linux-kernel - [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in update_permission

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Message-Id: <20180619192504.180479-1-mka@chromium.org>
Date:   Tue, 19 Jun 2018 12:25:04 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in update_permission_bitmask()

update_permission_bitmask() negates u8 bitmask values and assigns them
to variables of type u8. Since the MSB is set in the bitmask values the
compiler expands the negated values to int, which then is assigned to
an u8 variable. Cast the negated value back to u8.

This fixes several warnings like this when building with clang:

arch/x86/kvm/mmu.c:4266:39: error: implicit conversion from 'int' to 'u8'
  (aka 'unsigned char') changes value from -205 to 51 [-Werror,
  -Wconstant-conversion]
    u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
       ~~                               ^~

(gcc also raises a warning (see https://godbolt.org/g/6JWfWk), however it
doesn't seem to be universally enabled)

Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
---
Changes in v2:
- negate the bitmask at initialization and rename variables to not_X

 arch/x86/kvm/mmu.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index d634f0332c0f..ad0a8c35f27b 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4258,8 +4258,9 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
 {
 	unsigned byte;
 
-	const u8 x = BYTE_MASK(ACC_EXEC_MASK);
-	const u8 w = BYTE_MASK(ACC_WRITE_MASK);
+	const u8 not_x = (u8)~BYTE_MASK(ACC_EXEC_MASK);
+	const u8 not_w = (u8)~BYTE_MASK(ACC_WRITE_MASK);
+	const u8 not_u = (u8)~BYTE_MASK(ACC_USER_MASK);
 	const u8 u = BYTE_MASK(ACC_USER_MASK);
 
 	bool cr4_smep = kvm_read_cr4_bits(vcpu, X86_CR4_SMEP) != 0;
@@ -4275,11 +4276,11 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
 		 */
 
 		/* Faults from writes to non-writable pages */
-		u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
+		u8 wf = (pfec & PFERR_WRITE_MASK) ? not_w : 0;
 		/* Faults from user mode accesses to supervisor pages */
-		u8 uf = (pfec & PFERR_USER_MASK) ? ~u : 0;
+		u8 uf = (pfec & PFERR_USER_MASK) ? not_u : 0;
 		/* Faults from fetches of non-executable pages*/
-		u8 ff = (pfec & PFERR_FETCH_MASK) ? ~x : 0;
+		u8 ff = (pfec & PFERR_FETCH_MASK) ? not_x : 0;
 		/* Faults from kernel mode fetches of user pages */
 		u8 smepf = 0;
 		/* Faults from kernel mode accesses of user pages */
-- 
2.18.0.rc2.346.g013aa6912e-goog