[PATCH v7 14/26] x86/arch_prctl: Create ARCH_SET_XSTATE_ENABLE/ARCH_GET_XSTATE_ENABLE

From: Chang S. Bae
Date: Sat Jul 10 2021 - 09:09:34 EST

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arch_prctl(ARCH_SET_XSTATE_ENABLE)
Some XSTATE features, such as AMX, are unavailable to applications
until that process explicitly requests them via this call. Requests can
be made for any number of valid user XSTATEs in a single call. This
call is intended to be invoked very early in process initialization. A
forked child inherits access, but permission is reset upon exec. There
is no concept of un-requesting XSTATE access.
Return codes:
0: success (including repeated calls)
EINVAL: no hardware feature for the request
EBUSY: error in updating all threads in the process

arch_prctl(ARCH_GET_XSTATE_ENABLE)
Return the bitmask of permitted user XSTATE features.

The permission is checked at every XSTATE buffer expansion: e.g.
XFD-induced #NM event, and ptracer's XSTATE injection. When no permission
is found, inform userspace via SIGSEGV or with error code.

The notion of granted permission is broadcast to all threads in a process.
(This approach follows the PR_SET_FP_MODE prctl(2) implementation.)

Detect a fork race by aborting and returning -EBUSY if the number of
threads at the end of call changed.

[ An alternative implementation would not save the permission bitmap in
every task. But instead would extend the per-process signal data, and that
would not be subject to this race. ]

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

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 v6:
* Add state bitmap param to proposed syscall. (Thiago Macieira)
* Add companion syscall to return the current permission bitmap.
* Update the ptrace path to return EFAULT when no permission to write
XTILEDATA.
* Update do_arch_prctl_common().

Changes from v5:
* Switched to per-process permission. (Based on the discussion on LKML)
---
arch/x86/include/asm/fpu/types.h | 8 ++++
arch/x86/include/asm/fpu/xstate.h | 9 ++++
arch/x86/include/asm/proto.h | 2 +-
arch/x86/include/uapi/asm/prctl.h | 3 ++
arch/x86/kernel/fpu/regset.c | 17 +++++---
arch/x86/kernel/fpu/xstate.c | 68 +++++++++++++++++++++++++++++++
arch/x86/kernel/process.c | 8 +++-
arch/x86/kernel/process_64.c | 6 +++
arch/x86/kernel/traps.c | 8 ++--
9 files changed, 117 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index c0192e16cadb..03160a1a79ad 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -336,6 +336,14 @@ struct fpu {
*/
unsigned long avx512_timestamp;

+ /*
+ * @state_perm:
+ *
+ * The bitmap indicates the permission of using some state
+ * components which are dynamically stored in the per-task buffer.
+ */
+ u64 dynamic_state_perm;
+
/*
* @state_mask:
*
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index 45735441fbe8..f33ad441e42f 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -149,6 +149,15 @@ void *get_xsave_addr(struct fpu *fpu, int xfeature_nr);
unsigned int get_xstate_size(u64 mask);
int alloc_xstate_buffer(struct fpu *fpu, u64 mask);
void free_xstate_buffer(struct fpu *fpu);
+
+long set_process_xstate_perm(struct task_struct *tsk, u64 state_perm);
+void reset_task_xstate_perm(struct task_struct *tsk);
+static inline long get_task_xstate_perm(struct task_struct *tsk)
+{
+ return (xfeatures_mask_uabi() & ~xfeatures_mask_user_dynamic) |
+ tsk->thread.fpu.dynamic_state_perm;
+}
+
int xfeature_size(int xfeature_nr);
int copy_uabi_from_kernel_to_xstate(struct fpu *fpu, const void *kbuf);
int copy_sigframe_from_user_to_xstate(struct fpu *fpu, const void __user *ubuf);
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..5fb1393a289c 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_SET_XSTATE_ENABLE 0x1021
+#define ARCH_GET_XSTATE_ENABLE 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/regset.c b/arch/x86/kernel/fpu/regset.c
index 244e672c3e3d..ee71ffd7c221 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -166,22 +166,27 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
/*
* When a ptracer attempts to write any dynamic user state in the
* target buffer but not sufficiently allocated, it dynamically
- * expands the buffer.
+ * expands the buffer if permitted.
*
* Check if the expansion is possibly needed.
*/
if (xfeatures_mask_user_dynamic &&
((fpu->state_mask & xfeatures_mask_user_dynamic) != xfeatures_mask_user_dynamic)) {
- u64 state_mask;
+ u64 state_mask, dynstate_mask;

/* Retrieve XSTATE_BV. */
memcpy(&state_mask, (kbuf ?: tmpbuf) + offsetof(struct xregs_state, header),
sizeof(u64));

- /* Expand the xstate buffer based on the XSTATE_BV. */
- state_mask &= xfeatures_mask_user_dynamic;
- if (state_mask) {
- ret = alloc_xstate_buffer(fpu, state_mask);
+ /* Check the permission and expand the xstate buffer. */
+ dynstate_mask = state_mask & xfeatures_mask_user_dynamic;
+ if (dynstate_mask) {
+ if ((dynstate_mask & fpu->dynamic_state_perm) != dynstate_mask) {
+ ret = -EFAULT;
+ goto out;
+ }
+
+ ret = alloc_xstate_buffer(fpu, dynstate_mask);
if (ret)
goto out;
}
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 2fb08d1a96e6..cff0aa82ef53 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -961,6 +961,7 @@ void __init fpu__init_system_xstate(void)
goto out_disable;

