Re: [PATCH v2 2/8] x86/fpu/xstate: Separate user and supervisor xfeatures mask

Borislav Petkov
2020-02-21T10:34:44Z

On Tue, Jan 21, 2020 at 12:18:37PM -0800, Yu-cheng Yu wrote:
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 400a05e1c1c5..7c7f3efa3c57 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -254,11 +254,14 @@ static int copy_user_to_fpregs_zeroing(void __user *buf, u64 xbv, int fx_only)
> {
> if (use_xsave()) {
> if (fx_only) {
> - u64 init_bv = xfeatures_mask & ~XFEATURE_MASK_FPSSE;
> + u64 init_bv;
> +
> + init_bv = xfeatures_mask_user() & ~XFEATURE_MASK_FPSSE;
> copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
> return copy_user_to_fxregs(buf);
> } else {
> - u64 init_bv = xfeatures_mask & ~xbv;
> + u64 init_bv = xfeatures_mask_user() & ~xbv;
> +
> if (unlikely(init_bv))
> copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
> return copy_user_to_xregs(buf, xbv);

If you don't want to overflow 80 cols here in the fx_only() case, you can do:

---
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 7c7f3efa3c57..d7e94677cd31 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -252,15 +252,16 @@ sanitize_restored_xstate(union fpregs_state *state,
*/
static int copy_user_to_fpregs_zeroing(void __user *buf, u64 xbv, int fx_only)
{
+ u64 init_bv;
+
if (use_xsave()) {
if (fx_only) {
- u64 init_bv;
-
init_bv = xfeatures_mask_user() & ~XFEATURE_MASK_FPSSE;
+
copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
return copy_user_to_fxregs(buf);
} else {
- u64 init_bv = xfeatures_mask_user() & ~xbv;
+ init_bv = xfeatures_mask_user() & ~xbv;

if (unlikely(init_bv))
copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);

...

> @@ -210,19 +211,24 @@ void fpstate_sanitize_xstate(struct fpu *fpu)
> */
> void fpu__init_cpu_xstate(void)
> {
> - if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask)
> + if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask_all)
> return;
> /*
> * Unsupported supervisor xstates should not be found in
> * the xfeatures mask.
> */
> - WARN_ONCE((xfeatures_mask & UNSUPPORTED_XFEATURES_MASK_SUPERVISOR),
> + WARN_ONCE((xfeatures_mask_all & UNSUPPORTED_XFEATURES_MASK_SUPERVISOR),
> "x86/fpu: Found unsupported supervisor xstates.\n");

Let's say which to ease debugging:

u64 unsup_bits = xfeatures_mask_all & UNSUPPORTED_XFEATURES_MASK_SUPERVISOR;

...

WARN_ONCE(unsup_bits, "x86/fpu: Found unsupported supervisor xstates: 0x%llx\n",
unsup_bits);

>
> - xfeatures_mask &= ~UNSUPPORTED_XFEATURES_MASK_SUPERVISOR;
> + xfeatures_mask_all &= ~UNSUPPORTED_XFEATURES_MASK_SUPERVISOR;
>
> cr4_set_bits(X86_CR4_OSXSAVE);
> - xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask);

<---- newline here.

> + /*
> + * XCR_XFEATURE_ENABLED_MASK (aka. XCR0) sets user features
> + * managed by XSAVE{C, OPT, S} and XRSTOR{S}. Only XSAVE user
> + * states can be set here.
> + */
> + xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask_user());
> }
>
> /*
> @@ -232,7 +238,7 @@ void fpu__init_cpu_xstate(void)
> */
> static int xfeature_enabled(enum xfeature xfeature)

static bool

> {
> - return !!(xfeatures_mask & (1UL << xfeature));
> + return !!(xfeatures_mask_all & (1UL << xfeature));

return xfeatures_mask_all & BIT(xfeature);

while at it.

> }
>
> /*
> @@ -419,7 +425,7 @@ static void __init setup_init_fpu_buf(void)
>
> if (boot_cpu_has(X86_FEATURE_XSAVES))
> init_fpstate.xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
> - xfeatures_mask;
> + xfeatures_mask_all;
>
> /*
> * Init all the features state with header.xfeatures being 0x0
> @@ -479,7 +485,7 @@ int using_compacted_format(void)
> int validate_xstate_header(const struct xstate_header *hdr)
> {
> /* No unknown or supervisor features may be set */
> - if (hdr->xfeatures & ~(xfeatures_mask & SUPPORTED_XFEATURES_MASK_USER))
> + if (hdr->xfeatures & ~xfeatures_mask_user())
> return -EINVAL;
>
> /* Userspace must use the uncompacted format */
> @@ -614,7 +620,7 @@ static void do_extra_xstate_size_checks(void)
>
>
> /*
> - * Get total size of enabled xstates in XCR0/xfeatures_mask.
> + * Get total size of enabled xstates in XCR0 | IA32_XSS.
> *
> * Note the SDM's wording here. "sub-function 0" only enumerates
> * the size of the *user* states. If we use it to size a buffer
> @@ -704,7 +710,7 @@ static int __init init_xstate_size(void)
> */
> static void fpu__init_disable_system_xstate(void)
> {
> - xfeatures_mask = 0;
> + xfeatures_mask_all = 0;
> cr4_clear_bits(X86_CR4_OSXSAVE);
> setup_clear_cpu_cap(X86_FEATURE_XSAVE);
> }
> @@ -739,16 +745,22 @@ void __init fpu__init_system_xstate(void)
> return;
> }
>
> + /*
> + * Find user xstates supported by the processor.
> + */
> cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
> - xfeatures_mask = eax + ((u64)edx << 32);
> + xfeatures_mask_all = eax + ((u64)edx << 32);
> +
> + /* Place supervisor features in xfeatures_mask_all here */
>

Superfluous newline.

> - if ((xfeatures_mask & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {
> + if ((xfeatures_mask_user() & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {
> /*
> * This indicates that something really unexpected happened
> * with the enumeration. Disable XSAVE and try to continue
> * booting without it. This is too early to BUG().
> */
> - pr_err("x86/fpu: FP/SSE not present amongst the CPU's xstate features: 0x%llx.\n", xfeatures_mask);
> + pr_err("x86/fpu: FP/SSE not present amongst the CPU's xstate features: 0x%llx.\n",
> + xfeatures_mask_all);
> goto out_disable;
> }

...

--
Regards/Gruss,
Boris.