[PATCH] x86/fpu: Fix PKRU state saving for coredump

["Chang S. Bae" <chang.seok.bae@intel.com>]

Christina reported mismatched PKRU values from GDB coredump tests [1].

PKRU is stored in thread.pkru rather than in fpstate. Then the coredump
path currently syncs fpstate only and hands thread.pkru to
copy_xstate_to_uabi_buf(), so a task that still owns the register state
can end up dumping a stale PKRU value.

Save PKRU from the register when the target is the current task.

Fixes: e84ba47e313d ("x86/fpu: Hook up PKRU into ptrace()")
Closes: https://sourceware.org/bugzilla/show_bug.cgi?id=34646 # [1]
Reported-by: Christina Joos <christina.joos@intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Tested-by: Christina Joos <christina.joos@intel.com>
Cc: stable@vger.kernel.org
---
Testing:
Christina verified the fix through the GDB tests that originally exposed
the issue. Also a simple test that sets PKRU and triggers a coredump by
dereferencing a NULL pointer can verify the saved PKRU from the corefile
read.

The existing ptrace selftests cover most of this path but not completely,
e.g. not checking back from a corefile. So it may worth adding coredump
tests for XSTATEs in general. I'd follow up on this separately.

Code changes:
The PKRU saving could be folded into sync_fpstate() similar to
restore_fpregs_from_init_fpstate(). Instead, since this is on the saving
path and PKRU storage is separate, sync_pkru() makes that dinstinction
more explicit along with the fix.
---
 arch/x86/kernel/fpu/regset.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
index 0986c2200adc..213a06fd72f3 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -49,6 +49,13 @@ static void sync_fpstate(struct fpu *fpu)
 		fpu_sync_fpstate(fpu);
 }
 
+/* Keep the same invocation context with sync_fpstate() */
+static void sync_pkru(struct task_struct *target)
+{
+	if (target == current)
+		target->thread.pkru = read_pkru();
+}
+
 /*
  * Invalidate cached FPU registers before modifying the stopped target
  * task's fpstate.
@@ -135,6 +142,9 @@ int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
 
 	sync_fpstate(x86_task_fpu(target));
 
+	/* copy_xstate_to_uabi_buf() takes PKRU from thread.pkru */
+	sync_pkru(target);
+
 	copy_xstate_to_uabi_buf(to, target, XSTATE_COPY_XSAVE);
 	return 0;
 }
-- 
2.53.0