[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-06 
10:47:57.176796121 -0700
+++ b/arch/x86/kernel/head_64.S 2018-04-06 10:47:57.179796121 -0700
@@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_

[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-06 
10:47:57.176796121 -0700
+++ b/arch/x86/kernel/head_64.S 2018-04-06 10:47:57.179796121 -0700
@@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_

[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-02 
16:41:16.085605170 -0700
+++ b/arch/x86/kernel/head_64.S 2018-04-02 16:41:16.088605170 -0700
@@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_

[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-02 
16:41:16.085605170 -0700
+++ b/arch/x86/kernel/head_64.S 2018-04-02 16:41:16.088605170 -0700
@@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_

[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-02 
10:26:46.113661210 -0700
+++ b/arch/x86/kernel/head_64.S 2018-04-02 10:26:46.116661210 -0700
@@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_

[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-02 
10:26:46.113661210 -0700
+++ b/arch/x86/kernel/head_64.S 2018-04-02 10:26:46.116661210 -0700
@@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_

[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-03-21 
16:31:59.729192314 -0700
+++ b/arch/x86/kernel/head_64.S 2018-03-21 16:31:59.733192314 -0700
@@ -401,8 +401,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -433,6 +438,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_

[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

[Dave Hansen]

From: Dave Hansen 

I was mystified as to where the _PAGE_GLOBAL in the kernel page tables
for kernel text came from.  I audited all the places I could find, but
I missed one: head_64.S.

The page tables that we create in here live for a long time, and they
also have _PAGE_GLOBAL set, despite whether the processor supports it
or not.  It's harmless, and we got *lucky* that the pageattr code
accidentally clears it when we wipe it out of __supported_pte_mask and
then later try to mark kernel text read-only.

Comment some of these properties to make it easier to find and
understand in the future.

Signed-off-by: Dave Hansen 
Cc: Andrea Arcangeli 
Cc: Andy Lutomirski 
Cc: Linus Torvalds 
Cc: Kees Cook 
Cc: Hugh Dickins 
Cc: Juergen Gross 
Cc: x...@kernel.org
Cc: Nadav Amit 
---

 b/arch/x86/kernel/head_64.S |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff -puN arch/x86/kernel/head_64.S~comment-global-page 
arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~comment-global-page 2018-03-21 
16:31:59.729192314 -0700
+++ b/arch/x86/kernel/head_64.S 2018-03-21 16:31:59.733192314 -0700
@@ -401,8 +401,13 @@ NEXT_PAGE(level3_ident_pgt)
.quad   level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.fill   511, 8, 0
 NEXT_PAGE(level2_ident_pgt)
-   /* Since I easily can, map the first 1G.
+   /*
+* Since I easily can, map the first 1G.
 * Don't set NX because code runs from these pages.
+*
+* Note: This sets _PAGE_GLOBAL despite whether
+* the CPU supports it or it is enabled.  But,
+* the CPU should ignore the bit.
 */
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
 #else
@@ -433,6 +438,10 @@ NEXT_PAGE(level2_kernel_pgt)
 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
 *  If you want to increase this then increase MODULES_VADDR
 *  too.)
+*
+*  This table is eventually used by the kernel during normal
+*  runtime.  Care must be taken to clear out undesired bits
+*  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
 */
PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
KERNEL_IMAGE_SIZE/PMD_SIZE)
_