From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
PKRU has space in the task XSAVE buffer, but is not context-switched by
XSAVE/XRSTOR. It is switched more eagerly than other FPU state because
PKRU affects things like copy_to/from_user(). This is because PKRU
affects user *PERMISSION* accesses, not just accesses made from user
*MODE* itself.
Prepare to move PKRU away from being XSAVE-managed. Allocate space in
the thread_struct for it and save/restore it in the context-switch path
separately from the XSAVE-managed features.
Leave the XSAVE storage in place for now to ensure bisectability.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
---
b/arch/x86/include/asm/pkru.h | 5 +++++
b/arch/x86/kernel/cpu/common.c | 3 +++
b/arch/x86/kernel/process_64.c | 9 ++++-----
b/arch/x86/mm/pkeys.c | 2 ++
4 files changed, 14 insertions(+), 5 deletions(-)
diff -puN arch/x86/include/asm/pkru.h~pkru-stash-thread-value arch/x86/include/asm/pkru.h
--- a/arch/x86/include/asm/pkru.h~pkru-stash-thread-value 2021-06-22 14:49:06.594051763 -0700
+++ b/arch/x86/include/asm/pkru.h 2021-06-22 14:49:06.607051763 -0700
@@ -44,11 +44,16 @@ static inline void write_pkru(u32 pkru)
if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
return;
/*
+ * Update the actual register.
+ *
* WRPKRU is relatively expensive compared to RDPKRU.
* Avoid WRPKRU when it would not change the value.
*/
if (pkru != rdpkru())
wrpkru(pkru);
+
+ /* Update the thread-local, context-switched value: */
+ current->thread.pkru = pkru;
}
static inline void pkru_write_default(void)
diff -puN arch/x86/kernel/cpu/common.c~pkru-stash-thread-value arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~pkru-stash-thread-value 2021-06-22 14:49:06.596051763 -0700
+++ b/arch/x86/kernel/cpu/common.c 2021-06-22 14:49:06.608051763 -0700
@@ -482,6 +482,9 @@ static __always_inline void setup_pku(st
cr4_set_bits(X86_CR4_PKE);
/* Load the default PKRU value */
pkru_write_default();
+
+ /* Establish the default value for future tasks: */
+ init_task.thread.pkru = init_pkru_value;
}
#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
diff -puN arch/x86/kernel/process_64.c~pkru-stash-thread-value arch/x86/kernel/process_64.c
--- a/arch/x86/kernel/process_64.c~pkru-stash-thread-value 2021-06-22 14:49:06.599051763 -0700
+++ b/arch/x86/kernel/process_64.c 2021-06-22 14:49:06.608051763 -0700
@@ -349,15 +349,14 @@ static __always_inline void load_seg_leg
static __always_inline void x86_pkru_load(struct thread_struct *prev,
struct thread_struct *next)
{
- if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
- return;
+ u32 pkru = read_pkru();
/* Stash the prev task's value: */
- prev->pkru = rdpkru();
+ prev->pkru = pkru;
/*
- * PKRU writes are slightly expensive. Avoid them when not
- * strictly necessary:
+ * PKRU writes are slightly expensive. Avoid
+ * them when not strictly necessary:
*/
if (prev->pkru != next->pkru)
wrpkru(next->pkru);
diff -puN arch/x86/mm/pkeys.c~pkru-stash-thread-value arch/x86/mm/pkeys.c
--- a/arch/x86/mm/pkeys.c~pkru-stash-thread-value 2021-06-22 14:49:06.604051763 -0700
+++ b/arch/x86/mm/pkeys.c 2021-06-22 14:49:06.609051763 -0700
@@ -159,6 +159,8 @@ static ssize_t init_pkru_write_file(stru
return -EINVAL;
WRITE_ONCE(init_pkru_value, new_init_pkru);
+ WRITE_ONCE(init_task.thread.pkru, new_init_pkru);
+
return count;
}
_
