[PATCH v5 15/28] x86/arch_prctl: Create ARCH_GET_XSTATE/ARCH_PUT_XSTATE

From: Chang S. Bae
Date: Sun May 23 2021 - 15:39:22 EST

N.B. This interface is currently under active discussion on LKML. This v1
proposal implements a per-task system call with GET/PUT semantics. A
per-process system call without PUT semantics may be superior.

arch_prctl(ARCH_GET_XSTATE)
Each user task requests access to the hardware features associated
with the specified xstate bits.
Return codes:
0: success (including repeated calls)
EPERM: requested feature is not a user-supported feature.
EINVAL: too many GETs (>INT_MAX) without matching PUTS (reference
counter overflow)
ENOMEM: buffer or reference counter allocation failed.

arch_prctl(ARCH_PUT_XSTATE)
User task tells the kernel that it is no longer using the features
associated with the specified xstate bits.
Return codes:
0: success
EINVAL: imbalance with preceding ARCH_GET_XSTATE calls

The kernel enforces access to the specified using XFD hardware support. By
default, XFD is armed and results in #NM traps on un-authorized access.
Upon successful ARCH_GET_XSTATE, XFD traps are dis-armed and the user is
free to access the feature.

On the last ARCH_PUT_XSTATE, the kernel re-arms XFD. However, as an
optimization, the kernel does not immediately free the xstate buffer.
A future enhancement could be made to identify and free allocated but
un-used xstate buffers under constrained memory conditions.

Rename the third argument for do_arch_prctl_common() to reflect its generic
use.

Free up the dynamically-allocated buffer as well as the reference counters
when the task terminates.

Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes from v4:
* Added as a new patch. (Andy Lutomirski, Thomas Gleixner, and et al)
---
arch/x86/include/asm/fpu/internal.h | 2 +
arch/x86/include/asm/fpu/types.h | 8 +++
arch/x86/include/asm/fpu/xstate.h | 2 +
arch/x86/include/asm/proto.h | 2 +-
arch/x86/include/uapi/asm/prctl.h | 3 +
arch/x86/kernel/fpu/core.c | 14 ++++
arch/x86/kernel/fpu/xstate.c | 103 ++++++++++++++++++++++++++--
arch/x86/kernel/process.c | 13 +++-
8 files changed, 140 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index c250216320df..6460d52991ba 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -636,4 +636,6 @@ static inline void switch_fpu_finish(struct fpu *old_fpu, struct fpu *new_fpu)
update_pasid();
}

+void free_fpu(struct fpu *fpu);
+
#endif /* _ASM_X86_FPU_INTERNAL_H */
diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 6fc707c14350..387b96e0b643 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -343,6 +343,14 @@ struct fpu {
*/
u64 state_mask;

+ /*
+ * @refcount;
+ *
+ * Point a reference counter array for arch_prctl that tracks the xstate
+ * use request.
+ */
+ int *refcount;
+
/*
* @state:
*
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index cbb4795d2b45..30f83eb0aef4 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -116,6 +116,8 @@ unsigned int get_xstate_size(u64 mask);
int alloc_xstate_buffer(struct fpu *fpu, u64 mask);
void free_xstate_buffer(struct fpu *fpu);

+long do_arch_prctl_xstate(struct fpu *fpu, int option, unsigned long mask);
+
const void *get_xsave_field_ptr(int xfeature_nr);
int using_compacted_format(void);
int xfeature_size(int xfeature_nr);
diff --git a/arch/x86/include/asm/proto.h b/arch/x86/include/asm/proto.h
index 8c5d1910a848..feed36d44d04 100644
--- a/arch/x86/include/asm/proto.h
+++ b/arch/x86/include/asm/proto.h
@@ -40,6 +40,6 @@ void x86_report_nx(void);
extern int reboot_force;

long do_arch_prctl_common(struct task_struct *task, int option,
- unsigned long cpuid_enabled);
+ unsigned long arg2);

#endif /* _ASM_X86_PROTO_H */
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 5a6aac9fa41f..d3fce82845fd 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -10,6 +10,9 @@
#define ARCH_GET_CPUID 0x1011
#define ARCH_SET_CPUID 0x1012

+#define ARCH_GET_XSTATE 0x1021
+#define ARCH_PUT_XSTATE 0x1022
+
#define ARCH_MAP_VDSO_X32 0x2001
#define ARCH_MAP_VDSO_32 0x2002
#define ARCH_MAP_VDSO_64 0x2003
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 25c9c7dad3f9..016c3adebec3 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -254,6 +254,8 @@ int fpu__copy(struct task_struct *dst, struct task_struct *src)

WARN_ON_FPU(src_fpu != &current->thread.fpu);

+ dst_fpu->refcount = NULL;
+
/*
* The child does not inherit the dynamic states. Thus, use the buffer
* embedded in struct task_struct, which has the minimum size.
@@ -541,3 +543,15 @@ int fpu__exception_code(struct fpu *fpu, int trap_nr)
*/
return 0;
}
+
+/**
+ * free_fpu() - Free up memory that belongs to the FPU context.
+ * @fpu: A struct fpu * pointer
+ *
+ * Return: Nothing
+ */
+void free_fpu(struct fpu *fpu)
+{
+ kfree(fpu->refcount);
+ free_xstate_buffer(fpu);
+}
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index e60a20a1b24b..126c4a509669 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -21,6 +21,7 @@
#include <asm/tlbflush.h>
#include <asm/cpufeature.h>
#include <asm/trace/fpu.h>
+#include <asm/prctl.h>

