kvm: x86: use correct privilege level for sgdt/sidt/fxsave/fxrstor access

commit 3c9fa24ca7c9c47605672916491f79e8ccacb9e6 upstream.

The functions that were used in the emulation of fxrstor, fxsave, sgdt and
sidt were originally meant for task switching, and as such they did not
check privilege levels.  This is very bad when the same functions are used
in the emulation of unprivileged instructions.  This is CVE-2018-10853.

The obvious fix is to add a new argument to ops->read_std and ops->write_std,
which decides whether the access is a "system" access or should use the
processor's CPL.

Fixes: 129a72a0d3c8 ("KVM: x86: Introduce segmented_write_std", 2017-01-12)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.16: Drop change in handle_ud()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index c10f7175bef32ecc8ed700ed35321a02395d5d80..8a37c3b7302b5ca48a758cc64b130ae39c1c8aea 100644 (file)
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -104,11 +104,12 @@ struct x86_emulate_ops {
         *  @addr:  [IN ] Linear address from which to read.
         *  @val:   [OUT] Value read from memory, zero-extended to 'u_long'.
         *  @bytes: [IN ] Number of bytes to read from memory.
+        *  @system:[IN ] Whether the access is forced to be at CPL0.
         */
        int (*read_std)(struct x86_emulate_ctxt *ctxt,
                        unsigned long addr, void *val,
                        unsigned int bytes,
-                       struct x86_exception *fault);
+                       struct x86_exception *fault, bool system);
 
        /*
         * write_std: Write bytes of standard (non-emulated/special) memory.
@@ -116,10 +117,11 @@ struct x86_emulate_ops {
         *  @addr:  [IN ] Linear address to which to write.
         *  @val:   [OUT] Value write to memory, zero-extended to 'u_long'.
         *  @bytes: [IN ] Number of bytes to write to memory.
+        *  @system:[IN ] Whether the access is forced to be at CPL0.
         */
        int (*write_std)(struct x86_emulate_ctxt *ctxt,
                         unsigned long addr, void *val, unsigned int bytes,
-                        struct x86_exception *fault);
+                        struct x86_exception *fault, bool system);
        /*
         * fetch: Read bytes of standard (non-emulated/special) memory.
         *        Used for instruction fetch.

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 4ae2eb93c70dfea1e91541393b4f3db202e8e76e..ede6797ee2038c9c26364377c849369f4e24bae1 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -734,14 +734,14 @@ static int linearize(struct x86_emulate_ctxt *ctxt,
 static int linear_read_system(struct x86_emulate_ctxt *ctxt, ulong linear,
                              void *data, unsigned size)
 {
-       return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception);
+       return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception, true);
 }
 
 static int linear_write_system(struct x86_emulate_ctxt *ctxt,
                               ulong linear, void *data,
                               unsigned int size)
 {
-       return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception);
+       return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception, true);
 }
 
 static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
@@ -755,7 +755,7 @@ static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
        rc = linearize(ctxt, addr, size, false, &linear);
        if (rc != X86EMUL_CONTINUE)
                return rc;
-       return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception);
+       return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception, false);
 }
 
 static int segmented_write_std(struct x86_emulate_ctxt *ctxt,
@@ -769,7 +769,7 @@ static int segmented_write_std(struct x86_emulate_ctxt *ctxt,
        rc = linearize(ctxt, addr, size, true, &linear);
        if (rc != X86EMUL_CONTINUE)
                return rc;
-       return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception);
+       return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception, false);
 }
 
 /*
@@ -2472,12 +2472,12 @@ static bool emulator_io_port_access_allowed(struct x86_emulate_ctxt *ctxt,
 #ifdef CONFIG_X86_64
        base |= ((u64)base3) << 32;
 #endif
-       r = ops->read_std(ctxt, base + 102, &io_bitmap_ptr, 2, NULL);
+       r = ops->read_std(ctxt, base + 102, &io_bitmap_ptr, 2, NULL, true);
        if (r != X86EMUL_CONTINUE)
                return false;
        if (io_bitmap_ptr + port/8 > desc_limit_scaled(&tr_seg))
                return false;
-       r = ops->read_std(ctxt, base + io_bitmap_ptr + port/8, &perm, 2, NULL);
+       r = ops->read_std(ctxt, base + io_bitmap_ptr + port/8, &perm, 2, NULL, true);
        if (r != X86EMUL_CONTINUE)
                return false;
        if ((perm >> bit_idx) & mask)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c9fe8342b18427ef8c3f1f7cd53d5f84462f305d..5fa09ee9a12eab9da22cfd4b4efcf35b0562663a 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4191,10 +4191,15 @@ EXPORT_SYMBOL_GPL(kvm_read_guest_virt);
 
 static int emulator_read_std(struct x86_emulate_ctxt *ctxt,
                             gva_t addr, void *val, unsigned int bytes,
-                            struct x86_exception *exception)
+                            struct x86_exception *exception, bool system)
 {
        struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
-       return kvm_read_guest_virt_helper(addr, val, bytes, vcpu, 0, exception);
+       u32 access = 0;
+
+       if (!system && kvm_x86_ops->get_cpl(vcpu) == 3)
+               access |= PFERR_USER_MASK;
+
+       return kvm_read_guest_virt_helper(addr, val, bytes, vcpu, access, exception);
 }
 
 static int kvm_write_guest_virt_helper(gva_t addr, void *val, unsigned int bytes,
@@ -4229,12 +4234,17 @@ out:
 }
 
 static int emulator_write_std(struct x86_emulate_ctxt *ctxt, gva_t addr, void *val,
-                             unsigned int bytes, struct x86_exception *exception)
+                             unsigned int bytes, struct x86_exception *exception,
+                             bool system)
 {
        struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+       u32 access = PFERR_WRITE_MASK;
+
+       if (!system && kvm_x86_ops->get_cpl(vcpu) == 3)
+               access |= PFERR_USER_MASK;
 
        return kvm_write_guest_virt_helper(addr, val, bytes, vcpu,
-                                          PFERR_WRITE_MASK, exception);
+                                          access, exception);
 }
 
 int kvm_write_guest_virt_system(struct kvm_vcpu *vcpu, gva_t addr, void *val,