/* Make sure init_task does not include the dynamic user states. */
+ current->thread.fpu.dynamic_state_perm = 0;
current->thread.fpu.state_mask = (xfeatures_mask_all & ~xfeatures_mask_user_dynamic);

/*
@@ -1233,6 +1234,73 @@ int alloc_xstate_buffer(struct fpu *fpu, u64 mask)
return 0;
}

+/**
+ * set_process_xstate_perm() - Set a per-process permission to use dynamic
+ * user xstates.
+ * @task: A struct task_struct * pointer
+ * @state_perm: A bitmap to indicate which state's permission to be set.
+ * Return: 0 if successful; otherwise, error code.
+ */
+long set_process_xstate_perm(struct task_struct *tsk, u64 state_perm)
+{
+ u64 req_dynstate_perm, old_dynstate_perm;
+ struct task_struct *t;
+ int nr_threads = 0;
+
+ if (!boot_cpu_has(X86_FEATURE_FPU))
+ return -EINVAL;
+
+ if (state_perm & ~xfeatures_mask_uabi())
+ return -EINVAL;
+
+ req_dynstate_perm = state_perm & xfeatures_mask_user_dynamic;
+ if (!req_dynstate_perm)
+ return 0;
+
+ old_dynstate_perm = tsk->thread.fpu.dynamic_state_perm;
+
+ for_each_thread(tsk, t) {
+ t->thread.fpu.dynamic_state_perm |= req_dynstate_perm;
+ nr_threads++;
+ }
+
+ if (nr_threads != tsk->signal->nr_threads) {
+ for_each_thread(tsk, t)
+ t->thread.fpu.dynamic_state_perm = old_dynstate_perm;
+ pr_err("x86/fpu: ARCH_XSTATE_PERM failed as thread number mismatched.\n");
+ return -EBUSY;
+ }
+ return 0;
+}
+
+/**
+ * reset_task_xstate_perm() - Reset a task's permission to use dynamic user
+ * xstates.
+ *
+ * It is expected to call at exec in which one task runs in a process.
+ *
+ * @task: A struct task_struct * pointer
+ */
+void reset_task_xstate_perm(struct task_struct *tsk)
+{
+ struct fpu *fpu = &tsk->thread.fpu;
+
+ if (!xfeatures_mask_user_dynamic)
+ return;
+
+ WARN_ON(tsk->signal->nr_threads > 1);
+
+ fpu->state_mask = (xfeatures_mask_all & ~xfeatures_mask_user_dynamic);
+ free_xstate_buffer(fpu);
+ fpu->state = &fpu->__default_state;
+ if (boot_cpu_has(X86_FEATURE_XSAVES))
+ fpstate_init_xstate(&fpu->state->xsave, fpu->state_mask);
+
+ xfd_write(xfd_capable() ^ (fpu->state_mask & xfd_capable()));
+
+ fpu->dynamic_state_perm = 0;
+}
+
static void copy_feature(bool from_xstate, struct membuf *to, void *xstate,
void *init_xstate, unsigned int size)
{
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index b85fa499f195..e39fbaca536a 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -1012,13 +1012,17 @@ 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_SET_XSTATE_ENABLE:
+ return set_process_xstate_perm(task, arg2);
+ case ARCH_GET_XSTATE_ENABLE:
+ return get_task_xstate_perm(task);
}

return -EINVAL;
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 41c9855158d6..065ea28328b9 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -678,6 +678,9 @@ void set_personality_64bit(void)
so it's not too bad. The main problem is just that
32bit children are affected again. */
current->personality &= ~READ_IMPLIES_EXEC;
+
+ /* Make sure to reset the dynamic state permission. */
+ reset_task_xstate_perm(current);
}

static void __set_personality_x32(void)
@@ -723,6 +726,9 @@ void set_personality_ia32(bool x32)
/* Make sure to be in 32bit mode */
set_thread_flag(TIF_ADDR32);

+ /* Make sure to reset the dynamic state permission. */
+ reset_task_xstate_perm(current);
+
if (x32)
__set_personality_x32();
else
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index f45b2cefd6cf..88ad28c79dc2 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -1132,10 +1132,12 @@ DEFINE_IDTENTRY(exc_device_not_available)
int err = -1;

/*
- * Make sure not in interrupt context as handling a
- * trap from userspace.
+ * Make sure that dynamic buffer expansion is permitted
+ * and not in interrupt context as handling a trap from
+ * userspace.
*/
- if (!WARN_ON(in_interrupt())) {
+ if (((xfd_event & fpu->dynamic_state_perm) == xfd_event) &&
+ !WARN_ON(in_interrupt())) {
err = alloc_xstate_buffer(fpu, xfd_event);
if (!err)
xfd_write((fpu->state_mask & xfd_capable()) ^
--
2.17.1

Next message: Chang S. Bae: "[PATCH v7 13/26] x86/fpu/xstate: Support ptracer-induced XSTATE buffer expansion"
Previous message: Chang S. Bae: "[PATCH v7 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state"
In reply to: Thiago Macieira: "Re: [PATCH v7 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state"
Next in thread: Chang S. Bae: "[PATCH v7 13/26] x86/fpu/xstate: Support ptracer-induced XSTATE buffer expansion"
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