[PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state

Chang S. Bae
2021-05-23T19:39:36Z

Intel's Extended Feature Disable (XFD) feature is an extension of the XSAVE
architecture. XFD allows the kernel to enable a feature state in XCR0 and
to receive a #NM trap when a task uses instructions accessing that state.
In this way, Linux can control the access to that state.

XFD introduces two MSRs: IA32_XFD to enable/disable the feature and
IA32_XFD_ERR to assist the #NM trap handler. Both use the same
xstate-component bitmap format, used by XCR0.

Use XFD to detect unauthorized xstate access and raise a SIGSEGV.

Introduce helper functions:
xfd_write() - write IA32_XFD MSR
xfd_read() - read IA32_XFD MSR
xfd_switch() - switch IA32_XFD MSR
xfd_capable() - indicate XFD-capable features

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v4:
* Changed to use XFD to support the access request policy. Updated #NM
handler to raise a signal instead of buffer allocation.
* Updated helper functions.
* Updated function descriptions in a proper format.
* Updated some code comments.

Changes from v3:
* Removed 'no functional change' in the changelog. (Borislav Petkov)

Changes from v2:
* Changed to enable XFD only when the compacted format is used.
* Updated the changelog with task->fpu removed. (Borislav Petkov)

Changes from v1:
* Inlined the XFD-induced #NM handling code (Andy Lutomirski)
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/fpu/internal.h | 45 ++++++++++++++++++++++++++++-
arch/x86/include/asm/msr-index.h | 2 ++
arch/x86/kernel/cpu/cpuid-deps.c | 1 +
arch/x86/kernel/fpu/xstate.c | 45 +++++++++++++++++++++++++++--
arch/x86/kernel/process_32.c | 2 +-
arch/x86/kernel/process_64.c | 2 +-
arch/x86/kernel/traps.c | 12 ++++++++
8 files changed, 104 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index ac37830ae941..67a9e5282128 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -277,6 +277,7 @@
#define X86_FEATURE_XSAVEC (10*32+ 1) /* XSAVEC instruction */
#define X86_FEATURE_XGETBV1 (10*32+ 2) /* XGETBV with ECX = 1 instruction */
#define X86_FEATURE_XSAVES (10*32+ 3) /* XSAVES/XRSTORS instructions */
+#define X86_FEATURE_XFD (10*32+ 4) /* eXtended Feature Disabling */

/*
* Extended auxiliary flags: Linux defined - for features scattered in various
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index f964f3efc92e..c250216320df 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -557,11 +557,52 @@ static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
* Misc helper functions:
*/

+/* The Extended Feature Disable (XFD) helpers: */
+
+static inline void xfd_write(u64 value)
+{
+ wrmsrl_safe(MSR_IA32_XFD, value);
+}
+
+static inline u64 xfd_read(void)
+{
+ u64 value;
+
+ rdmsrl_safe(MSR_IA32_XFD, &value);
+ return value;
+}
+
+static inline u64 xfd_capable(void)
+{
+ return xfeatures_mask_user_dynamic;
+}
+
+/**
+ * xfd_switch() - Switches the MSR IA32_XFD context if needed.
+ * @prev: The previous task's struct fpu pointer
+ * @next: The next task's struct fpu pointer
+ *
+ * Returns: Nothing
+ */
+static inline void xfd_switch(struct fpu *prev, struct fpu *next)
+{
+ u64 prev_xfd_mask, next_xfd_mask;
+
+ if (!static_cpu_has(X86_FEATURE_XFD) || !xfd_capable())
+ return;
+
+ prev_xfd_mask = prev->state_mask & xfd_capable();
+ next_xfd_mask = next->state_mask & xfd_capable();
+
+ if (unlikely(prev_xfd_mask != next_xfd_mask))
+ xfd_write(xfd_capable() ^ next_xfd_mask);
+}
+
/*
* Load PKRU from the FPU context if available. Delay loading of the
* complete FPU state until the return to userland.
*/
-static inline void switch_fpu_finish(struct fpu *new_fpu)
+static inline void switch_fpu_finish(struct fpu *old_fpu, struct fpu *new_fpu)
{
u32 pkru_val = init_pkru_value;
struct pkru_state *pk;
@@ -571,6 +612,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)

set_thread_flag(TIF_NEED_FPU_LOAD);

+ xfd_switch(old_fpu, new_fpu);
+
if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
return;

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 211ba3375ee9..f9f92ab71fc1 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -626,6 +626,8 @@
#define MSR_IA32_BNDCFGS_RSVD 0x00000ffc

#define MSR_IA32_XSS 0x00000da0
+#define MSR_IA32_XFD 0x000001c4
+#define MSR_IA32_XFD_ERR 0x000001c5

