLUSH_BIT \scratch_reg
  
+.Lwrcr3_pcid_\@:
+	/* Flip the ASID to the user version */
+	orq	$(PTI_USER_PCID_MASK), \scratch_reg
+
  .Lwrcr3_\@:
-	/* Flip the PGD and ASID to the user version */
-	orq     $(PTI_SWITCH_MASK), \scratch_reg
+	/* Flip the PGD to the user version */
+	orq     $(PTI_USER_PGTABLE_MASK), \scratch_reg
  	mov	\scratch_reg, %cr3
  .Lend_\@:
  .endm
@@ -263,17 +270,12 @@ For 32-bit we have the following convent
  	movq	%cr3, \scratch_reg
  	movq	\scratch_reg, \save_reg
  	/*
-	 * Is the "switch mask" all zero?  That means that both of
-	 * these are zero:
-	 *
-	 *	1. The user/kernel PCID bit, and
-	 *	2. The user/kernel "bit" that points CR3 to the
-	 *	   bottom half of the 8k PGD
-	 *
-	 * That indicates a kernel CR3 value, not a user CR3.
+	 * Test the user pagetable bit. If set, then the user page tables
+	 * are active. If clear CR3 already has the kernel page table
+	 * active.
  	 */
-	testq	$(PTI_SWITCH_MASK), \scratch_reg
-	jz	.Ldone_\@
+	bt	$PTI_USER_PGTABLE_BIT, \scratch_reg
+	jnc	.Ldone_\@
  
  	ADJUST_KERNEL_CR3 \scratch_reg
  	movq	\scratch_reg, %cr3
@@ -290,7 +292,7 @@ For 32-bit we have the following convent
  	 * KERNEL pages can always resume with NOFLUSH as we do
  	 * explicit flushes.
  	 */
-	bt	$X86_CR3_PTI_SWITCH_BIT, \save_reg
+	bt	$PTI_USER_PGTABLE_BIT, \save_reg
  	jnc	.Lnoflush_\@
  
  	/*
--- a/arch/x86/include/asm/processor-flags.h
+++ b/arch/x86/include/asm/processor-flags.h
@@ -40,7 +40,7 @@
  #define CR3_NOFLUSH	BIT_ULL(63)
  
  #ifdef CONFIG_PAGE_TABLE_ISOLATION
-# define X86_CR3_PTI_SWITCH_BIT	11
+# define X86_CR3_PTI_PCID_USER_BIT	11
  #endif
  
  #else
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -81,13 +81,13 @@ static inline u16 kern_pcid(u16 asid)
  	 * Make sure that the dynamic ASID space does not confict with the
  	 * bit we are using to switch between user and kernel ASIDs.
  	 */
-	BUILD_BUG_ON(TLB_NR_DYN_ASIDS >= (1 << X86_CR3_PTI_SWITCH_BIT));
+	BUILD_BUG_ON(TLB_NR_DYN_ASIDS >= (1 << X86_CR3_PTI_PCID_USER_BIT));
  
  	/*
  	 * The ASID being passed in here should have respected the
  	 * MAX_ASID_AVAILABLE and thus never have the switch bit set.
  	 */
-	VM_WARN_ON_ONCE(asid & (1 << X86_CR3_PTI_SWITCH_BIT));
+	VM_WARN_ON_ONCE(asid & (1 << X86_CR3_PTI_PCID_USER_BIT));
  #endif
  	/*
  	 * The dynamically-assigned ASIDs that get passed in are small
@@ -112,7 +112,7 @@ static inline u16 user_pcid(u16 asid)
  {
  	u16 ret = kern_pcid(asid);
  #ifdef CONFIG_PAGE_TABLE_ISOLATION
-	ret |= 1 << X86_CR3_PTI_SWITCH_BIT;
+	ret |= 1 << X86_CR3_PTI_PCID_USER_BIT;
  #endif
  	return ret;
  }








