git: 153643a5bc8a - main - amd64: do not enable PKRU if user disabled saving PKRU register in xsave mask

The branch main has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=153643a5bc8ac3b1d47fc8e4de18d8c8a59817da

commit 153643a5bc8ac3b1d47fc8e4de18d8c8a59817da
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2023-01-27 10:45:02 +0000
Commit:     Konstantin Belousov <kib@FreeBSD.org>
CommitDate: 2023-01-27 17:44:49 +0000

    amd64: do not enable PKRU if user disabled saving PKRU register in xsave mask
    
    This is done by reverting CR4_PKE bit, because we perform %CR4
    initialization in initializecpu(), and the function is called before
    xsave_mask is read.  To not redo the whole early initialization
    sequence for the corner case, this should be good enough.
    
    Reported by:    jhb
    Reviewed by:    jhb, markj
    Sponsored by:   The FreeBSD Foundation
    MFC after:      1 week
    Differential revision:  https://reviews.freebsd.org/D38219
---
 sys/amd64/amd64/fpu.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/sys/amd64/amd64/fpu.c b/sys/amd64/amd64/fpu.c
index e9b058f175b0..64974a7210a9 100644
--- a/sys/amd64/amd64/fpu.c
+++ b/sys/amd64/amd64/fpu.c
@@ -372,6 +372,7 @@ void
 fpuinit(void)
 {
 	register_t saveintr;
+	uint64_t cr4;
 	u_int mxcsr;
 	u_short control;
 
@@ -379,7 +380,22 @@ fpuinit(void)
 		fpuinit_bsp1();
 
 	if (use_xsave) {
-		load_cr4(rcr4() | CR4_XSAVE);
+		cr4 = rcr4();
+
+		/*
+		 * Revert enablement of PKRU if user disabled its
+		 * saving on context switches by clearing the bit in
+		 * the xsave mask.  Also redundantly clear the bit in
+		 * cpu_stdext_feature2 to prevent pmap from ever
+		 * trying to set the page table bits.
+		 */
+		if ((cpu_stdext_feature2 & CPUID_STDEXT2_PKU) != 0 &&
+		    (xsave_mask & XFEATURE_ENABLED_PKRU) == 0) {
+			cr4 &= ~CR4_PKE;
+			cpu_stdext_feature2 &= ~CPUID_STDEXT2_PKU;
+		}
+
+		load_cr4(cr4 | CR4_XSAVE);
 		load_xcr(XCR0, xsave_mask);
 	}