#define MSR_IA32_APICBASE 0x0000001b
#define MSR_IA32_APICBASE_BSP (1<<8)
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index defda61f372d..7f891d2eb52e 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -75,6 +75,7 @@ static const struct cpuid_dep cpuid_deps[] = {
{ X86_FEATURE_SGX_LC, X86_FEATURE_SGX },
{ X86_FEATURE_SGX1, X86_FEATURE_SGX },
{ X86_FEATURE_SGX2, X86_FEATURE_SGX1 },
+ { X86_FEATURE_XFD, X86_FEATURE_XSAVE },
{}
};

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 299373669a5d..e60a20a1b24b 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -175,6 +175,26 @@ static bool xfeature_is_supervisor(int xfeature_nr)
return ecx & 1;
}

+/**
+ * xfd_supported() - Check if the feature supports Extended Feature Disable (XFD).
+ * @feature_nr: The feature number.
+ *
+ * Returns: Nothing
+ */
+static bool xfd_supported(int feature_nr)
+{
+ u32 eax, ebx, ecx, edx;
+
+ if (!boot_cpu_has(X86_FEATURE_XFD))
+ return false;
+
+ /*
+ * If state component 'i' supports XFD, ECX[2] return 1; otherwise, 0.
+ */
+ cpuid_count(XSTATE_CPUID, feature_nr, &eax, &ebx, &ecx, &edx);
+ return ecx & 4;
+}
+
/**
* get_xstate_comp_offset() - Find the feature's offset in the compacted format
* @mask: This bitmap tells which components reserved in the format.
@@ -366,6 +386,9 @@ void fpu__init_cpu_xstate(void)
wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor() |
xfeatures_mask_supervisor_dynamic());
}
+
+ if (boot_cpu_has(X86_FEATURE_XFD))
+ xfd_write(xfd_capable());
}

static bool xfeature_enabled(enum xfeature xfeature)
@@ -565,8 +588,9 @@ static void __init print_xstate_offset_size(void)
for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
if (!xfeature_enabled(i))
continue;
- pr_info("x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d\n",
- i, xstate_comp_offsets[i], i, xstate_sizes[i]);
+ pr_info("x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d (%s)\n",
+ i, xstate_comp_offsets[i], i, xstate_sizes[i],
+ (xfeatures_mask_user_dynamic & BIT_ULL(i)) ? "dynamic" : "default");
}
}

@@ -999,9 +1023,18 @@ void __init fpu__init_system_xstate(void)
}

xfeatures_mask_all &= fpu__get_supported_xfeatures_mask();
- /* Do not support the dynamically allocated buffer yet. */
xfeatures_mask_user_dynamic = 0;

+ for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
+ u64 feature_mask = BIT_ULL(i);
+
+ if (!(xfeatures_mask_user() & feature_mask))
+ continue;
+
+ if (xfd_supported(i))
+ xfeatures_mask_user_dynamic |= feature_mask;
+ }
+
/* Enable xstate instructions to be able to continue with initialization: */
fpu__init_cpu_xstate();
err = init_xstate_size();
@@ -1053,6 +1086,12 @@ void fpu__resume_cpu(void)
wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor() |
xfeatures_mask_supervisor_dynamic());
}
+
+ if (boot_cpu_has(X86_FEATURE_XFD)) {
+ u64 fpu_xfd_mask = current->thread.fpu.state_mask & xfd_capable();
+
+ xfd_write(xfd_capable() ^ fpu_xfd_mask);
+ }
}

/**
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 4f2f54e1281c..7bd5d08eeb41 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -213,7 +213,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)

this_cpu_write(current_task, next_p);

- switch_fpu_finish(next_fpu);
+ switch_fpu_finish(prev_fpu, next_fpu);

/* Load the Intel cache allocation PQR MSR. */
resctrl_sched_in();
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index d08307df69ad..5375a869f3f3 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -594,7 +594,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
this_cpu_write(current_task, next_p);
this_cpu_write(cpu_current_top_of_stack, task_top_of_stack(next_p));

- switch_fpu_finish(next_fpu);
+ switch_fpu_finish(prev_fpu, next_fpu);

/* Reload sp0. */
update_task_stack(next_p);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 853ea7a80806..7482448fcdca 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -1111,6 +1111,18 @@ DEFINE_IDTENTRY(exc_device_not_available)
{
unsigned long cr0 = read_cr0();

+ if (boot_cpu_has(X86_FEATURE_XFD)) {
+ u64 event_mask;
+
+ rdmsrl_safe(MSR_IA32_XFD_ERR, &event_mask);
+ wrmsrl_safe(MSR_IA32_XFD_ERR, 0);
+
+ if (event_mask) {
+ force_sig(SIGSEGV);
+ return;
+ }
+ }
+
#ifdef CONFIG_MATH_EMULATION
if (!boot_cpu_has(X86_FEATURE_FPU) && (cr0 & X86_CR0_EM)) {
struct math_emu_info info = { };
--
2.17.1