[PATCH v6 1/2] x86/fpu: Allow PKRU to be (once again) written by ptrace.

[Kyle Huey]

From: Kyle Huey <me@xxxxxxxxxxxx>

When management of the PKRU register was moved away from XSTATE, emulation
of PKRU's existence in XSTATE was added for reading PKRU through ptrace,
but not for writing PKRU through ptrace. This can be seen by running gdb
and executing `p $pkru`, `set $pkru = 42`, and `p $pkru`. On affected
kernels (5.14+) the write to the PKRU register (which gdb performs through
ptrace) is ignored.

There are three APIs that write PKRU: sigreturn, PTRACE_SETREGSET with
NT_X86_XSTATE, and KVM_SET_XSAVE. sigreturn still uses XRSTOR to write to
PKRU. KVM_SET_XSAVE has its own special handling to make PKRU writes take
effect (in fpu_copy_uabi_to_guest_fpstate). Push that down into
copy_uabi_to_xstate and have PTRACE_SETREGSET with NT_X86_XSTATE pass in
a pointer to the appropriate PKRU slot. copy_sigframe_from_user_to_xstate
depends on copy_uabi_to_xstate populating the PKRU field in the task's
XSTATE so that __fpu_restore_sig can do a XRSTOR from it, so continue doing
that.

This also adds code to initialize the PKRU value to the hardware init value
(namely 0) if the PKRU bit is not set in the XSTATE header provided to
ptrace, to match XRSTOR.

Fixes: e84ba47e313d ("x86/fpu: Hook up PKRU into ptrace()")
Signed-off-by: Kyle Huey <me@xxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 5.14+
---
 arch/x86/kernel/fpu/core.c   | 20 +++++++++-----------
 arch/x86/kernel/fpu/regset.c |  2 +-
 arch/x86/kernel/fpu/signal.c |  2 +-
 arch/x86/kernel/fpu/xstate.c | 25 ++++++++++++++++++++-----
 arch/x86/kernel/fpu/xstate.h |  4 ++--
 5 files changed, 33 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 3b28c5b25e12..c273669e8a00 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -391,8 +391,6 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
 {
 	struct fpstate *kstate = gfpu->fpstate;
 	const union fpregs_state *ustate = buf;
-	struct pkru_state *xpkru;
-	int ret;
 
 	if (!cpu_feature_enabled(X86_FEATURE_XSAVE)) {
 		if (ustate->xsave.header.xfeatures & ~XFEATURE_MASK_FPSSE)
@@ -406,16 +404,16 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
 	if (ustate->xsave.header.xfeatures & ~xcr0)
 		return -EINVAL;
 
-	ret = copy_uabi_from_kernel_to_xstate(kstate, ustate);
-	if (ret)
-		return ret;
+	/*
+	 * Nullify @vpkru to preserve its current value if PKRU's bit isn't set
+	 * in the header.  KVM's odd ABI is to leave PKRU untouched in this
+	 * case (all other components are eventually re-initialized).
+	 * (Not clear that this is actually necessary for compat).
+	 */
+	if (!(ustate->xsave.header.xfeatures & XFEATURE_MASK_PKRU))
+		vpkru = NULL;
 
-	/* Retrieve PKRU if not in init state */
-	if (kstate->regs.xsave.header.xfeatures & XFEATURE_MASK_PKRU) {
-		xpkru = get_xsave_addr(&kstate->regs.xsave, XFEATURE_PKRU);
-		*vpkru = xpkru->pkru;
-	}
-	return 0;
+	return copy_uabi_from_kernel_to_xstate(kstate, ustate, vpkru);
 }
 EXPORT_SYMBOL_GPL(fpu_copy_uabi_to_guest_fpstate);
 #endif /* CONFIG_KVM */
diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
index 75ffaef8c299..6d056b68f4ed 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -167,7 +167,7 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 	}
 
 	fpu_force_restore(fpu);
-	ret = copy_uabi_from_kernel_to_xstate(fpu->fpstate, kbuf ?: tmpbuf);
+	ret = copy_uabi_from_kernel_to_xstate(fpu->fpstate, kbuf ?: tmpbuf, &target->thread.pkru);
 
