[PATCH 21/27] x86/fpu: Let __fpu__restore_sig() restore the !32bit+fxsr frame from kernel memory

Sebastian Andrzej Siewior
2019-04-03T16:42:38Z

The !32bit+fxsr case loads the new state from user memory. In case we
restore the FPU state on return to userland we can't do this. It would
be required to disable preemption in order to avoid a context switch
which would set TIF_NEED_FPU_LOAD. If this happens before the "restore"
operation then the loaded registers would become volatile.

Disabling preemption while accessing user memory requires to disable the
pagefault handler. An error during XRSTOR would then mean that either a
page fault occured (and we have to retry with enabled page fault
handler) or a #GP occured because the xstate is bogus (after all the
sig-handler can modify it).

In order to avoid that mess, copy the FPU state from userland, validate
it and then load it. The copy_users_…() helper are basically the old
helper except that they operate on kernel memory and the fault handler
just sets the error value and the caller handles it.

copy_user_to_fpregs_zeroing() and its helpers remain and will be used
later for a fastpath optimisation.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
arch/x86/include/asm/fpu/internal.h | 43 ++++++++++++++++++++
arch/x86/kernel/fpu/signal.c | 62 +++++++++++++++++++++++------
2 files changed, 92 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index b12874b7cf0cf..037472d0be140 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -121,6 +121,21 @@ extern void fpstate_sanitize_xstate(struct fpu *fpu);
err; \
})

+#define kernel_insn_err(insn, output, input...) \
+({ \
+ int err; \
+ asm volatile("1:" #insn "\n\t" \
+ "2:\n" \
+ ".section .fixup,\"ax\"\n" \
+ "3: movl $-1,%[err]\n" \
+ " jmp 2b\n" \
+ ".previous\n" \
+ _ASM_EXTABLE(1b, 3b) \
+ : [err] "=r" (err), output \
+ : "0"(0), input); \
+ err; \
+})
+
#define kernel_insn(insn, output, input...) \
asm volatile("1:" #insn "\n\t" \
"2:\n" \
@@ -149,6 +164,14 @@ static inline void copy_kernel_to_fxregs(struct fxregs_state *fx)
kernel_insn(fxrstorq %[fx], "=m" (*fx), [fx] "m" (*fx));
}

+static inline int copy_kernel_to_fxregs_err(struct fxregs_state *fx)
+{
+ if (IS_ENABLED(CONFIG_X86_32))
+ return kernel_insn_err(fxrstor %[fx], "=m" (*fx), [fx] "m" (*fx));
+ else
+ return kernel_insn_err(fxrstorq %[fx], "=m" (*fx), [fx] "m" (*fx));
+}
+
static inline int copy_user_to_fxregs(struct fxregs_state __user *fx)
{
if (IS_ENABLED(CONFIG_X86_32))
@@ -162,6 +185,11 @@ static inline void copy_kernel_to_fregs(struct fregs_state *fx)
kernel_insn(frstor %[fx], "=m" (*fx), [fx] "m" (*fx));
}

+static inline int copy_kernel_to_fregs_err(struct fregs_state *fx)
+{
+ return kernel_insn_err(frstor %[fx], "=m" (*fx), [fx] "m" (*fx));
+}
+
static inline int copy_user_to_fregs(struct fregs_state __user *fx)
{
return user_insn(frstor %[fx], "=m" (*fx), [fx] "m" (*fx));
@@ -361,6 +389,21 @@ static inline int copy_user_to_xregs(struct xregs_state __user *buf, u64 mask)
return err;
}

+/*
+ * Restore xstate from kernel space xsave area, return an error code instead an
+ * exception.
+ */
+static inline int copy_kernel_to_xregs_err(struct xregs_state *xstate, u64 mask)
+{
+ u32 lmask = mask;
+ u32 hmask = mask >> 32;
+ int err;
+
+ XSTATE_OP(XRSTOR, xstate, lmask, hmask, err);
+
+ return err;
+}
+
/*
* These must be called with preempt disabled. Returns
* 'true' if the FPU state is still intact and we can
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index f733425c66d8d..8ee54a1958308 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -234,7 +234,8 @@ sanitize_restored_xstate(union fpregs_state *state,
*/
xsave->i387.mxcsr &= mxcsr_feature_mask;

- convert_to_fxsr(&state->fxsave, ia32_env);
+ if (ia32_env)
+ convert_to_fxsr(&state->fxsave, ia32_env);
}
}

@@ -337,28 +338,63 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
kfree(tmp);
return err;
} else {
+ union fpregs_state *state;
+ void *tmp;
int ret;

+ tmp = kzalloc(sizeof(*state) + fpu_kernel_xstate_size + 64, GFP_KERNEL);
+ if (!tmp)
+ return -ENOMEM;
+ state = PTR_ALIGN(tmp, 64);
+
/*
* For 64-bit frames and 32-bit fsave frames, restore the user
* state to the registers directly (with exceptions handled).
*/
- if (use_xsave()) {
- if ((unsigned long)buf_fx % 64 || fx_only) {
+ if ((unsigned long)buf_fx % 64)
+ fx_only = 1;
+
+ if (use_xsave() && !fx_only) {
+ u64 init_bv = xfeatures_mask & ~xfeatures;
+
+ if (using_compacted_format()) {
+ ret = copy_user_to_xstate(&state->xsave, buf_fx);
+ } else {
+ ret = __copy_from_user(&state->xsave, buf_fx, state_size);
+
+ if (!ret && state_size > offsetof(struct xregs_state, header))
+ ret = validate_xstate_header(&state->xsave.header);
+ }
+ if (ret)
+ goto err_out;
+ sanitize_restored_xstate(state, NULL, xfeatures,
+ fx_only);
+
+ if (unlikely(init_bv))
+ copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
+ ret = copy_kernel_to_xregs_err(&state->xsave, xfeatures);
+
+ } else if (use_fxsr()) {
+ ret = __copy_from_user(&state->fxsave, buf_fx, state_size);
+ if (ret)
+ goto err_out;
+
+ if (use_xsave()) {
u64 init_bv = xfeatures_mask & ~XFEATURE_MASK_FPSSE;
copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
- ret = copy_user_to_fxregs(buf_fx);
- } else {
- u64 init_bv = xfeatures_mask & ~xfeatures;
- if (unlikely(init_bv))
- copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
- ret = copy_user_to_xregs(buf_fx, xfeatures);
}
- } else if (use_fxsr()) {
- ret = copy_user_to_fxregs(buf_fx);
- } else
- ret = copy_user_to_fregs(buf_fx);
+ state->fxsave.mxcsr &= mxcsr_feature_mask;

+ ret = copy_kernel_to_fxregs_err(&state->fxsave);
+ } else {
+ ret = __copy_from_user(&state->fsave, buf_fx, state_size);
+ if (ret)
+ goto err_out;
+ ret = copy_kernel_to_fregs_err(&state->fsave);
+ }
+
+err_out:
+ kfree(tmp);
if (ret) {
fpu__clear(fpu);
return -1;
--
2.20.1