/*
* Although we spell it out in here, the Processor Trace
@@ -78,6 +79,11 @@ static unsigned int xstate_supervisor_only_offsets[XFEATURE_MAX] = { [ 0 ... XFE
* byte boundary. Otherwise, it follows the preceding component immediately.
*/
static bool xstate_aligns[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = false};
+/*
+ * Remember the index number in the reference counter array that supports
+ * access request. '-1' indicates that a state component does not support it.
+ */
+static unsigned int xstate_refcount_idx[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};

/**
* struct fpu_xstate_buffer_config - xstate per-task buffer configuration
@@ -969,8 +975,7 @@ void __init fpu__init_system_xstate(void)
{
unsigned int eax, ebx, ecx, edx;
static int on_boot_cpu __initdata = 1;
- int err;
- int i;
+ int err, i, j;

WARN_ON_FPU(!on_boot_cpu);
on_boot_cpu = 0;
@@ -1025,14 +1030,17 @@ void __init fpu__init_system_xstate(void)
xfeatures_mask_all &= fpu__get_supported_xfeatures_mask();
xfeatures_mask_user_dynamic = 0;

- for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
+ for (i = FIRST_EXTENDED_XFEATURE, j = 0; i < XFEATURE_MAX; i++) {
u64 feature_mask = BIT_ULL(i);

if (!(xfeatures_mask_user() & feature_mask))
continue;

- if (xfd_supported(i))
+ if (xfd_supported(i)) {
xfeatures_mask_user_dynamic |= feature_mask;
+ xstate_refcount_idx[i] = j;
+ j++;
+ }
}

/* Enable xstate instructions to be able to continue with initialization: */
@@ -1339,6 +1347,93 @@ int alloc_xstate_buffer(struct fpu *fpu, u64 mask)
return 0;
}

+/**
+ * do_arch_prctl_xstate() - Handle xstate-related arch_prctl requests.
+ *
+ * @fpu: A struct fpu * pointer
+ * @option: A subfunction of arch_prctl()
+ * @mask: A xstate-component bitmap
+ *
+ * Return: 0 if successful; otherwise, return a relevant error code.
+ */
+long do_arch_prctl_xstate(struct fpu *fpu, int option, unsigned long mask)
+{
+ bool need_xfd_update = false;
+ int i;
+
+ switch (option) {
+ case ARCH_GET_XSTATE: {
+ int err = 0;
+
+ if (mask & ~xfeatures_mask_user())
+ return -EPERM;
+
+ if (!fpu->refcount) {
+ fpu->refcount = kcalloc(hweight64(xfeatures_mask_user_dynamic),
+ sizeof(int), GFP_KERNEL);
+ if (!fpu->refcount)
+ return -ENOMEM;
+ }
+
+ for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
+ unsigned int idx = xstate_refcount_idx[i];
+
+ if ((idx == -1) || !(BIT_ULL(i) & mask))
+ continue;
+
+ if (fpu->refcount[idx] == INT_MAX)
+ return -EINVAL;
+
+ fpu->refcount[idx]++;
+ }
+
+ if ((mask & fpu->state_mask) == mask)
+ return 0;
+
+ err = alloc_xstate_buffer(fpu, mask);
+ if (!err)
+ need_xfd_update = true;
+ else
+ return err;
+ break;
+ }
+ case ARCH_PUT_XSTATE: {
+ if (mask & ~xfeatures_mask_user())
+ return -EPERM;
+
+ if (!fpu->refcount)
+ return -EINVAL;
+
+ for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
+ int idx = xstate_refcount_idx[i];
+ u64 feature_mask = BIT_ULL(i);
+
+ if ((idx == -1) || !(feature_mask & mask))
+ continue;
+
+ if (fpu->refcount[idx] <= 0)
+ return -EINVAL;
+
+ fpu->refcount[idx]--;
+ if (!fpu->refcount[idx]) {
+ need_xfd_update = true;
+ fpu->state_mask &= ~(feature_mask);
+ }
+ }
+ break;
+ }
+ default:
+ return -EINVAL;
+ }
+
+ if (need_xfd_update) {
+ u64 fpu_xfd_mask = fpu->state_mask & xfd_capable();
+
+ xfd_write(xfd_capable() ^ fpu_xfd_mask);
+ }
+ return 0;
+}
+
static void fill_gap(struct membuf *to, unsigned *last, unsigned offset)
{
if (*last >= offset)
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 5252464a27e3..c166243f64e4 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -98,6 +98,12 @@ void arch_thread_struct_whitelist(unsigned long *offset, unsigned long *size)
*size = get_xstate_config(XSTATE_MIN_SIZE);
}

+void arch_release_task_struct(struct task_struct *tsk)
+{
+ if (boot_cpu_has(X86_FEATURE_FPU))
+ free_fpu(&tsk->thread.fpu);
+}
+
/*
* Free thread data structures etc..
*/
@@ -990,13 +996,16 @@ unsigned long get_wchan(struct task_struct *p)
}

long do_arch_prctl_common(struct task_struct *task, int option,
- unsigned long cpuid_enabled)
+ unsigned long arg2)
{
switch (option) {
case ARCH_GET_CPUID:
return get_cpuid_mode();
case ARCH_SET_CPUID:
- return set_cpuid_mode(task, cpuid_enabled);
+ return set_cpuid_mode(task, arg2);
+ case ARCH_GET_XSTATE:
+ case ARCH_PUT_XSTATE:
+ return do_arch_prctl_xstate(&task->thread.fpu, option, arg2);
}

return -EINVAL;
--
2.17.1