 out:
 	vfree(tmpbuf);
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 91d4b6de58ab..558076dbde5b 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -396,7 +396,7 @@ static bool __fpu_restore_sig(void __user *buf, void __user *buf_fx,
 
 	fpregs = &fpu->fpstate->regs;
 	if (use_xsave() && !fx_only) {
-		if (copy_sigframe_from_user_to_xstate(fpu->fpstate, buf_fx))
+		if (copy_sigframe_from_user_to_xstate(tsk, buf_fx))
 			return false;
 	} else {
 		if (__copy_from_user(&fpregs->fxsave, buf_fx,
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c8340156bfd2..8f14981a3936 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1197,7 +1197,7 @@ static int copy_from_buffer(void *dst, unsigned int offset, unsigned int size,
 
 
 static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
-			       const void __user *ubuf)
+			       const void __user *ubuf, u32 *pkru)
 {
 	struct xregs_state *xsave = &fpstate->regs.xsave;
 	unsigned int offset, size;
@@ -1246,6 +1246,21 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
 		}
 	}
 
+	/*
+	 * Update the user protection key storage. Allow KVM to
+	 * pass in a NULL pkru pointer if the mask bit is unset
+	 * for its legacy ABI behavior.
+	 */
+	if (pkru)
+		*pkru = 0;
+
+	if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
+		struct pkru_state *xpkru;
+
+		xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU);
+		*pkru = xpkru->pkru;
+	}
+
 	/*
 	 * The state that came in from userspace was user-state only.
 	 * Mask all the user states out of 'xfeatures':
@@ -1264,9 +1279,9 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
  * Convert from a ptrace standard-format kernel buffer to kernel XSAVE[S]
  * format and copy to the target thread. Used by ptrace and KVM.
  */
-int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf)
+int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf, u32 *pkru)
 {
-	return copy_uabi_to_xstate(fpstate, kbuf, NULL);
+	return copy_uabi_to_xstate(fpstate, kbuf, NULL, pkru);
 }
 
 /*
@@ -1274,10 +1289,10 @@ int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf)
  * XSAVE[S] format and copy to the target thread. This is called from the
  * sigreturn() and rt_sigreturn() system calls.
  */
-int copy_sigframe_from_user_to_xstate(struct fpstate *fpstate,
+int copy_sigframe_from_user_to_xstate(struct task_struct *tsk,
 				      const void __user *ubuf)
 {
-	return copy_uabi_to_xstate(fpstate, NULL, ubuf);
+	return copy_uabi_to_xstate(tsk->thread.fpu.fpstate, NULL, ubuf, &tsk->thread.pkru);
 }
 
 static bool validate_independent_components(u64 mask)
diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
index 5ad47031383b..a4ecb04d8d64 100644
--- a/arch/x86/kernel/fpu/xstate.h
+++ b/arch/x86/kernel/fpu/xstate.h
@@ -46,8 +46,8 @@ extern void __copy_xstate_to_uabi_buf(struct membuf to, struct fpstate *fpstate,
 				      u32 pkru_val, enum xstate_copy_mode copy_mode);
 extern void copy_xstate_to_uabi_buf(struct membuf to, struct task_struct *tsk,
 				    enum xstate_copy_mode mode);
-extern int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf);
-extern int copy_sigframe_from_user_to_xstate(struct fpstate *fpstate, const void __user *ubuf);
+extern int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf, u32 *pkru);
+extern int copy_sigframe_from_user_to_xstate(struct task_struct *tsk, const void __user *ubuf);
 
 
 extern void fpu__init_cpu_xstate(void);
-- 
2.37.2

Changelog since v5:
- Avoids a second copy from the uabi buffer as suggested.
- Preserves old KVM_SET_XSAVE behavior where leaving the PKRU bit in the
  XSTATE header results in PKRU remaining unchanged instead of
  reinitializing it.
- Fixed up patch metadata as requested.

Changelog since v4:
- Selftest additionally checks PKRU readbacks through ptrace.
- Selftest flips all PKRU bits (except the default key).

Changelog since v3:
- The v3 patch is now part 1 of 2.
- Adds a selftest in part 2 of 2.

Changelog since v2:
- Removed now unused variables in fpu_copy_uabi_to_guest_fpstate

Changelog since v1:
- Handles the error case of copy_to_buffer().